BIO214 Lecture Notes Part 3 PDF

Protein Metabolism Including the Nitrogen Web Importance of Nitrogen in Biochemistry - Nitrogen (with H, O, and C) is a major elemental constituent of living organisms - Mostly in nucleic acids and proteins (amino acids) - But also found in: - several cofactors (NAD, FAD, biotin …...

Protein Metabolism Including the Nitrogen Web Importance of Nitrogen in Biochemistry - Nitrogen (with H, O, and C) is a major elemental constituent of living organisms - Mostly in nucleic acids and proteins (amino acids) - But also found in: - several cofactors (NAD, FAD, biotin … ) - many small hormones (epinephrine) - many neurotransmitters (serotonin) - many pigments (chlorophyll) - many defense chemicals (amanitin; see Figure) Biochemistry of Molecular Nitrogen Molecular nitrogen (N2) comprises 80% of the Earth’s atmosphere but is virtually unusable by most organisms due to the highly resistant N≡N bond - Need N2 + 3 H2 → 2 NH3 (NH3 + H2O → NH4+ + OH- ) Nitrogen fixation - Main process by which nitrogen-fixing bacteria and archea convert N2 to NH3 (biologically useful form) Some nitrogen-fixing bacteria (diazotrophs) and archea live in symbiosis with other organisms - Plants (e.g., proteobacteria with legumes such as peanuts, beans) - Animals (e.g., spirochaete with termites) Combined, this represents 60% of newly fixed nitrogen - Ammonia can be used by all organisms either directly or indirectly (after it has been converted to nitrites, nitrates or amino acids) Nitrogen-Fixing Nodules Rhizobium leguminosarum is N2-fixing a bacterium associated with legumes Biochemistry of Molecular Nitrogen A few non-biological processes can convert N2 to biologically useful forms - N2 and O2 → NO via lightning (represents 25% of newly fixed N2) - N2 and H2 → NH3 (ammonia) via the industrial Haber (Haber-Bosch) process - (requires T > 400°C, P > 200 atm; represents 15% of newly fixed N2) - Biological nitrogen fixation must occur at biological temperatures and at 0.8 atm of nitrogen… → partly helped through ATP hydrolysis The Nitrogen Web (don’t need to memorise, but need to understand what’s happening) - Movement of nitrogen through the biosphere has been viewed historically as a cycle - Our evolving understanding makes it clear that nitrogen moves through a complex web, rather than in a neat cycle - Fixation (key step) - Nitrification - Denitrification - Anammox - Assimilation (key step) Chemical transformations maintain a balance between N2 and biologically useful forms of nitrogen 1. Fixation: Reduction of N2 → NH3/NH4+ (ammonia/ammonium) 2. Nitrification: Bacteria oxidize NH3 into NO2 – (nitrite) and NO3 – (nitrate) 3. Denitrification: NO3 – / NO2 – is reduced to N2 under anaerobic conditions 4. Anammox: Anaerobic ammonia oxidation = alternative pathway back to N2 5. Assimilation: Plants and microorganisms reduce NO3 – and NO2 – to NH3/NH4+ via nitrate reductases and nitrite reductases Following Fixation or Assimilation ammonia in its reduced form (NH4+) is assimilated into amino acids and biomolecules - NH4+ in plants is incorporated into amino acids and other nitrogen-containing biomolecules - Animals eat plants as source of amino acids - Organisms die, returning NH4+ to soil - Nitrifying bacteria again convert NH4+/NH3 to NO2 – and NO3 – Ammonia is Incorporated into Biomolecules through Glutamate and Glutamine - Glutamate and glutamine are the crucial entry points for incorporating NH4+ into biomolecules (NH4+ assimilation), which his mediated by two key reactions Reaction 1: (Don’t need to know full reactions only the bold parts) Glutamate + NH4+ + ATP → Glutamine + ADP + Pi + H+ (via glutamine synthetase) - Glutamine synthetase is present in all organisms Reaction 2: α-Ketoglutarate + Glutamine + NAD(P)H + H+ → 2 Glutamate + NAD(P)+ (via glutamate synthase) - Glutamate synthase is present in bacteria and plants, but animals maintain glutamate amounts through transamination of α-ketoglutarate (see Slide 17) - Glutamine is made from glutamate by glutamine synthetase in a two-step process - Phosphorylation of glutamate creates a good leaving group that can be easily displaced by ammonia Glutamine Synthetase is a Primary Regulatory Point in Nitrogen Metabolism (don’t need to know details) Regulation of Glutamine Synthetase by Allosteric Inhibitors Glutamine synthetase: - Found in all organisms - Plays a central role in the conversion of toxic free NH3 to glutamine and in the metabolism of amino acids - Undergoes cumulative regulation by six end products of glutamine metabolism Glutamine Synthetase is also Inhibited by Adenylylation (don’t need to know details)(need to know stimulates and what inhibits) - Adenylylation is a post-translational modification whereby AMP binds to a hydroxyl group of a molecule via adenylyltransferase (AT) - Adenylylation inhibits glutamine synthetase (off switch) - Adenylylation is indirectly: - Stimulated by glutamine and Pi - Inhibited by α-ketoglutarate and ATP Transamination as a Nitrogen Source (don’t need to know details) - Transamination is the transfer of an amino group from one molecule to another - Catalysed by aminotransferases and which use pyridoxal phosphate (PLP) as a cofactor - Glutamate acts as a temporary storage of nitrogen - Glutamate donates the amino group when needed for amino acid biosynthesis, thus readily reversible - Glutamate is the source of amino groups for most other amino acids in mammals α-Ketoglutarate + L-Amino acid ⇄ L-Glutamate + α-Ketoacid - Note: reaction is reversible Ammonia is Incorporated into Biomolecules through Glutamate and Glutamine - A third but minor reaction of glutamate formation is mediated via glutamate dehydrogenase α-Ketoglutarate + NH4+ + NAD(P)H + H+ → Glutamate + NAD(P)+ + H2O Amino Acid Biosynthesis (Anabolism) - Source of nitrogen is glutamate or glutamine - Via transamination - Glutamine synthetase - Carbon skeletons of amino acids come from intermediates of - Glycolysis - Citric acid cycle - Pentose phosphate pathway - Bacteria can synthesize all 20 amino acids - Mammals require nine in diet (essential amino acids; histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) Amino Acid Biosynthetic Families Can be Grouped by Metabolic Precursors - Non-essential amino acids can be made from one of six precursors - Essential amino acids must be supplied in the diet - A deficiency of one amino acid impacts the synthesis of all proteins required for life Amino Acids Made from Intermediates of Major Pathways (don’t need to know details) Amino Acid Biosynthesis (Anabolism) Nonessential amino acids (except arginine) are synthesised by simple reactions - 1-5 steps Essential amino acids are synthesised by more complex reactions - 5-16 steps Feedback Inhibition Regulates Amino Acid Biosynthesis (don’t need to recall definitions) - Inhibition of the First Irreversible Reaction: In linear pathways, the first irreversible or 'committed' step is often tightly regulated. This serves as a bottleneck, controlling the flow of the entire pathway - Feedback Inhibition and Activation in Branched Pathways: In pathways that split into multiple directions, the