Lecture 2.1 Energy Production Carbohydrates PDF - University of Buckingham

Lecture 2.1 Energy Production from Carbohydrates (1) Dr Andrew Irving [email protected]  Learning Outcomes Describe the general structures and functions of carbohydrates Describe how dietary carbohydrates are digested and absorbed Explain why cellulose is not...

Lecture 2.1 Energy Production from Carbohydrates (1) Dr Andrew Irving [email protected]  Learning Outcomes Describe the general structures and functions of carbohydrates Describe how dietary carbohydrates are digested and absorbed Explain why cellulose is not digested in the human gastrointestinal tract Describe the glucose-dependency of some tissues Describe the key features of glycolysis Explain the biochemical basis of the clinical conditions of lactose intolerance Understand the metabolic requirements for lactic acid/lactate generation Explain how the blood concentration of lactate is controlled  Lecture Outline Overview of catabolism Carbohydrates Stage 1 of carbohydrate catabolism Lactose intolerance Stage 2 of carbohydrate of catabolism Glycolysis Anaerobic glycolysis and lactic acid production  The sweetest thing Which natural sugar is the sweetest? What is the difference between artificial sweeteners sugar?  The sweetest thing Sugar-free victoria sponge Contains 1 tbsp xylitol Overview of Catabolism Stage 1: Dietary macronutrients are broken down to cellular fuel molecules Stage 2: Transformation of fuel molecules to metabolic intermediates (reducing power and some energy release) Stage 3: The tricarboxylic acid (TCA), also called Krebs cycle or citric acid cycle (reducing power and some energy release) Stage 4: Oxidative phosphorylation (conversion of reducing power into ATP)  Key Points About Metabolism From Week 1 Metabolism involves a series of sequential reactions in defined metabolic pathways: Catabolic (breakdown) Anabolic (synthetic) Catabolism involves the breakdown of chemicals to release: Organic precursors e.g. pyruvate Reducing power (e.g. NADH⁺ + H⁺) oxidative Energy (ATP) phosphorylation  Catabolism - Stage 1 Extracellular (GI tract) Carbohydrates, fats, and proteins are digested to monosaccharides, fatty acids and glycerol, and amino acids respectively Fuel molecules are absorbed from GI tract into circulation No energy produced  Catabolism - Stage 2 Intracellular (cytosol & mitochondria) Fuel molecules are transported to tissues and are converted into various metabolites Oxidative Requires H+ carriers which are then reduced (e.g. NAD+ → NADH⁺ + H⁺) Reducing power is released Some energy as ATP is produced  Catabolism - Stage 3 The TCA cycle (Krebs or citric acid cycle) Intracellular (mitochondria) Oxidative Acetyl CoA is oxidised to CO2 Requires NAD+, FAD Reducing power is released Some energy (as GTP=ATP) is produced  Catabolism – Stage 4: Electron transport and ATP synthesis (Oxidative Phosphorylation) Intracellular (mitochondria) NADH⁺ + H⁺ & FADH2 re‐oxidised O2 required (reduced to H2O) Free energy from electron transport is used to synthesise large amounts of energy (ATP) Summary of Catabolic Metabolism Amino Acids Glucose Fatty Acids Alcohol NH3 Keto-acids Pyruvate Acetyl CoA Urea CO2  Body Composition And Dietary Intake - Carbohydrates 70kg male 55kg female Dietary Intake Carbohydrate 1% 1% 15% Lipid 16% 25% 8% Protein 16% 15% 5%  What Are Carbohydrates? General formula: (CH2O)n Contain aldehyde: (‐CHO) C=O or keto: (‐C=O) group H Contain many –OH groups Hydrophilic Do not pass across cell membranes without help Partially oxidised Need less oxygen than fatty acids for complete oxidation  Monosaccharides Single sugar units (3‐9 C atoms): Triose – 3 carbons (most common) e.g. glyceraldehyde Pentose – 5 carbons (ribose) Hexose – 6 carbons (glucose, fructose, galactose) They are: aldehyde‐containing sugars (aldoses) e.g. glucose, galactose keto‐containing sugars (ketoses) e.g. fructose  3‐Carbon Monosaccharides (Trioses) (CH2O)3 aldoses ketose Asymmetric C-atom → stereoisomers Naturally occurring forms → D isomers D-Glucose vs L-Glucose Fischer projection 5 or > 5 Carbon Monosaccharides Exist mainly as ring structures C6H(CH O) 12O26 6 D-glucose Fischer projection Haworth projection Carbonyl group reacts with alcohol group to form a ring  α- and β- D- Glucose The position of OH group on C1 determines whether D-glucose has α- or β- structure A glucose solution contains about one third of α-D glucose, two thirds of β-D-glucose and a very small amount of the straight chain D-glucose  Polymers of Monosaccharides Disaccharides (2 monosaccharides) – lactose (galactose & glucose) – sucrose (fructose & glucose) – maltose (glucose & glucose) Oligosaccharides (3 –10 monosaccharides) – Dextrins (consists of glucose monomers) Polysaccharides (10 – 1000 monosaccharides) – glycogen, starch, cellulose (polymers of glucose monomers) Formation of Polymers of Monosaccharides Formed by condensation of monosaccharides: H2O is eliminated -O‐glycosidic bond is formed  α- and β- Glycosidic Bonds 1,4 α: OH group was below C1 1,4 β: OH group was above C1  Polysaccharides Glycogen – Polymer of glucose found in animals – Major store of glucose in mammals (liver, skeletal muscle) – 1‐4 and 1‐6 glycosidic bonds – Highly branched Starch – Polymer of glucose found in plants – Mixture of amylose (1‐4 bonds) and amylopectin (1‐4 and 1‐6 glycosidic bonds) – Less branched than glycogen – GI tract enzymes release glucose and maltose Cellulose – Structural polymer of glucose in plants – β1‐4 glycosidic bonds – No GI enzymes to digest β1‐4 bonds in cellulose – It forms dietary fibre that is important for GI function Polysaccharides - Glycogen Taken from Marks’ Basic Medical Biochemistry Polysaccharides - Glycogen Glycogen: a core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain around 30,000 glucose units  Catabolism of Carbohydrates: Stage 1 Extracellular (the GI tract) Polysaccharides are broken down to monosaccharides Monosaccharides are absorbed from GI to blood Digestion of Dietary Carbohydrates Mouth – Salivary α-amylase (1-4 bonds): Starch, Glycogen → dextrins and disaccharides Pancreas – Pancreatic α-amylase (1-4 bonds) Small intestine: Pancreatic amylase (1-4 bonds) dextrins → disaccharides Disaccharidases attached to brush border of epithelial cells (enterocytes): lactase (lactose) (β1-4 bonds) sucrase (sucrose) (1-2 bonds) maltase (maltose) (1-4 bonds) isomaltase (1-6 bonds) You will learn more about it in ….  Small Intestine: Brush Border http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/anatomy.html Brush border (microvilli)- apical membrane extensions of epithelial cells covering villi  Monosaccharide Transport Glucose, galactose and fructose are transported to enterocytes by facilitated or active transport SGLT1 GLUT2 GLUT2 (glucose transporter type 2) GLUT5 SGLT1 (Na+/glucose/galactose cotransporter) GLUT5 (fructose transporter type 5) They are transported from enterocytes to GLUT2 the blood by GLUT2 They are then transported to target tissues via various transporters (GLUT1-14)  GLUTs Kidne ys Lactose Intolerance- Consequences Undigested lactose is passed to the large intestine Colonic bacteria ferment lactose and produce organic acids and gases Lactose and organic acids increase osmotic pressure and draw in water causing diarrhoea Gases cause abdominal cramps and bloating  Lactose Intolerance - Causes Loss/reduction of lactase activity → lactose is not hydrolysed to glucose and galactose Genetic cause: Lactase activity is high in infants but decreases in childhood in most populations (especially African and Asian) Nongenetic cause: injury to the small intestine (e.g. inflammatory bowel disease, surgery, infection)  Lactose intolerance - Manifestation and Management NOT an allergic reaction to lactose -no immune system involvement Symptoms: − abdominal pain, discomfort, bloating, diarrhoea, nausea; appear in 30 to 120 minutes following consumption of lactose Diagnosis: − Positive hydrogen breath test − Positive stool acidity test Management: − Decrease or elimination of the amount of lactose in diet (lactose free diet) − Consumption of lactase-treated foods or lactase supplements  Glucose Requirements of Tissues Around 180g of glucose is needed per day Glucose is the major fuel molecule and is metabolised by all tissues Blood glucose is regulated (~5 mM) some tissues (RBC, WBC, kidney medulla, testes, lens and cornea of the eye) have an absolute requirement for glucose and glucose uptake by these tissues depends on its concentration in blood (approx. 40g/day) CNS (brain) prefers glucose as fuel (approx. 140g/day) Some tissues need it for specialised functions (liver, adipose)  Catabolism of Carbohydrates: Stage 2- Glycolysis Intracellular (cytosol) Occurs in all tissue types Oxidative Interactive Glycolysis  10- Step Glycolysis & its Phases Phase 1: Preparative (ATP-consuming) phase Phase 1 of glycolysis (reactions 1-3) 2 moles of ATP per mole glucose are used Phase 2: ATP-generating phase of glycolysis (reactions 4-10) Phase 2 4 moles of ATP per mole of glucose are synthesised Glycolysis-video Phase 1 of Glycolysis (Reactions 1 ‐ 3) Reaction 1: Phosphorylation of glucose to glucose‐6‐ phosphate (G‐6‐P) by hexokinase (glucokinase in liver) Prevents glucose from going back through the plasma