Bioenergetics Lecture Notes PDF

Unit 2: Bioenergetics Bioenergetics Bioenergetics Unit outline: Biological Energy Transfer Enzymes Fuel Sources, ATP and the Metabolic Road Map ATP-Phosphocreatine Non-Oxidative Metabolism – Glycolysis and Lactate Oxidative Metabolism – Krebs Cycle Oxidative Metabolism – Electron Tra...

Unit 2: Bioenergetics Bioenergetics Bioenergetics Unit outline: Biological Energy Transfer Enzymes Fuel Sources, ATP and the Metabolic Road Map ATP-Phosphocreatine Non-Oxidative Metabolism – Glycolysis and Lactate Oxidative Metabolism – Krebs Cycle Oxidative Metabolism – Electron Transport Chain Lipid Metabolism Interaction Carbohydrate and Lipid Metabolism Protein Metabolism Summary: Integration of Metabolic Pathways Biological Energy Transfer Bioenergetics What is Bioenergetics? o The study of energy transfer in biological systems What is Metabolism? o All the reactions that take place in a living cell! 1. Catabolism Reactions that breakdown large biomolecules to release energy 2. Anabolism Reactions that use energy to synthesize biomolecules Energy What is energy? o Very difficult to define, but generally energy is the capacity to do work What is work? o Product of a given force acting through a given distance What is power? o Rate of work (i.e., work/time) The ability of the human body to perform internal and/or external work (e.g., exercise!) depends entirely on the conversion of one form of energy to another Thermodynamics Describe the First Law of Thermodynamics o States that the total amount of energy in a closed system is constant o Also known as the law of conservation of energy, it means that energy can neither be created nor destroyed, but can only be converted from one type to another Describe the second law of thermodynamics o States that natural spontaneous processes move from a state of order to a state of disorder o This state of disorder, or randomness is known as entropy Decreasing entropy (i.e., maintaining order) requires energy o What does this mean for the cell? Transfer of Energy The most fundamental processes of energy transfer for life are photosynthesis and respiration Photosynthesis Requires 689 kcal of energy to produce 1 mole of glucose Figure 5.4 (McArdle) Transfer of Energy Cellular Respiration Chemical potential energy is stored in glucose is released and transferred Oxidation of 1 mole glucose releases 689 kcal Figure 5.5 (McArdle) Thermodynamics The first law of thermodynamics is often written in the form of an equation describing the relationships among energy, heat, and work DE = Efinal - Einitial = ± q – w Where: o DE is the change in internal energy of the system o q is the amount of heat absorbed or given off between the system and the surroundings o w is the work done by the system on the surroundings Thermodynamics During metabolic processes in individuals, the total change in internal energy (i.e., oxidation of fuels and of energy from ATP and PCr) is negative and equal to the sum of the heat given off and external work done DE = – q – w What does this mean for us? o Energy from foodstuffs can be transferred to heat and work Energy in = Energy out + Energy out ± Energy stored (food) (work) (heat) (fat) Chemical Reactions Energy is transferred through a system by a series of chemical reactions A+B C+D Important characteristics of reactions: Reaction Rate = rate of disappearance of reactants or the rate of appearance of products (M/s) Free Energy = the potential energy stored in chemical bonds Free Energy Change Products will have higher or lower free energy than the reactants, where: Exergonic Reactions release energy (-ve ∆G) o To do work o Dissipated as heat o Stored as potential energy Endergonic Reactions require a net input of energy (+ve ∆G) o Remains trapped in bonds during synthesis reactions Exergonic (- ∆G) vs. Endergonic (+ ∆G) Figure 4.3 (Silverthorn) Glucose Metabolism Exergonic or Endergonic? Figure 6.1 (McArdle, 7th) Glucose