final products can often inhibit or activate the initial steps, preventing wasteful overproduction or ensuring efficient supply - Inhibition by Own Product and Activation by Another: An enzyme in the common initial step can be inhibited by its own product while being activated by the product of a different pathway. This creates a balanced network of regulation - Enzyme Multiplicity: In branched pathways, multiple enzymes can catalyse the same reaction but are regulated differently - Inhibition of the Committed Step by Multiple Enzymes: >1 enzyme can catalyse the committed step, and these can be inhibited differently, adding another layer of control - Cumulative Feedback Inhibition: In branched pathways, multiple end products may each partially inhibit the initial steps, offering nuanced control over the pathway's activity Feedback Inhibition and Activation Regulates Amino Acid Biosynthesis: Example Roles of Amino Acids Energy production - The TCA cycle - Gluconeogenesis - Ketogenesis Synthesis - Proteins - Nucleotides - Porphyrins - Creatine - Neurotransmitters - Hormones - Nitric oxide - Glutathione - Rigid polymers - Polyamines Roles of Amino Acids: Energy Production (don’t need to know details) - Catabolism of amino acids yield 7 major products (carbon skeletons), which enter the TCA cycle, gluconeogenesis or ketogenesis for energy production - Most amino acids do not directly enter the TCA cycle - Catabolism of amino acids accounts for only 10-15% of energy production - Glycolysis and fatty acid oxidation are more active Roles of Amino Acids: Neurotransmitter and Hormone Synthesis (don’t need to know details) - Catecholamines: Dopamine (motor coordination, reward behaviour); Nnorepinephrine and epinephrine (increase blood pressure, heart rate, blood flow glucose release) - GABA: Key neurotransmitter, with its dysregulation linked to many disorders - Histamine: vasodilation, sleep-wake regulation and gastric acid release - Serotonin: mood, cognition, reward behaviour, learning, memory, vomiting and vasoconstriction Roles of Amino Acids: Glutathione Synthesis(don’t need to know details) Glutathione (GSH) is present in most cells at high amounts Reducing agent/antioxidant - Keeps proteins and metal cations reduced - Keeps redox enzymes in reduced state - Removes toxic reactive oxygen species (ROS) Roles of Amino Acids: Nitric Oxide Synthesis (don’t need to know details) - Mid-80’s discovery that pollutant nitric oxide (NO) played important role in neurotransmission, blood pressure regulation, blood clotting and immunity - Synthesized from arginine via nitric oxide synthase using NADPH Summary - Nitrogen is a major elemental constituent of living organisms that needs to be “fixed” - Glutamate and glutamine are the crucial entry points for incorporating nitrogen (NH4+ ) into biomolecules - Essential amino acids need to obtained via the diet, non-essential amino acids can be synthesized - Carbon skeletons of amino acids come from intermediates of other pathways - Various feedback inhibition pathways regulate amino acid biosynthesis - Catabolism of amino acids results in products which can enter the TCA cycle, gluconeogenesis and/or ketogenesis - In addition to protein formation and assembly, metabolism of amino acids can yield various molecules such as neurotransmitters, glutathione and nitric oxide Protein Metabolism Including the Urea Cycle Overview of Amino Acid Catabolism in Mammals - Amino acids from dietary proteins are the main source of amino groups - Most amino acids are metabolised in the liver - Amino acids are used to: - Produce glucose or lipids - Produce energy - Provide carbon skeletons - Produce proteins - Excess ammonium (NH4+) must be excreted Dietary Protein is Enzymatically Degraded Through the Digestive Tract - Eating signals secretion of gastrin → lowers stomach pH (more acidic) - Low pH triggers proteolytic activation of pepsinogen → pepsin (breaks down proteins) - Peptides/amino acids pass through the small intestine - Pancreatic zymogens are secreted into the intestine for further digestion of peptides - Acute pancreatitis: obstruction of pancreatic secretions results in proteolytic activation of zymogens within the pancreas which attack pancreatic tissue when they have no other proteins/peptides to digest causing pain and cellular damage/inflammation. - Amino acids are absorbed through the epithelial cell layer (intestinal mucosa) of the villi and enter the capillaries. Digestive Enzymes are Secreted as Zymogens or Proenzymes - Enteropeptidase is secreted from the small intestine in response to proteins entering the stomach to begin this cascade - Pancreas can synthesise and secrete trypsin inhibitors - Inhibition of trypsin is a key regulatory step in preventing overactivation of zymogens The Digestion and Absorption of Proteins - Pancreatic proteases (proteolytic enzymes) (previous slide) hydrolyze proteins into oligopeptides (and amino acids) - Peptidases further cleave oligopeptides into tri/dipeptides and amino acids - Amino acids are transported by several transporters into intestinal cells - Peptidases cleave tri/dipeptides further - Free amino acids are releated into the blood for use by other tissues Amino Acid Catabolism in Vertebrates - Removal of the amino group (i.e. transamination) is the first step of degradation for all amino acids - Glutamate, glutamine, alanine and aspartate are central in nitrogen metabolism - Glu, Gln, Ala and Asp readily converted into TCA cycle intermediates - Gln and Ala are important in the transport of amino groups from extrahepatic tissues and skeletal muscle to the liver - Asp provides the second amino group in the urea cycle Ammonia is Safely Transported in the Bloodstream as Glutamine - NH4+ is protonated and cannot cross the plasma membrane - Excess NH4+ in tissues is added to glutamate to form glutamine - Glutamine is transported in the bloodstream to the liver - NH4+ is released in the liver and enters the urea cycle - Glutamine has two amino groups The Glucose-Alanine Cycle - Vigorously working muscles operate nearly anaerobically and rely on glycolysis for energy - Glycolysis yields pyruvate (if not eliminated lactate will build up) - This pyruvate can be converted to alanine for transport into the liver - This alanine in the liver can enter the urea cycle or the gluconeogenesis pathway Excretory Forms of Nitrogen - When amino acids are not used to produce new amino acids or other products they are removed as a single excretory end-product - Ammonium (NH4+) is highly toxic to animals - Animals excrete nitrogenous wastes in different forms: ammonia, urea or uric acid - In ammonotelic organisms, NH4+ can be excreted as NH4+ - In ureotelic organisms, NH4+ is converted to urea for excretion - In uricotelic organisms, NH4+ is converted to uric acid for excretion Excretory Forms of Nitrogen The Urea Cycle - Excess NH4+ is removed via the urea cycle in ureotelic animals - Discovered by Hans Kreb (who also discovered the TCA cycle) - Urea is produced from NH4+ in five steps - Enzymes catalyzing these reactions are distributed between the mitochondrial matrix and the