membrane Increases the reactivity of glucose to permit subsequent steps Reaction 2: Isomerisation of G-6-P to fructose-6- phosphate (F-6-P) by phosphoglucose isomerase Reaction 3: Phosphorylation of F-6-P to fructose-1,6- bis phosphate (F-1,6-bis P) by phosphofructokinase-1 ‘Committing step’: first step that commits glucose to glycolysi s Reaction 1 and 3 have -ve G and are irreversible Phase 2 of Glycolysis (Reactions 4 ‐ 6) Reaction 4: Cleavage of (F-1,6-bis P) into two C3 units by aldolase: -DHAP (dihydroxyacetone phosphate) -glyceraldehyde 3- phosphate (G-3-P) Reaction 5: DHAP is rapidly converted to G-3-P by triose phosphate isomerase Therefore 2 moles of G-3-P pass through the rest of glycolysis Reaction 6: REDOX reaction Oxidation of aldehyde group of G-3-P to carboxyl group and addition of inorganic phosphate forming 1,3-bis phosphoglycerate (1,3-BPG) (catalysed by G-3-P dehydrogenase) Reduction of NAD⁺ to NADH⁺ + H⁺ Phase 2 of Glycolysis (Reactions 7 ‐ 10) 2 Reaction 7: Substrate level phosphorylation Transfer of phosphoryl group from 1,3- BPG to ADP to give ATP and 3- phosphoglycerate (3-PG) catalysed by phosphoglycerate kinase Reaction 8: Isomerisation of 3-PG to 2- PG by phosphoglyceromutase Reaction 9: Dehydration of 2-PG to form phosphoenolpyruvate (PEP), catalysed by enolase Reaction 10: Substrate level phosphorylation 2 Transfer of phosphoryl group from PEP to ADP to form pyruvate and ATP, catalysed by pyruvate kinase Large ‐ve G and is irreversible ATP Synthesis and Reducing Power Release Glucose (C6) + 2 Pi + 2 ADP + 2 NAD+ → 2 pyruvate (C3) + 2 ATP + 2 NADH⁺ + 2 H+ + 2 H2O Reactions 1 & 3 - 2 moles of ATP are used per mole of glucose to initiate pathway Reactions 7 & 10 - 4 moles of ATP are produced per mole of glucose Overall - net of 2 ATP per mole of glucose Anaerobic Glycolysis Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen are available. It is a means of energy production in cells that cannot produce adequate energy through oxidative phosphorylation. This occurs naturally during exercise and in certain disease states (see later). Anaerobic Glycolysis Under anaerobic conditions, pyruvate does not undergo oxidative phosphorylation in mitochondria. Instead, the cytosolic enzyme lactate dehydrogenase converts pyruvate to lactate. Although lactate itself is not utilized by the cell as a direct energy source, this reaction also allows for the regeneration of NAD+ from NADH. NAD+ is an oxidizing cofactor necessary to maintain the flow of glucose through glycolysis. Anaerobic glycolysis produces 2 ATP per glucose molecule, and thus provides a direct means of producing energy in the absence of oxygen  Anaerobic Glycolysis Regeneration of NAD+ during anaerobic glycolysis (step 11) 2 NADH⁺ + 2 H+ + 2 pyruvate 2 NAD+ + 2 lactate Lactate dehydrogenase CH3CO.COOH CH3CHOH.COOH Overall reaction of the 11- steps of anaerobic glycolysis: Glucose + 2 Pi + 2 ADP → 2 lactate + 2 ATP + 2 H2O  Where Does Lactate Go? Lactate is released into blood and is metabolised by liver, heart and kidney NADH⁺ + H⁺ : NAD⁺ ratio in liver and heart is lower than in exercising muscle  Cori Cycle (Lactic Acid Cycle) Cori cycle is the cycling of lactate and glucose between peripheral tissues (muscle) and liver Lactic released into blood is taken up by liver, oxidised to pyruvate which is then converted to glucose (gluconeogenesis) and released back into the blood The rate of lactate production = the rate of its utilisation Lactate utilisation is impaired in: Liver disease Vitamin deficiencies ‐ e.g. thiamine High alcohol intake Enzyme deficiencies  Lactate Production Without exercise - 40 ‐50 g/day RBC, skin, brain, skeletal muscle, G.I. tract Strenuous exercise (includes hearty eating!) - 30 g/5 min Plasma levels x 10 in 2 ‐ 5 min Back to normal by 90 min Also pathological situations involving hypoxia, e.g. SHOCK (insufficient blood flow to maintain tissue perfusion) eg SEPSIS (Septic shock due to infection) CONGESTIVE HEART DISEASE (failure of heart to pump blood effectively) ARTERIAL DISEASE (localised poor perfusion e.g. intermittent claudication) Elevations of Plasma Lactate Concentration Plasma lactate concentration determined by relative rates of: Production (↑ in strenuous exercise, hearty eating, shock and congestive heart disease and arterial disease) Utilisation (↓ in liver disease, vitamin and enzyme deficiencies and high alcohol intake) Disposal (kidney) Hyperlactatemia Lactic acidosis 2 - 5 mM in blood Above 5 mM in blood Below renal threshold Above renal threshold No change in blood pH Blood pH lowered (buffering capacity) 0 2 4 mM 6 8 10 Blood concentration normally constant