Metabolism Coupled Reactions Where do endergonic reactions get the required energy? Exergonic Endergonic Exergonic Endergonic Coupled Reactions Energy can also be ‘trapped’ for later use in the form of high-energy electrons carried on nucleotides (NADH & FADH2) Figure 6.2 (McArdle) Reversible Reactions Many reactions in the cell are reversible, for example: A+B C+D Reversible reactions will proceed towards a state of equilibrium, where the ratio of products to substrates is always equal to the reaction’s equilibrium constant: [C][D] Keq = [A][B] This is often referred to as the law of mass action Reversibility of Reactions The net free energy change (∆G) plays a critical role in determining the reversibility of a reaction. Why? A+B C+D -∆G +∆G ∆G, Keq, Reversibility and Control The change in free energy (DG) available to do work from a chemical reaction is related to the equilibrium (Keq) constant of the reaction Table 2-1 (Brooks) High negative DG reactions require significant energy for the reverse reaction. o In biological systems these are essentially, irreversible This makes these reactions (more specifically, the enzymes involved) an important point of control during metabolic processes Enzymes Enzymes Enzymes are highly specific protein catalysts that accelerate the rate of chemical reactions without being consumed or changed These reactions would occur spontaneously at a slower rate Enzymes reduce the activation energy by binding the reactant molecules and bringing them together in the best position to react Enzymes Enzymes Substrates attach to enzymes at a specific binding site The two models used to explain enzyme and substrate binding are the lock and key model and the induced-fit model Most enzymes react with only one set of substrates (or with a group of very similar substrates) Figure 2.10 (Silverthorn) Modulating Enzyme Activity There are several factors that can increase or decrease enzyme activity: Temperature and pH Chemical Modulators o Competitive Inhibition o Allosteric Activation o Allosteric Inhibition Modulating Enzyme Activity Temperature pH Modulating Enzyme Activity Figure 2.12 (Silverthorn) Classification of Enzymes Oxidoreductases o Catalyze oxidation-reduction reactions o Dehydrogenases, oxidases, peroxidases, etc.. Redox reactions involve the transfer of hydrogen atoms or electrons (e-) o Gain of e-’s or decrease in valence is reduced o Loss of e-’s or increase in valence is oxidized X-H2 NADH + H+ NADH + H+ Y-H2 X NAD+ NAD+ Y NAD+ is an Oxidizing Agent NADH is a Reducing Agent (i.e., accepts e-’s) (i.e., donates e-’s) Classification of Enzymes Transferases o Transfer elements of one molecule to another o e.g., Kinases, transcarboxylases, transaminases Hydrolases o Cleave bonds by adding water o e.g., Phosphatases, peptidases Lyases o Groups of elements are removed to form a double bond or added to a double bond o e.g., Synthases, deaminases, decarboxylases Classification of Enzymes Isomerases o Rearrangement of the structure of molecules o e.g., Mutases, isomerases Ligases o Catalyze bond formation between substrate molecules o Involves breakdown of ATP o e.g., synthetase, carboxylase Rate of Reaction and Enzymes Enzymes impact the rate of reaction o This makes enzymes important control points in metabolism and other biochemical processes Exercise training (and other physiological adaptations) can significantly impact enzyme concentration and activity o Training will increase enzyme concentration and activity and so increase the rate of energy turnover (i.e., rate of ATP production and turnover) but NOT the amount of energy from ATP o Disease or disuse will decrease the enzyme concentration and activity Rate of Reaction and Enzymes Velocity of reactions and maximal reaction velocity (Vmax) are significantly impacted by enzyme concentration o A hallmark adaption to training is increased concentration of enzymes, especially those that are key regulating enzymes