cytosol (should know this cycle ←) Mitochondrial Reactions of the Urea Cycle - Excess NH4+ is metabolised in the mitochondria of liver cells - NH4+ arrives in liver mitochondria as glutamine and glutamate - One amino group enters the urea cycle as carbamoyl phosphate, formed in the matrix from glutamine or glutamate - Another amino group enters the urea cycle as aspartate, formed in the matrix by transamination of glutamate and oxaloacetate Mitochondrial carbamoyl phosphate synthetase I: - Converts free NH4+ and HCO3 - to carbamoyl phosphate - First step leading into the urea cycle - Allosterically activated by N-acetylglutamate - First step of pyrimidine synthesis and with aspartate provides orotate - Further reactions yield urea Cytosolic Reactions of the Urea Cycle - Carbamoyl phosphate and ornithine form citrulline in the mitochondrial matrix - Citrulline enters cytosol - Citrulline and aspartate form argininosuccinate - Argininosuccinate is converted to arginine and fumarate - TCA (Kreb’s) cycle intermedidate - Arginine is converted to urea and ornithine - Ornithine “reenters” mitochondrial matrix Defects of the Urea Cycle - The liver is the site of urea synthesis - Defects in any of the urea cycle enzymes result in elevated levels of NH3/NH4+ in the blood (hyperammonemia) - Elevated blood NH3/NH4+ causes nervous system malfunction and possible death - Liver damage caused by excessive alcohol consumption can be fatal, in part because the liver is unable to synthesize urea and consequently NH3/NH4+ appears in the blood Catabolism of Amino Acids: Revision - Catabolism of amino acids yield 7 major products (carbon skeletons), which enter the TCA cycle, gluconeogenesis or ketogenesis for energy production - Most amino acids do not directly enter the TCA cycle - Catabolism of amino acids accounts for only 10-15% of energy production - Glycolysis and fatty acid oxidation are more active Phenylketonuria (PKU) is Caused by a Defect in Phenylalanine Degradation - Inborn error of metabolism - Genetic defect in phenylalanine hydroxylase - Results in buildup of phenylalanine (and phenylpyruvate) - Impairs neurological development leading to intellectual deficits and premature death if untreated - Controlled by limiting dietary intake of phenylalanine (including artificial sweeteners) and dietary non-phenylalanine protein supplementation Muscles and Catabolism of Amino Acids: Proteins as a Source of Energy - Muscles are the primary storage site of protein in the body - Protein within muscle can be degraded to amino acids during times of long-term fasting - Amino acids are secreted from muscle for production of glucose and ketones in the liver - Glucose and ketones are then available as an energy source for the brain Problem with Starvation Diets - Glucose derived primarily from glycogen stores in liver via glycogenolysis - Maintaining adequate blood glucose levels is essential - When glycogen stores are depleted (dieting), maintaining blood glucose levels requires gluconeogenesis - No intake of food, so muscle protein must be catabolised for glucose production - Thus, primarily muscle loss rather than loss of fat reserves Problems with the Atkins Diet High fat and protein diet (low carbohydrate) thought to reduce appetite and overall consumption Metabolism is like fasting but the following result: - Decreased glucose - Increased ketones (secreted in breath and urine leading to a net calorie loss) - Increased serum lipids - Increased NH4+ Potential heath risks: - Hypoglycemia (low glucose in blood) causing neural effects - Ketoacidosis causing acidification of blood - High serum lipids causing heart disease - Increased ammonia causing liver damage Summary - Amino acids from dietary proteins are the main source of amino groups - Proenzymes and peptidases degrade proteins and oligopeptides in the digestive tract - Glutamate, glutamine, alanine and aspartate are central in nitrogen metabolism - Excess ammonium (NH4+) must be excreted, with NH4+ secreted as urea in humans and other ureotelic organisms - The urea cycle involves both mitochondrial and cytosolic reactions - Catabolism of amino acids results in products which can enter the TCA cycle, gluconeogenesis and/or ketogenesis - Phenylketonuria (PKU) results from a defect in phenylalanine hydroxylase and is an example of an inborn error of metabolism - Proteins can be used as an energy source but can result in muscle loss and excess NH4+ in some diets Photosynthesis I: The Light Reactions The Importance of Photosynthesis - Biochemical energy in animals comes from the oxidation (in mitochondria) of organic fuel from food - This food comes ultimately from plants - Plants get their energy from the sun - The process which converts solar energy to biochemical energy is photosynthesis - Photosynthesis is the entry point for energy into the biosphere - Photosynthesis provides carbohydrate building blocks: - Biomass - coal deposits - crude oil - gas deposits - Photosynthesis liberates oxygen and produces carbohydrates which are used by animals in the oxidative processes of respiration - Almost all energy flow and all fixed carbon in the biosphere, originated from photosynthesis - Almost all oxygen in the atmosphere is generated by photosynthesis The Basic Equation for Photosynthesis - Water + sunlight + carbon dioxide → carbohydrates + oxygen 2H2O + hu + CO2 → (CH2O)n + O2 - Use of the stable isotope 18O proved that the oxygen involved in photosynthesis is derived from water 2H2 18O + hu + CO2 → (CH2O)n + 18O2 - Photosynthetic bacteria eg. purple sulphur bacteria can use alternative electron donors such as H2S 2H2S + hu + CO2 → (CH2O)n + 2S - h, Planck’s constant (J/s); u, frequency of light (cycles) The Reactions of Photosynthesis - Pigments absorb light energy and convert it to NADPH, ATP and O2 - NADPH and ATP are used to reduce CO2 to form (CH2O)n Photosynthesis Occurs in Chloroplasts Chloroplasts The role of chloroplasts in plants is to capture light energy and convert it to chemical energy “Light reactions” - electron transport/proton gradient - Take place in the thylakoid membranes - Absorption of light by pigments - Conversion of solar energy to ATP and NADPH “Dark reactions” - Fixation of CO2 into organic carbon - Catalysed by aqueous enzymes - Driven by ATP and NADPH Photosynthesis Relies on Electron Flow and Electrochemical Gradients Why Leaves are Green (don’t need to know graph) - Chlorophyll is the main photoreceptor in chloroplasts - Have delocalized electrons that are easily excited by photons of visible light. - Chlorophyll absorbs light in the blue and red regions of visible light - Light in the green region is reflected Other Light Harvesting Pigments - Secondary light absorbing pigments are called carotenoids - The two most important are: - β-Carotene (red-orange) - Lutein (yellow) - These pigments absorb light at wavelengths not absorbed by chlorophyll and thus serve as supplementary light receptors to expand the range of wavelengths absorbed - Different proportions of these pigments give plant species their characteristic colour - Carotenoids also function as anti-oxidants Photosystems Harvest Light - The light-absorbing pigments are arranged in arrays called photosystems embedded in the thylakoid membrane - Absorption of light (photon) by antenna chlorophyll leads to excitation of the reaction centre - Specialized chlorophyll molecules associated with the photochemical reaction centre can transduce light into chemical energy Excitation and Electron Transfer (don’t need to know details) Light Reactions of Photosynthesis Photosystems - Plants have two types which act in tandem: - Photosystem II (PSII) - Photosystem I (PSI) - Each contains multiple antenna pigment molecules and a pair of reaction centre chlorophylls - Each contains a series of electron transfer molecules - PSII and PSI are linked by the cytochrome b6f complex - Combined these 3 complexes generate NADPH, and protons to drive ATP production (via ATP synthase) (as well as O2) Cytochrome b6f Complex - e- transfer from PSII through plastoquinone (PQ) to Cyt b6f - Plastocyanin carries e- to PSI - Four protons (4H+) are released into lumen to contribute to proton gradient (to drive ATP synthase to generate ATP; not shown) Integration of Photosystems - Z-scheme outlines the pathway of e- flow between PSII and PSI - P680/P700 are the reaction centre PS which receives light energy - Cytochrome b6f (Cyt b6f) links PSII and PSI - Cooperation between PSII and PSI creates an e- flow from water to NADP+ - Cyclic transfer of e- - Transfer from ferredoxin (Fd) back to Cyt b6f - Results in more ATP (increased proton gradient) but reduced NADPH compared to noncyclic transfer Light Reactions of Photosynthesis Photosynthesis II: The Dark Reactions NADPH and ATP feed into the “Dark” (Carbon-assimilation) Reactions - Can occur in the absence of light, but accelerated by light - Occur in chloroplast stroma - Fix atmospheric CO2 into carbohydrate - Driven by ATP and NADPH that was produced by the “light reactions” Assimilation of CO2 Into Biomass in Plants - The simple product of photosynthesis is the precursor of more complex biomolecules including sugars, polysaccharides and metabolites derived from them CO2 Assimilation (The Calvin Cycle) - The Calvin cycle is used to assimilate carbon dioxide into sugars - The Calvin cycle has three stages - The net product is 1 triose phosphate - 3 CO2 are required for the net synthesis of one molecule of glyceraldehyde 3-phosphate (triose phosphate) - 9 ATP and 6 NADPH provided by light reactions - 6 triose phosphates are produced and 5 are used to regenerate 3 molecules of ribulose 1,5- bisphosphate (RubP) Stage 1: Carbon Fixation Plants in which this 3-carbon compound is the first intermediate are called C3 plants RuBisCO - Ribulose 1,5-bisphosphate carboxylase/oxygenase - One of the world’s most important enzymes - Catalyses the first step of atmospheric CO2 fixation - Rate-limiting step in hexose synthesis - Only 3 CO2 molecules fixed per rubisco per second (low efficiency) - Triose phosphate isomerase acts on 4300 molecules of glyceraldehyde 3-phosphate per second (just for reference) - The most abundant protein in the biosphere: - 35-50% of soluble chloroplast protein Rubisco Activase Activates Rubisco - Rubisco must be carbamoylated by CO2 to be active - BUT over time, rubisco binds to ribulose 1,5-bisphosphate which prevents rubisco activation - Rubisco activase overcomes this using energy from ATP hydrolysis to displace the ribulose 1,5- bisphosphate Stage 2: Conversion of 3-Phosphoglycerate to Glyceraldehyde 3-Phosphate - Note both ATP and NADPH are used - G3P has different fates depending on the needs of the plant - NOTE: this is not the entire calvin cycle, this image only shows the second stage (reduction) Stage 3: Regeneration of Ribulose 1,5- Bisphosphate from Triose Phosphates - 80% of the triose phosphates must be recycled to RuBP Rubisco is also an Oxygenase (Photorespiration) - Rubisco is not just specific for CO2 but can also fix O2 - Before photosynthetic organisms raised the O2 content of the atmosphere, there was no selection pressure for rubisco to discriminate between CO2 and O2 - Increased O2 concentration = greater amount available to be fixed - Significant amounts of O2 are also fixed with a ratio of about 1 O2 per 4 CO2 incorporated into the cycle - Oxygenase activity results in a significant loss of carbon and waste of energy (photorespiration) Reactions Catalysed by Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase Ribulose 1,5-bisphosphate carboxylase/oxygenase - Oxygenase activity results in photorespiration and production of metabolically useless 2-phosphoglycolate Salvage of 2-Phosphoglycolate - Complex process involving three organelles - Chloroplast stroma - Peroxisomes - Mitochondra - Energy is required for salvage - Consumes O2 at two points - The process of carbon fixation is reversed - But a carbon is lost as CO2 without production of energy = wasteful Improving Rubisco - Rubisco is rate limiting for photosynthesis - Improved efficiency would have a large impact on CO2 reduction in the atmosphere, and food and biofuel production - Improve specificity: Replace crop gene with rubisco from red algae or purple photosynthetic bacteria - Increase catalytic activity: Introduction of altered rubisco with higher activity - Increase levels of expression - Current attempts to improve rubisco efficiency have been met with limited success but are ongoing C3 and C4 Photosynthesis - C3 plants use the conventional pathway of CO2 uptake in which the first intermediate produced is a C3 compound - C4 plants use an alternative pathway of CO2 uptake in which the first intermediate produced is a C4 compound - C4 pathway was discovered by Australian scientists Hal Hatch and Roger Slack - C4 pathway is found in tropical grasses (eg. maize, sugarcane) - C4 pathway is more efficient in intense sunlight and high temperatures Characteristics of C4 Plants - High photosynthetic rates - High growth rates - Low photorespiration rates (better efficiency) - Low rates of water loss - Specialised leaf structure C4 Photosynthesis - Phosphoenolpyruvate (PEP) carboxylase incorporates atmospheric CO2 via HCO3 - - PEP carboxylase has no affinity for O2 (more efficient) - C4 plants have a different anatomy which allows them to concentrate CO2 and overcome photorespiration - Concentrates CO2 for Rubisco in the Calvin Cycle but requires additional energy - Used when the gains in efficiency outweigh the increased energy requirement - C4 pathway requires high sunlight for energy production (increased ATP synthesis) - Rubisco’s affinity for CO2 decreases with increased temperature; above 30°C the C4 pathway provides a significant gain in the efficiency of CO2 fixation - Concentration of CO2 allows stomata to partially close during the day, conserving water - Therefore, photosynthesis in C4 plants is more efficient in hot, arid or tropical climates Biochemistry of Energy Flow: Cells Metabolism (don’t need to memorise) - Metabolism is the set of chemical reactions in cells that sustain life - Required for growth, reproduction, maintenance and responses to the environment - Chemical reactions, including digestion and the