o De-training and disuse causes the opposite adaptation Higher [Enzyme] Vmax (after training) Reaction Rate (product / time) Lower [Enzyme] Vmax (before training) Substrate Concentration (S) Fuel Sources, ATP and the Metabolic Road Map Food Sources of Energy Carbohydrate (CHO) Converted to glucose to be transported in the blood o Stored in liver and muscle as glycogen which is readily available for ATP production o ‘Starting’ substrate for glycolysis These reserves are limited, therefore must be replenished with diet Yields ~4 kcal/g Food Sources of Energy Lipid Only triglycerides are used for metabolism o Must be broken down from its triglyceride form to glycerol and free fatty acids (FFA) o FFAs undergo β-oxidation Much larger body stores compared to CHO Yields ~9 kcal/g Food Sources of Energy Protein Proteins must be broken down to amino acids to be used as a source of energy Can be used as starting point for glucogenesis Amino acids can also be used to create Krebs cycle intermediates Can generate FFAs in times of starvation through lipogenesis Yields ~4-6 kcal/g Fuel Storage in the Body g kcal Carbohydrates Liver glycogen 110 451 Muscle glycogen 250 1,025 Glucose in body fluids 15 62 Total 375 1,538 Lipid Subcutaneous 7,800 70,980 Intramuscular 161 1,465 Total 7,961 72,445 Note. These estimates are based on a body weight of 65 kg with 12% body fat. Fuel Use In General, oAt rest, the body uses CHO and Lipid for energy oDuring mild to severe muscular effort, the body relies mostly on CHO for fuel oLipids provide substantial energy during prolonged, low-intensity activity oProtein (a.a.) can provide as much as 10-15% of energy for cellular activity oAlso serve as building blocks for the body's tissues High Energy Phosphates ATP consists of an adenosine ring combined with three phosphate groups Figure 3.10 (Powers) The third phosphate group is attached to ADP by a high energy covalent bond ATP: The Currency of Energy Hydrolysis of ATP ATPase ATP + H2O ADP + Pi Exergonic or Endergonic? ∆G = -7.3 kcal/mol This is an exergonic reaction, therefore provides energy which can be used to do work ATP Homeostasis ATP is the energy ‘currency’ used to power muscle contractions and other forms of cellular work (chemical, transport, mechanical) This requires maintenance of [ATP] over a very wide range of turnover rates The essence of metabolism is to maintain constant [ATP] and therefore the energetic state of the cell despite changing energy needs of the cell o The cell must be able to modify rate of ATP production to match rate of ATP use over a wide range of conditions o The [ATP] and/or the ratio of ATP/ADP are important regulators of metabolism; several key enzymes are affected by this There are three main ‘pathways’ to maintain [ATP] over the wide range of required turnover rates and durations during activities…. Energy (ATP) Pathways 1. The ATP – PCr System Anaerobic alactic – ATP is supplied immediately and provides energy at a high rate but has a low capacity 2. The Glycolytic System Anaerobic/nonoxidative – ATP is supplied at an intermediate rate and capacity and does not require oxygen, but is intimately involved in aerobic metabolism 3. The Oxidative System Aerobic – ATP is supplied at a slower rate, but very high capacity Energy (ATP) Pathways These three pathways work together to meet the needs of the cell as opposed to switching ‘on’ and ‘off’ Table 3.5 Figure 3.2 (Brooks) (Brooks) Metabolism Atlas Figure 6.8 (McArdle) Metabolic Road Map Figure 4.8 (Silverthorn) Regulation of Metabolism The flow of molecules/energy through metabolic pathways is regulated in several ways: 1. By controlling enzyme concentration 2. By producing allosteric modulators 3. By using two different enzymes to catalyze reversible reactions 4. By isolating enzymes within organelles 5. By maintaining an optimum ratio of ATP-to-ADP ATP-Phosphocreatine ATP-PCr: The Phosphagen System ‘Simplest’ energy system used Takes place in the cytosol adjacent the the contractile apparatus Phosphocreatine (PCr) o aka creatine phosphate (CP) o Like ATP, has a high-energy phosphate bond o Not used directly for work, rather to produce ATP Creatine Kinase PCr + ADP ATP + Creatine ATPase ATP ADP + Pi + Energy Work The Phosphagen System The CK reaction and ATP Hydrolysis are coupled at the sarcomere o CK is located on the M-line The overall outcome is: o [PCr] decreases o Pi Increases o [ATP] remains constant o Heat is produced o Work is done PCr and ATP Changes with Exercise McCann DJ et al. Med Sci Sports Exerc. 1995 Mar;27(3):378-89 Recovery of PCr Recovery of PCr occurs quickly Approximately 60-70% replenished 30 sec after exercise o Full recovery in ~2-4 min Recovery of PCr How do we replenish PCr?! o Requires ATP – recall, the energy released from ATP hydrolysis is required to produce PCr o Where is the ATP from? o Aerobic Metabolism o EPOC So, is the phosphagen system really anaerobic/nonoxidative? Recovery of PCr PCr Shuttle Figure 6-12 (Brooks) Adenylate Kinase Reaction Adenylate Kinase ADP + ADP ATP + AMP a.k.a. the myokinase reaction, this reaction occurs as energy requirement increases o Also occurs in the cytosol AMP increases the activity of the enzyme phosphofructokinase (PFK) and glycogen phosphorylase kinase Essentially, this signals for the increased activity of glycolysis Non-oxidative Metabolism: Glycolysis and Lactate Glucose Transport (Silverthorn) (Silverthorn) Figure 5.21 Figure 5.13 Vmax and KM Vmax is maximal velocity of reaction/transport o Related to enzyme/transporter concentration KM is the substrate concentration at which ½ Vmax is achieved o Related to enzyme/transporter affinity for substrate o High KM = low affinity o Low KM = high affinity Glucose Transporters Approx. KM Transporter Location Characteristics (mM) Ubiquitous (brain, Constitutive glucose 5-20 heart, muscle transporter; basal GLUT 1 (as low as 1?) endothelium, 𝛽-cells, glucose uptake; high RBC, etc.) affinity 15-20 Liver, kidney, small Bi-directional; low GLUT 2 intestine, 𝛽-cells affinity (as high as 42) Neurons, brain, Very high affinity GLUT 3 1-10 placenta Skeletal muscle, heart, Insulin-responsive GLUT 4 2-10 adipocytes Small intestine, kidney, Fructose transporter GLUT 5 NA brain, adipocytes, muscle Overview of Glucose Metabolism Liver Vs. Skeletal Muscle Hexokinase (in muscle) has a low KM (i.e., high affinity for glucose) o G-6-P reaction is not reversible in muscle Glucokinase (in liver) has a high KM (i.e., low affinity for glucose) o Ensures storing glucose in liver only happens at higher [Glucose] o G-6-P reaction is reversible in liver Note the KM of the GLUT (2 vs. 4) matches the KM of the enzyme in the cell Glycolysis All enzymes for glycolysis are found in the cytosol See Figure 6.10 (McArdle) Glycolysis There are two (primary) sources of glucose for glycolysis: o Glucose from the blood (via digestion or hepatic glucose production) o Glycogen stored in muscle (the majority of G-6-P in muscle originates from glycogen) Figure 3.14 (Powers) Glycogen Glycogen is a glucose polysaccharide stored in muscle and liver Glycogenolysis is really a two-step process: 1. Removal of a glucose monomer from glycogen by way of cleavage with inorganic phosphate to produce glucose-1-phosphate using glycogen phosphorylase 2. Conversion of G-1-P to G-6-P using phosphoglucomutase Glycogenesis is the reverse process by which glucose is joined to produce glycogen o G-1-P is converted to UDP-Glucose, then assembled into glycogen using glycogen synthase Glycogenolysis In the liver, glucose-6-phosphatase can remove the Pi from G-6-P...which leaves?? o Glucose released into blood (recall: GLUT 2 is bi-directional) Skeletal Muscle does not have this enzyme...so? o G-6-P goes through glycolysis o No role in releasing glucose into the blood Glycogen phosphorylase is activated by phosphorylation involving PKA in liver and muscle o Epinephrine and