transport of substances into and between different cells - Highly coordinated activity - Catabolism (degradative) breaks down organic matter, to harvest energy in cellular respiration. - Anabolism uses energy to construct components of cells such as proteins and nucleic acids Energy Relationships in Cells - Cellular catabolic pathways deliver chemical energy in the form of ATP, NADH, NADPH and FADH - These energy carriers are used in anabolic pathways to convert small precursor molecules into cellular macromolecules Energy and Thermodynamics - Energy transformations in living systems are governed by the laws of thermodynamics - First law of thermodynamics states: - Energy can neither be created or destroyed - It can be transformed from one form to another - No energy transfer is 100% efficient. Some is always lost as heat and is not available for work - Entropy (S) is the measure of disorder caused by the continual loss of energy during transformations (unusable energy) Enthalpy and Free Energy - Chemical reactions occur by chemical bonds breaking and forming. - Amount of energy required to break chemical bonds is equivalent to total potential energy of the system - known as ENTHALPY (H) ENTHALPY = Free Energy + ENTROPY (total energy) = (Usable energy) + (Unusable energy) - Free energy (G) is the amount of energy that is available to do work (Gibbs free energy) - The change in free energy (∆G) is the chemical potential of a reaction (∆G = ∆H – T ∆S) Free Energy and Enzyme Reactions - When ∆G is negative, energy is released in an exergonic reaction - Enzymes are still required to lower the activation energy and increase the rate of reaction - When ∆G is positive, energy is needed for an endergonic reaction - Energy from an exergonic reaction must be coupled to an endergonic reaction - The sum of the ∆G from the two reactions still must be negative - Enzymes increase the rate of reaction for exergonic and endergonic reactions Free Energy Changes are Additive Synthesis of glucose 6-phosphate - Glucose + Pi → Glucose 6-phosphate + H2O ∆G = 13.8 kJ/mol (endergonic) - ATP + H2O → ADP + Pi ∆G = -30.5 kJ/mol (exergonic) Sum of coupled reactions: - ATP + glucose → ADP + glucose 6-phosphate ∆G = -16.7 kJ/mol - The sum of the ∆G from the two reactions is negative Free Energy Summary - The change in free energy ∆G determines the chemical potential of a reaction - Exergonic reactions have -∆G = energy is released - Endergonic reactions have +∆G = require energy input - If the sum of ∆G from two reactions is negative - Then enzymes can catalyse endergonic reactions by coupling them to exergonic reactions - Conformational state is important - Enzymes increase the rate of reactions by orientation of reactants at the active site - Lowers the activation energy The Energy Stored in Nucleotide Triphosphates Drives Endergonic Reactions - Reaction still requires an enzyme catalyst - Enzymes brings reactants together at the active site - ATP + glucose → glucose 6-phosphate + ADP - Breaking of a phosphate bond on ATP is directly coupled to formation of a phosphate bond on glucose Drugs that Target the ATP Binding Site - Drugs that inhibit reactions by binding to the ATP binding site of enzymes are a new class of drugs - Problem - Drugs are often not specific for a given enzyme - The medical potential is significant if more selective drugs an be produced ATP Synthesis - Synthesis of ATP is thermodynamically unfavourable ADP + Pi + H+ ↔ -30.5 kJ/mol ↔ ATP + H2O - How is the pool of ATP restored from ADP + Pi? - The potential energy from a transmembrane proton gradient and ATP synthase is used to drive the reaction - ATP synthesis is coupled to diffusion of protons across a gradient (chemiosmosis) ATP Synthesis - Production of potential energy (proton gradient) still requires breaking and forming of chemical bonds (reduction-oxidation reactions) ↓ - Electron Transport Chain (Electrical energy) ↓ - Proton Pumping (Potential energy) ↓ - Oxidative Phosphorylation Photophosphorylation (Chemical energy ATP) Reduction-Oxidation Reactions - These reactions involve: - Loss of electrons by one chemical species (oxidised) &; - Gain of electrons by another (reduced). - The flow of electrons from one molecule to another in redox reactions underlies all energy transductions in cells Electron Transport Chain Reduction-Oxidation - Electrons are most commonly transferred in the form of hydride ions (proton(s) coupled with 2 electrons). - Coenzymes, NAD+, NADP+, FAD → NADH, NADPH and FADH2 act as universal electron carriers Reduction-Oxidation Reactions: Catabolism Involves NAD+ ⇌ NADH - Oxidation of reduced fuel molecules (i.e. pyruvate) by dehydrogenases Reduction-Oxidation Reactions: Anabolism Involves NADP+ ⇌ NADPH - NADPH produced by the pentose phosphate pathway - In both plants and animals - Electrons moved from NADPH to macromolecules - Oxidation of NADPH by reductases for the production of reduced macromolecules Disruption of Energy Flow in Cells: Metabolic Effects of Alcohol Consumption - Ethanol is metabolised to acetate in the liver - Excess NADH inhibits NAD+ requiring reactions such as fatty acid oxidation - Triacylglycerols increase in liver and contribute to fatty liver disease Disruption of Energy Flow in Cells: Alcohol Toxicity Alcohol intolerance: - Alcohol intolerance is due to lack of acetaldehyde dehydrogenase - Acetaldehyde is toxic and build up causes can cause severe illness Methanol Poisoning: - Methanol, present as a byproduct, is metabolised to formaldehyde by alcohol dehydrogenase. - Ethanol is used to treat methanol poisoning Methanol Poisoning: - Methanol, present as a byproduct, is metabolised to formaldehyde by alcohol dehydrogenase. - Ethanol is used to treat methanol poisoning Biochemistry of Energy Flow: Organisms Energy Relationships in Cells - Cellular catabolic pathways deliver chemical energy in the form of ATP, NADH, NADPH and FADH - These energy carriers are used in anabolic pathways to convert small precursor molecules into cellular macromolecules Energy Relationships in Cells Energy and Thermodynamics - A diet can vary substantially but needs to contain a mixture of the 3 main energy sources - Requires digestion to amino acids, monosaccharides, or fatty acids and monoglycerides for absorption in the intestinal tract Absorption in the Intestine: The Liver Acts as a Metabolic Hub 1. Absorption in the intestine 2. Enters the hepatic portal system - Carried to the portal vein and into the liver before entering other tissues First Pass Metabolism in the liver: - Relates to the bioavailability of pharmaceuticals in systemic circulation - The concentration of a drug is greatly reduced before it reaches the systemic circulation through metabolism - Critical parameter for drug design and development (oral dose, alternative routes) Carbohydrates - The primary role of dietary carbohydrate is the provision of energy to cells, particularly the brain that requires glucose for its metabolism - Carbohydrates provide 17 kJ per gram of weight (4 kcal/g) - Sugars (Mono & Disaccharides) - Starch (Polysaccharides) - Dietary fibre - Complex carbohydrates, like