glucagon via GPCR and adenylyl cyclase In muscle, glycogen phosphorylase kinase is allosterically activated by AMP and Ca2+ Glycogenolysis Figure 6.8 (Silverthorn) ….back to Glycolysis Glucose + 2 NAD+ + 2 ADP + 2 Pi 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H20 There are 3 main control points in glycolysis 1. Hexokinase 2. Phosphofructokinase – PFK-1: glycolysis – PFK-2/FBP-2: gluconeogenesis 3. Pyruvate Kinase These are essentially irreversible reactions under physiological conditions – why might this be? o Any issues with this? o What about gluconeogenesis in the liver and kidney? Regulation of Glycolysis in Muscle Hexokinase is allosterically inhibited by G-6-P Glycogen Phosphorylase is activated by Ca2+, AMP, Pi, Epinephrine Phosphofructokinase-1 (PFK-1) is the rate limiting enzyme of glycolysis o Allosterically inhibited by ATP and citrate o Allosterically activated by AMP, ADP and Pi o Also inhibited by decreased pH Pyruvate Kinase is allosterically inhibited by ATP o Allosterically activated by PEP and F-1,6-BP Note the importance of the energy state of the cell (i.e., the ATP/ADP + Pi ratio) in regulating key enzymes Regulation of Glycolytic Enzymes Summary Glucose Glycogen Hexokinase Glycogen Phosphorylase Very high affinity for glucose Activated by Pi, AMP, Ca2+, G-6-P G-1-P Inhibited by G-6-P glucagon & EPI Phosphofructokinase (PFK) Inhibited by ATP and citrate F-1,6-BP Activated by AMP, ADP, Pi Inhibited by decreased pH PEP Pyruvate Kinase Inhibited by ATP Pyruvate Activated by PEP and F-1,6-BP Products of Glycolysis How the products of glycolysis are handled by the cell depends on the availability of oxygen and the redox potential of the cell The redox potential is another indication of the energy state of the cell o It is a ratio of NADH/NAD+ o High levels of NADH relative to NAD+ leads to a low oxidative potential and less ability to transfer electrons o Lower levels of NADH relative to NAD+ leads to a balanced or high oxidative potential means greater ability to transfer electrons If oxygen availability is sufficient, the 2 NADH + 2 H+ produced in glycolysis are shuttled to mitochondria and feed into electron transport chain If the supply of oxygen is not sufficient (or the cell has met its capacity to use it….), these are used to produce lactic acid from pyruvate Fate of Pyruvate Figure 3.16 (Powers) This reaction is one of the mechanisms that regenerates NAD+ in the cytoplasm o This is crucial in managing the redox potential in the cytoplasm and allowing glycolysis to continue (i.e., Glyceraldehyde-3-Phosphate dehydrogenase) Some lactic acid is always produced even with adequate oxygen present simply because substrate for the LDH reaction is available o The reaction must follow the law of mass action Lactic Acid Vs. Lactate The ionization of lactic acid forms the conjugate base called lactate Figure 3.12 (Powers) These terms are often used interchangeably o Not entirely correct to do so The lactic acid produced in glycolysis rapidly disassociates to lactate and H+ Fate of Lactate – Waste Product? Lactate can exit the exercising muscle by way of the monocarboxylate translocase (MCT) in the sarcolemma o Carried in blood to other tissues In other muscle cells (heart and skeletal), LDH reaction forms pyruvate o Pyruvate can be used to produce Acetyl Coenzyme A In the liver, lactate can be used to produce glucose/glycgen o The Cori cycle and the ‘glucose paradox’ Fate of Lactate – Waste Product? Intracellular Lactate Shuttle Figure 5-14 (Brooks) Oxidative Metabolism: Krebs Cycle Oxidative Metabolism a.k.a. aerobic metabolism, oxidative phosphorylation Lipid, carbohydrate and protein (a.a.) may all be fuel for oxidative metabolism, while lipid and protein have no nonoxidative metabolic pathways Oxidative metabolism occurs in the mitochondria Pyruvate’s Oxidative Fate If sufficient oxygen is available and the redox potential is favourable, pyruvate enters the mitochondria and is converted to acetyl Coenzyme A (acetyl CoA) via the pyruvate dehydrogenase (PDH) complex The PDH complex involves a series of reactions which are irreversible and yields one CO2 and one NADH + H+ Recall: one glucose yields two pyruvate Pyruvate’s Oxidative Fate The acyl unit from Acetyl CoA (2C) reacts with oxaloacetate (4C) to form citrate (6C) MCT Citrate (or citric acid) then proceeds through the Krebs Cycle – a.k.a. the Citric Acid Cycle or The Tricarboxylic Acid Cycle (Silverthorn) Figure 4.16 The Krebs Cycle The purposes of the Krebs ‘cycle’ are decarboxylation (CO2 production), ATP formation and, most importantly, NADH and FADH2 production NADH and FADH2 are reducing agents o Able to transfer electrons to the electron transport chain where they are oxidized o The term oxidative phosphorylation refers to coupling their oxidation to phosphorylation of ADP to produce ATP See Figure 6.14 (McArdle) Krebs Cycle Tricarboxylic Acid Cycle Figure 6-7 (Brooks) Regulation of the Krebs Cycle The PDH complex is inactivated by phosphorylation o Uses ATP, and so PDH is inhibited by high ATP/ADP o High NADH/NAD+ and Acetyl CoA/CoA also inhibit PDH It is activated by dephosphorylation o Insulin, pyruvate and Ca2+ Figure 6-9 (Brooks) Regulation of the Krebs Cycle Isocitrate dehydrogenase (IDH) is the rate limiting enzyme of the Krebs cycle IDH, citrate synthase (CS) and α-ketogluterate dehydrogenase (α-KDH) are all allosterically activated by ADP and inhibited by ATP (so, inhibited by high ATP/ADP) All dehydrogenase reactions are sensitive to the redox potential in the mitochondria o Dehydrogenases are inhibited by high redox potential and stimulated by a decline in redox potential (So, inhibited by high NADH/NAD+) Recall: PFK can be inhibited by citrate o Serves as a signal to glycolysis how much energy transfer is occurring in the Krebs cycle o Serves as an important regulatory mechanism for balancing lipid and glucose use The Krebs Cycle: Summary Citrate is metabolized to oxaloacetate o Two CO2 molecules given off Produces three molecules of NADH and one FADH2 Also forms one molecule of GTP o Produces one ATP Enzymes are controlled primarily by ATP/ADP and redox potential Oxygen is not directly needed for the Krebs cycle, although NAD+ and FAD are o These are regenerated in the electron transport chain, which does require oxygen  Oxidative Metabolism: The Electron Transport Chain The Electron Transport Chain Intermembrane space matrix The Electron Transport Chain ‘Chemiosmotic Coupling’ ATP/ADP translocase See Figure 3.21 (Powers) The Electron Transport Chain Complex I o NADH dehydrogenase oxidizes NADH to NAD+ o 2 e-’s are carried by a series of redox reactions involving FMN/FMNH2, Fe2+/Fe3+ and coenzyme Q/QH2 o ∆G from these reactions powers active pumping of 4 H+ from the mitochondrial matrix into the intermembrane space Complex II o Succinate Dehydrogenase o FADH2 is oxidized to FAD o 2 e-’s carried as in complex I, also involving coenzyme Q/QH2 o No protons are pumped The Electron Transport Chain Complex III o Receives 2 e-’s from both complex I and II via coenzyme QH2 (i.e., e- pathways from NADH and FADH2 converge here) o Redox reactions pass e-’s through cytochromes b, c and c1 (this involves Fe2+/Fe3+) o Again, 4 H+ are ‘moved’ into the intermembrane space for each pair of e-’s Complex IV o e-’s passed through redox reactions involving cytochromes a and a3 o Oxygen serves as the final e- acceptor, producing H20 o 2 H+ pumped for each pair of e-’s ATP Synthase The F0F1 ATP Synthase uses the H+ gradient to produce ATP o F0 portion resides in the inner membrane o F1 portion extend into matrix The kinetic energy of H+ movement provides the energy needed to form ATP o 1 ATP formed for every 4 H+ Each NADH ‘adds’ 10 H+ to the gradient while FADH2 ‘adds’ 6 H+ o 2.5 ATP/NADH o 1.5 ATP/FADH2 https://www.youtube.com/watch?v=_GPDsQnnvrA Cytoplasmic NADH + H+ The Glycerol Phosphate Shuttle The