disaccharides and starch must first be broken down to simpler sugars Sugars - Sugars are absorbed into the bloodstream as monosaccharides Monosaccharides - The end product of carbohydrate digestion - Found in fruits, certain roots, corn and honey - Also produced as a product of starch digestion in the body - E.g. glucose, fructose, and galactose Disaccharides - Double sugars e.g. sucrose (composed of glucose and fructose), lactose (milk), maltose (malt) Disaccharide Metabolism - Disaccharides are hydrolysed by enzymes on the surface of intestinal epithelial cells - Absorbed into the portal vein and transported to the liver - The liver is important for control of glucose levels in the blood Lactose Intolerance - Lactose: a sugar found in milk (5%) - Lactose Intolerance: Lactose not digested due to lack of expression of the enzyme lactase - What are the common symptoms? - Nausea, cramps, bloating, gas, and diarrhoea (fermentation of sugars in lower intestine) - Who is affected? - Approx 75% of African and Asian populations Starch Catabolism - a-amylase: cleavage of a(1-4) bonds on non-reducing ends - Found in the mouth, small intestine - Products: - Maltose → (maltase) → glucose + glucose - Debranching enzyme: a(1-6) glucosidase - cleavage of branch points Dietary Fibre - Most complex form of carbohydrate - Two types of fibre: soluble & insoluble - Resistant to digestion and absorption in the small intestine - Usually with complete or partial fermentation in the large intestine - Dietary fibre includes polysaccharides, oligosaccharides (degree of polymerisation >2) and lignins, and promotes one or more of the following beneficial physiological effects: - Laxation - Reduction in blood cholesterol - Modulation of blood glucose Protein - Nitrogen-containing macromolecules - Provide 17kJ/g (4 kcal/g) - A source of amino acids - For the synthesis of many essential proteins - Animals cannot synthesise the amine group - Through the process of digestion, animals break down ingested protein into free amino acids that are then used in metabolism - Some ingested amino acids are used for protein biosynthesis, while others are converted to other forms of energy, and/or excreted Dietary Protein - The body needs proteins - For tissue maintenance and replacement - To function and grow - Protein is the primary component of most cells - Muscle, connective tissues and skin are all built of protein - Physical activity and exertion, as well as enhanced muscular mass, increase the need for protein - The body cannot store excess protein Fats Fats are the most concentrated form of energy for the body (37 kJ/g or 9 kcal/g) - Fatty acids - Triacylglycerol - Phospholipids - Sterols Lipids Used as fuel in the body Fat Metabolism 1. Fats ingested 2. Bile salts & lipases degrade triacylglycerols 3. Fatty acids taken up by the intestinal mucosa & converted to triacylglycerols 4. Chylomicrons are formed 5. Chylomicrons move through the lymphatic system & bloodstream to tissues 6. Lipoprotein lipase releases fatty acids and glycerol 7. Fatty acids enter cells 8. Fatty acids are oxidised as fuel or re-esterified for storage Storage of Calorie-Rich Fat in Adipose Tissue Energy Flow: Organs - Different organs work together to maintain caloric homeostasis, a constant availability of fuels in the blood - The major fuel depots in animals are glycogen (liver and muscle), triacylgylcerols (adipose tissue) and protein (most in skeletal muscle) - There are clear metabolic divisions of labour among the major organs and they exhibit a unique metabolic profile The Liver is a Metabolic Hub - Carbohydrate consumed in diet is absorbed then enters the liver through the portal vein - Liver synthesises glycogen from glucose - Stored for regulation of blood glucose during fasting - Liver synthesises triacylglycerols and releases very low-density lipoprotein (VLDL) for transport to adipose tissue - Regulation of cholesterol and triacylglycerols in the blood - Hormonal regulation The Liver is the Primary Storage Reserve of Glucose by Synthesis of Glycogen - Starch from diet converted to glucose - Transported to the liver - Glucose in diet stored in the liver as glycogen - Main storage polysaccharide in animals - Glycogen stored in the liver maintains blood glucose during short-term fasting - Glycogenolysis used to raise blood glucose - Breakdown of glycogen to glucose Energy Flow to the Brain - The brain consumes large amounts of energy but does not store or release energy - Preferred energy source is glucose. During starvation, ketone bodies can be used for energy but are insufficient for proper function - Brain uses 25% of total body glucose - Blood glucose must not fall below 3.5 mmol/L in humans or person may lose consciousness Energy Flow to Muscles Energy for Muscle Contraction Energy storage - Glycogen synthesis from glucose - Phosphocreatine - Protein content in fibres Different fuels are used for ATP synthesis - During bursts of heavy activity, & - During light activity or rest. Phosphocreatine can rapidly supply ATP during periods of intense exercise - Catalysed by creatine kinase Energy Flow in Animals - Muscle cell fuel choice in exercise depends on intensity and duration of activity - Heart muscle also efficiently uses fatty acids as an energy source Diet and Energy Flow - Metabolism is controlled to a great extent by the availability of substrates (carbohydrate vs lipid vs protein levels) - Regulated by the endocrine system - Catabolism for energy depends on the concentration of available substrates. - Catabolism also depends on the preferred substrate of particular organ systems. Fatty acids are generally not a preferred substrate except in muscles during aerobic exercise. - Problem: reduction in fat is what many diets are trying to achieve. Fasting and Energy Flow 1. The depletion of glycogen reserves in the liver 2. The breakdown of protein in muscle stores for export of amino acids to the liver 3. The intake of amino acids by the liver for gluconeogenesis to raise blood glucose 4. The release of fatty acids from stores of triacylglycerols in adipose tissue 5. The intake of fatty acids in the liver and production of ketone bodies 6. The brain is deprived of glucose and has to rely on ketone bodies as a secondary energy source Exercise and Energy Flow - Sustained aerobic exercise gives you what you want - Metabolism of fats - Fat is primarily a storage compound and is not a preferred substrate for energy production in most organs. - Sustained low intensity muscle use (aerobic respiration, betaoxidation of fatty acids) is a rare situation where fatty acids can act as a preferred substrate Integration of Metabolism I: Signal Transduction Signal Transduction Mechanisms of Hormone Action 1. Water soluble peptide and amine hormones bind extracellular receptors - Alters enzyme activity: fast acting - Alters gene expression: slow acting - E.g. insulin, glucagon, epinephrine 2. Water insoluble hormones pass through plasma membrane and bind intracellular receptors - Alters gene expression: slow acting - E.g. cortisol, thyroid hormones, sex hormones Mechanisms of Hormone Action