Malate-Asparate Shuttle Figure 5-12 (Brooks) Generates FADH2 in the mitochondria Generates NADH in the mitochondria Skeletal muscle Heart and liver ATP Transport Figure 6-12 (Brooks) ATP Yield From Oxidation of Glucose ATP Yield from Oxidation of Glucose Efficiency Complete oxidation of 1 mole of glucose: C6H12O6 + 6O2 → 6CO2 + 6H20 -∆G=689 kcal Recall, 7.3 kcal is needed to synthesize each mole of ATP…theoretically, glucose should yield 94 moles ATP! With ‘only’ 30-32 moles ATP formed, oxidative metabolism of glucose is 32-35% efficient – the remaining energy is dissipated as heat Lipid Metabolism Lipid Metabolism Figure 6.17 (McArdle) Lipid Metabolism Only triglycerides are major lipid fuel sources for metabolism Composed of a glycerol (3C) backbone along with 3 long-chain fatty acids (FAs) Most common FAs are stearic acid (18C), oleic acid (18C) and palmitic acid (16C) Triglycerides Adipocytes are the major storage cells for triglycerides in the body o Occupy 95% of the cell volume Triglycerides are broken down (via lipolysis) in the adipocyte by the enzyme lipase: Triglyceride + 3 H2O → glycerol + 3 FAs FAs diffuse into the blood and are carried bound to albumin as free fatty acids (FFAs) Mobilization of FFAs Overview of lipolysis and FFA Transport in the Blood Mobilization of FFAs FFAs and glycerol are delivered via the blood o Therefore, dependent on blood flow o Movement into the cell also depends on [FFA] in blood FFAs enter the cell by way of the sarcolemmal fatty acid binding protein (S- FABP) o FFAs need to be transported to the mitochondria to undergo β-oxidation Glycerol is transformed into G-3-P o Glycerol kinase not identified in muscle to date, only in liver o What could this mean for the potential use of glycerol? FFA Activation and Translocation FFAs must be activated in the cytoplasm before entering the mitochondria o Addition of a coenzyme A group producing a fatty acyl-CoA molecule o Requires 2 Pi from ATP (leaving AMP) The fatty acyl-CoA is transported into the mitochondria using the enzyme carnitine acyltransferase and the carnitine transporter Carnitine Transporter Carnitine Transporter β-Oxidation Figure 7-8 (Brooks) β-Oxidation β-Oxidation continues until the entire fatty acyl-CoA has been broken down into 2C acetyl-CoA units For example: o Palmitic acid has 16 carbons o How many cycles through β-Oxidation are needed? β-Oxidation Acetyl-CoA from β-oxidation of FAs has the same fate as that which is produced by the PDH complex following glycolysis Figure 6.18 (McArdle) From here, metabolism of lipids and CHO share the same pathway ATP Yield Example: Total ATP yield from one chain of palmitic acid with 16C Activation of FFA - 2 ATP -2 ATP β-Oxidation 7 X 1 NADH + H+ 17.5 ATP 7 X 1 FADH2 10.5 ATP Krebs Cycle 8 X 1 ATP 8 ATP 8 X 3 NADH + H+ 60 ATP 8 X 1 FADH2 12 ATP TOTAL: 106 ATP Efficiency Complete oxidation of 1 mole of palmitic acid is as follows: C16H32O2 + 23O2 → 16CO2 + 16H20 -∆G = ~2333 kcal Recall, 7.3 kcal is needed to synthesize each mole of ATP…theoretically, palmitic acid should yield ~319 moles ATP! With only 106 moles of ATP formed, oxidative metabolism of palmitic acid is ~33% efficient – the remaining energy is dissipated as heat Interaction Between Carbohydrate and Lipid Metabolism Interaction Between Carbohydrate and Lipid ‘Glucose-Fatty Acid Cycle’ HK PFK PDH Figure 7-12 (Brooks) Carbohydrate Vs. Lipid The “Crossover Concept” Figure 4.11 Figure 7-13 (Powers) (Brooks)  in an increased rate crease in lipolysis results in an increase in blood and on (19). Interestingly, muscle levels of FFA and promotes fat metabolism. In ck phosphorylase and general, lipolysis is a slow process, and an increase ical Applications 4.1). in fat metabolism occurs only after several minutes of te inhibits fat metabo- exercise. This point is illustrated in Fig. 4.12 by the Carbohydrate Vs. Lipid of fat as a substrate slow increase in fat metabolism across time during e for working muscles prolonged submaximal exercise. hat carbohydrate will 5 for more details on 70 se as a means of keep- % Fat or carbohydrate metabolism 65 n check, and you are % Fat exercise equipment 60 ng” workout. In such 55 w intensity and long best way to burn fat? 50 swer. 45 40 % CHO Selection 35 than 30 minutes), V̇O2max) exercise the 30 g a gradual shift from 0 20 40 60 80 100 120 Figure 4.12 an increasing reliance Exercise time (min) (Powers) 6, 88, 96). Figure 4.12 Figure 4.12Shift from carbohydrate metabolism toward fat metabolism during prolonged exercise. Chapter Four Exercise Metabolism 79 Carbohydrate Vs. Lipid Recall that fat provides/stores more energy by weight than does CHO (9 kcal/g vs. 4 kcal/g) However, fat oxidation provides less usable energy per litre of oxygen oFat yields 4.70 kcal/LO2 oCHO yields 5.05 kcal/LO2 Since oxygen delivery is limited as intensity increases and lipid oxidation is ‘slow,’ CHO is the preferred fuel for higher intensity exercise o Also need to consider muscle fibre type Carbohydrate Vs. Lipid The glucose-fatty acid cycle is understood to have its primary function during mild to moderate exercise o i.e., when both substrates are used It will also play a key role in recovery Why might these be beneficial to exercise and recovery? o Glycogen depletion can lead to fatigue, therefore fatty acid metabolism’s suppression of glycogenolysis can assist in preserving glycogen stores o In recovery, this can help in restoration glycogen stores Carbohydrate Vs. Lipid The heart and liver are highly specialized for lipid utilization Brain and RBCs rely almost exclusively on glycolytic means Skeletal muscle fuel use is dictated by fibre type o Red muscle (I, SO) has a high capacity to use lipid o rich capillarization, high [myoglobin], high mitochondrial density, and large population of FABPs o White muscle (IIb/x, FG) has a low capacity to use lipid o FOG (IIa) fibres are intermediate in many ways Intramuscular Lipid Intramuscular Lipid Intramuscular triglycerides: Are not mobilized during most activities o Probably recruited after glycogen depletion o A “reserve tank” of sorts Are mobilized during recovery from glycogen depleting exercise o Likely to help in restoration of glycogen Increase with endurance training o Likely to spare glycogen use during exercise (?) o May also help increase rate of glycogen replenishment Protein Metabolism Protein Metabolism Only amino acids can be used in metabolism Protein Metabolism General Structure of Amino Acids Anything familiar? – COO- – Where else have we seen these? Need to remove the amine group by transamination or oxidative deamination Protein Metabolism Transamination Oxidative Deamination Amino Acid Metabolism Figure 8 -8 (Brooks) Amino Acid Metabolism Amino acids contribute 5-10% of the substrate supply and become increasingly important in negative energy balance (prolonged exercise, starvation, etc…) The Krebs cycle supports several functions and so intermediates can become depleted o Amino acids can help contribute to the replenishment of intermediates which supports lipid and carbohydrate metabolism Alanine is an important gluconeogenic precursor o Alanine can be used in the Cori cycle to maintain blood glucose during exercise High levels of cortisol, a proteolytic hormone, are released during prolonged exercise or starvation o Need dietary replenishment of several amino acids  Summary: Integration of Metabolic Pathways Integration of Metabolic Pathways Figure 25.1 (Silverthorn) “The Metabolic Mill” See Figure 6.18 (McArdle) “Nutrient Pools” Figure 22.3 (Silverthorn) Meeting Increasing Energy Demand When metabolic demand increases, energy supply is not merely the result of a series of energy systems that switch “on” and “off” in some order Rather, there is a smooth blending of the systems with considerable overlap from one mode of energy supply to the next Consider exercise of increasing intensity... Figure 11.2 (McArdle)