Four types of receptors important for metabolism 1. Growth factor receptors: membrane receptors with extracellular ligand binding and intracellular enzyme activity - e.g. insulin receptor for insulin 2. G protein-coupled receptors: membrane receptors that act through G proteins to produce “second messengers” - e.g. adrenergic receptor for epinephrine 3. Intracellular steroid receptors: nuclear hormone receptors - e.g. glucocorticoids, sex hormones i.e. testosterone, oestrogen 4. Gated ion channels: membrane receptors that respond to ligands to mediate intracellular signalling events - e.g. P2X channels (not discussed further in this lecture apart from next slide) Receptor Signalling From Important Metabolic Hormones Insulin receptor: receptor tyrosine kinases (e.g of growth factor receptor) - Signals glucose uptake by liver, muscle and adipose tissue - Signals glycogen synthesis in liver and muscle - Signals decreased appetite Glucagon and epinephrine receptors: adrenergic receptors (e.g of GPCRs) - Signals glucose production in the liver - Signals lipolysis of triacylglycerols to fatty acids and glycerol in adipose tissue Glucocorticoid receptor: receptor that binds cortisol (e.g. of hormone receptor) - Signals glucose production - Signals lipolysis in adipose tissue - Signals catabolism of proteins to amino acids in muscle Formation of Insulin - Insulin is generated from a larger precursor molecule - Synthesized on ribosomes in the pancreas as preproinsulin ① - Proteolytic removal of 23 amino acid signal sequence - proinsulin ② - Proinsulin is stored in secretory vesicles - Proteolytic removal of the C peptide - Occurs when blood glucose is elevated - A and B chain joined by two disulphide bonds to form mature insulin ③ Insulin Receptor (only need to know insulin receptor) Insulin can act as a growth factor - The insulin receptor is structurally similar to other growth factor receptors (receptor tyrosine kinases) - Effector tyrosine kinase domain The insulin receptor has the receptor and effector domains in the same molecule - Extracellular α subunits contain the insulin binding domain - Intracellular enzymatic domains of the β subunits contain the protein kinase activity These αβ monomers form a dimer to produce the active insulin receptor protein Insulin Binding and Receptor Activation - When insulin binds the receptor sub-units come together to form a dimer - Receptor tyrosine kinase (effector) domain is activated via autophosphorylation - Results in a cascade of protein (Tyr) phosphorylation reactions Insulin Receptor Activation Promotes Glucose Uptake and Glycogen Synthesis (don’t need to know details) - Typical of signal transduction from receptors for growth factors - Phosphorylation of insulin receptor substrate 1 (IRS1) by insulin activates a kinase cascade ① - Initiates movement of glucose transporters to the plasma membrane - Increased glucose uptake ② - Phosphorylation of the glycogen synthase kinase 3 (GSK3) - Activates glycogen synthase (GS) to increase synthesis of glycogen ③ Control of the Insulin Signal by Endocytosis of Activated Receptor - Insulin binding causes internalisation of the receptor, limiting the duration of the signal - Receptor signalling → Internalisation of receptor → Lysosomal degradation Signalling Pathway of Leptin in the Hypothalamus via the JAK-STAT System 1. Leptin binding induces dimerisation of the receptor 2. Phosphorylation of specific Tyr residues by Janus kinase (JAK) 3. Signal transducers and activators of transcription (STATs) dimerize 4. STATs enter the nucleus to regulate transcription G Protein-Coupled Receptors (GPCRs) - The largest family of cell surface receptors - The most important class of receptors in drug development (pathway is associated with approximately 30% of approved drugs) - Glucagon, epinephrine, ghrelin and GLP-1 all act through GPCRs - Signal transduction from GPCRs follows a basic pattern involving 3 protein components and a second messenger - Act through a member of the guanosine nucleotidebinding protein (G protein) family Signal Transduction is Similar For All G protein Coupled Receptors - Hormones that act through GPCRs invoke the activity of 3 protein modules: 1. Receptor (or seven transmembrane domain receptors) 2. Transducer (G protein) 3. Effector (eg. Adenylyl cyclase) - A second messenger i.e. cAMP activates or inhibits downstream targets - Diversity of responses provided by different receptors, G proteins, effector proteins and second messengers Transducer: G-proteins - Heterotrimeric G proteins are a conserved family of signaling proteins with three subunits: α, β, and γ - α subunit is the binding site for GDP or GTP - G proteins link membrane receptor stimulation to activation of the effector molecule - G proteins stimulate (or inhibit) different membrane proteins that act as effectors - Adenylyl cyclase is the effector for epinephrine and glucagon - Adenylyl cyclase is a membrane protein that catalyses the production of cAMP from ATP Transduction of the Epinephrine Signal The GTPase Switch - G protein stimulation is self-limiting - G protein is inactive when GDP bound - Activated when hormone binds to GPCR - GDP displaced by GTP - Signal is inactivated by GTPase activity of asubunit - Hydrolyses GTP back to GDP - GTPase switch Cholera Toxin Interferes With G-protein Signalling Cholera toxin is a protein enzyme that can enter intestinal epithelial cells to cause high cAMP, which signal fluid release Receptor Internalisation Limits the Duration of the Signal GPCRs are internalised via endocytosis by arrestin proteins and the receptors are recycled Steroid Hormones - Steroid hormones can diffuse across plasma membranes - Corticosteroids: cortisol - Sex hormones: testosterone - Cell uptake can also occur by specific transporters - After diffusing across plasma membrane, the hormone binds to specific receptors in the cytoplasm or nucleus - Ligand receptor complex acts as a transcription factor and can activate or inhibit many genes - Hence steroid hormones can alter cells and introduce long term changes Glucocorticoid Receptor Signalling - Glucocorticoids cross the cell membrane - Carrier protein complex (corticosteroid-binding globulin) containing glucocorticoid receptors are present in the cytoplasm bound to heat shock proteins (chaperones) - Glucocorticoids bind glucocorticoid receptor and displaces chaperones – revealing the nuclear localization sequence (NLS) - Receptor forms a dimer and translocates to the nucleus where it binds to glucocorticoid response element (GRE) - Signals the modulation of transcription - Upregulates genes important for gluconeogenesis i.e. genes for aminotransferases - Can also downregulate anti-inflammatory genes - Regulation response is very specific to individual cell/tissue types Summary - Both water soluble and insoluble molecules can act as hormones by binding to their respective receptors - Growth factor receptors, G protein-coupled receptors, intracellular steroid receptors and gated ion channels represent the four major hormone receptor types - Each receptor family has unique downstream signalling properties - Insulin and leptin bind growth factor receptors - Glucagon and epinephrine bind G protein-coupled receptors - Cortisol and testosterone bind intracellular steroid receptors Integration of Metabolism II: Hormonal Regulation Neuroendocrine System Co-ordinates metabolism in mammals - Neuronal signalling: nerve cells release neurotransmitters that act on nearby cells - Endocrine signalling: hormones can act on nearby cells or are carried by the bloodstream to act on cells other organs Similar modes of action - Cells sense a change in circumstances of organism - Respond by secreting a chemical messenger Example: neural response to stress results in hormonal release and energy production Hormones - Small molecules or proteins that: - Are produced in one tissue - Secreted directly into the bloodstream (endocrine) - carried to other tissues - Act through specific receptors to bring about changes in cellular activities in remote targets - Maintain energy homeostasis in response to environmental factors - Act at low concentrations and effects are generally short-lived The Diversity of Hormones Hormones Control Responses to Feeding, Fasting and Stress ‘Food’ Hormones - Feeding: energy intake: increase in blood glucose - Insulin, leptin and GLP-1 (glucagon-like peptide 1) - Fasting: low energy intake: decrease in blood glucose - Glucagon, glucocorticoids and ghrelin - Environmental Stress: Increased energy for survival, fight or flight - Epinephrine, glucocorticoids Homeostatic Regulation of Cellular Fuel Hormonal Regulation of Blood Glucose: Insulin - Insulin: major regulator of glucose uptake - Protein secreted by β-cells of the pancreas in response to high blood glucose - Promotes: - Uptake of glucose into liver, muscle, adipose tissue. - Synthesis of fatty acids, triacylglycerols in liver, adipose tissue - Synthesis of glycogen in liver and muscle (storage) - Inhibits gluconeogenesis and decreases appetite Hormonal Regulation of Blood Glucose: Insulin During Feeding Hormonal Regulation of Blood Glucose: Insulin During Feeding in Detail Hormonal Regulation of Blood Glucose: Glucagon - Glucagon: Regulates glucose production - Protein secreted by α-cells of the pancreas in response to low blood glucose levels - Glucagon acts primarily on liver and adipose tissue - Stimulates: - Conversion of glycogen to glucose (glycogenolysis) in the liver - Glucose production (gluconeogenesis) in the liver - Conversion of triacylglycerols to fatty acids (lipolysis) Hormonal Regulation of Blood Glucose: Glucagon During Fasting Hormonal Regulation of Blood Glucose: Glucagon During Fasting in Detail Responses to Food are Controlled by Complex Signalling Between Organs Messages of Hunger and Satiety from Tissues Feeling hungry - Ghrelin: produced in the gut (cells lining the stomach and intestines) - Acts in brain to stimulate feeding (or hunger) Feeling full (satiety) - Leptin: produced in adipose tissue - Peptide YY (PYY3–36): produced in intestine - Signal to the brain to reduce hunger - Incretins: produced in the intestine - Glucagon-like peptide 1 (GLP-1): stimulates insulin secretion Neuronal Messages of Hunger and Satiety Feeling hungry - Neuropeptide Y (NPY) - “Eat more – do less” signal Feeling full (satiety) - α-Melanocyte-stimulating hormone (α-MSH) - “Eat less – do more” signal Ghrelin Stimulates Appetite - The first gut hormone shown to stimulate food intake - Produced in stomach and small intestine epithelia when stomach is empty - Ghrelin levels peak sharply in anticipation of a meal - Levels suppressed immediately after by ingested nutrients - Ghrelin activates NPY-expressing neurons (appetite simulating) Variations in Hormones Relative to Mealtimes Rise in ghrelin before normal times for meals - Drop after mealtime - Parallels feelings of hunger Blood glucose concentration rises during/after meals - Followed immediately by insulin release to coordinate metabolic homeostasis Depressing Your Appetite: Leptin - Leptin is a peptide hormone that regulates both appetite and metabolism - Adipokine - produced in adipose tissue - Leptin levels are increased by feeding and decreased by fasting - ↑ leptin = satiety - Leptin increases body temperature and energy expenditure - Leptin stimulates the release of α -MSH from appetite -suppressing neurons to inhibit eating Leptin and Obesity - Mice defective in leptin gene can be controlled by leptin replacement therapy - Leptin injections have no effect on animals without a defective leptin gene - Obesity results in ↓ secretion of PYY - Less satiety/appetite suppression - Blood levels of leptin are much higher in obese animals - Obesity can lead to a resistance to leptin receptor sensitivity and signalling Peptide Hormones That Act on Feeding Behavior and Fuel Selection in Mammals (summary) Regulation of Metabolism in Response to Environmental Stress - Two main hormones used to increase available energy in response to environmental stress - Epinephrine (adrenaline) = preparation for action - Glucocorticoids (mainly cortisol) = long-term adjustment - Glucocorticoids are also produced in response to fasting, particularly during longterm fasting - Both hormones oppose the actions of insulin and increase glucose and fatty acids in the blood = energy for stress response i.e. fight or flight Regulation of Metabolism in Response to Environmental Stress (don’t need to know details) Epinephrine (Adrenaline) - A catecholamine: which are derivatives of the amino acid tyrosine - Involved in regulation of blood glucose: - Stimulates glycogen breakdown in the liver and muscle - Stimulates gluconeogenesis in the liver - Fatty acid release from adipose tissue (lipolysis) - Provides fuel for fight or flight response Epinephrine (Adrenaline) - Epinephrine produces short term effects in many tissues in the body - Primary role is to increase chances of survival in an immediately life threatening situation Glucocorticoids – “The Stress Hormone” Cortisol Metabolic effects of cortisol are similar to epinephrine, so stimulates: - Gluconeogenesis in the liver - Glucose release from liver - Fatty acid release from adipose tissue (lipolysis) Cortisol also leads to increased amino acids in blood from breakdown of muscle protein Glucocorticoids – Catabolic Steroid Hormones - Like epinephrine, cortisol impacts many tissues, but effects are long term - An important factor in chronic stress - Inhibits cell growth - Suppresses the immune system - Long term increase in blood glucose, which can promote the development of type 2 diabetes Stress as a Risk Factor - Cortisol levels increase with age and cortisol promotes the formation of abdominal (visceral) fat - Stress can oppose the actions of insulin through cortisol - Stimulates breakdown of TAG and protein to raise the level of blood glucose, ketones, fatty acids, amino acids Summary - The neuroendocrine system regulates metabolism - Hormones represent a diverse group of molecules - Hormones control responses to feeding, fasting and stress - Insulin and glucagon regulate blood glucose and metabolism - Catecholamines and glucocorticoids in response to environmental stress regulate to metabolism