Chapter 3 Carbohydrate Metabolism PDF

Chapter 3 CARBOHYDRATE METABOLISM  The study of the biochemical reactions in an organism, including the coordination, WHAT IS METABOLISM? regulation and energy needs Definition:...

Chapter 3 CARBOHYDRATE METABOLISM  The study of the biochemical reactions in an organism, including the coordination, WHAT IS METABOLISM? regulation and energy needs Definition: Metabolism is the sum total of the chemical reactions of biomolecules in an organism Metabolism consists of 1. catabolism: the breakdown of larger molecules into smaller ones; an oxidative process that releases energy 2. anabolism: the synthesis of larger molecules from smaller ones; a reductive process Catabolism: thethat requires oxidative energy breakdown of nutrients Anabolism: the reductive synthesis of biomolecules  TERMINOLOGY OF METABOLISM light Eg. 6 CO2(g) + 6 H2O(l) → C6H12O6(aq) + 6 O2(g) Anabolism photosynthesis C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O respiration Catabolism ⚫ Metabolic pathway: A sequence of reactions, where the product of one reaction becomes the substrate for the next reaction. - either linear pathway or cyclic pathway - metabolic pathways proceed in many stages, allowing for efficient use of energy ⚫ Metabolites: intermediates in metabolic pathway METABOLIC PATHWAYS  THE ROLE OF OXIDATION AND REDUCTION (redox) Oxidation-Reduction IN METABOLISM reactions are those in which electrons are transferred from a donor to an acceptor – oxidation: the loss of electrons; the substance that loses the electrons is called a reducing agent – reduction: the gain of electrons; the substance that gains the electrons is called an oxidizing agent Reduced CarbonOxidized in most reduced form- alkane Carbon in most oxidized form- CO2 (final product of catabolism)  OXIDATION AND REDUCTION IN METABOLISM Reduction – gain e Oxidation – less e Oxidizing agent – e acceptor reducing agent – e donor  A group of noncovalently associated enzymes that catalyze 2 or more METABOLISM: FEATURES sequential steps in metabolic/biochemical pathway Metabolic pathway: 1. Enzymes – multienzymes 2. Coenzymes 3. ATP – produced or used Regulation of metabolic pathway: ⚫ Feedback inhibition or ⚫ Feed-forward activation  METABOLISM: REGULATION Regulation of metabolic pathway: 1. Feedback inhibition = product (usually ultimate product) of a pathway controls the rate of synthesis through inhibition of an early step (usually the firstE1step) E2 E3 E4 E5 — A→B→C→D→E→P 2. Feed-forward activation = metabolite produced early in pathway activates enzyme that catalyzes a reaction E1 E2 further E3 E4down E5 the pathway A+ →B→C→D→E→P  COENZYMES Coenzymes in metabolism: ⚫ NAD+/NADH ⚫ NADP+/NADPH ⚫ FAD+/FADH2 Electron carriers ⚫ Coenzyme A (CoASH) – activation of metabolites  NAD+/NADH: AN IMPORTANT COENZYME ⚫ Nicotinamide adenine dinucleotide (NAD+) is an important coenzyme ⚫ Acts as a biological oxidizing agent ⚫ The structure of NAD+/NADH is comprised of a nicotinamide portion. ⚫ It is a derivative of nicotinic acid ⚫ NAD+ is a two-electron oxidizing agent, and is Reduced form, NADH carries 2 electrons reduced to NADH  NADP+/NADPH: ALSO COMPRISED OF NICOTINAMIDE PORTION 🞭 Nicotinamide adenine dinucleotide phosphate (NADP+) – oxidizing agent 🞭 NADPH involves in reductive biosynthesis 🞭 Differ with NAD+ at ribose (C2 contain a phosphoryl group, PO32- 🞭 As electron carrier in photosythesis and Reduced form, pentose phosphate NADPH carries 2 Anabolis pathway  THE STRUCTURES FLAVIN ADENINE DINUCLEOTIDE (FAD) 🞭 FAD is also a biological The terminal e oxidizing acceptor (O2) can agent accept only unpaired e (e must be 🞭 FAD – can transferred to O2 one accept one- at a time) electron or two-electron FADH carries 1 electron, FADH2 carries 2 electrons  FAD/FADH2 FADH (semiquinone form) carries 1 electron, FADH2 (fully reduced hydroquinone form) carries 2 electrons Formation of fully reduced hydroquinone form bypass the  COENZYME A IN ACTIVATION OF METABOLIC PATHWAYS A step frequently encountered in metabolism is activation – activation: the formation of a more reactive substance – A metabolite is bonded to some other molecule and the free-energy change for breaking the new bond is negative. – Causes next reaction to be exergonic  COENZYME A (COASH) 🞭 Coenzyme A – functions as a carrier of acetyl and other acyl groups 🞭 Has sulfhydryl/thiol group Thioester bond Acetyl-CoA: is a “high-energy” compound because of the presence of phospho anhydrous bond – hydrolysis will release energy  ATP- HIGH ENERGY COMPOUND 🞭 ATP is essential high energy bond- containing compound 🞭 Phosphorylation of ADP to ATP requires energy 🞭 Hydrolysis of ATP to ADP releases energy nucleotide Phosphorylation: the addition of phosphoryl (PO32-) group/Pi (inorganic phosphate) THE PHOSPHORIC ANHYDRIDE BONDS IN ATP ARE “HIGH ENERGY” BONDS 🞭 “High Energy” bonds- bonds that require or release convenient amounts of energy, depending on the direction of the reaction 🞭 Couple reactions: the energy released by one reaction, such as ATP hydrolysis, provides energy for another reactions to completion – in metabolic pathway COUPLE REACTION: EXAMPLE BREAK-DOWN OF GLUCOSE TO GENERATE ENERGY - Also known as Respiration. - Comprises of these different processes depending on type of organism: I. Anaerobic Respiration II. Aerobic Respiration ANAEROBIC RESPIRATION Comprises of these stages: 🞭 glycolysis: glucose 2 pyruvate + NADH 🞭 fermentation: pyruvate lactic acid or ethanol 🞭 cellular respiration:  AEROBIC RESPIRATION Comprises of these stages: 🞭 Oxidative decarboxylation of pyruvate 🞭 Citric Acid cycle 🞭 Oxidative phosphorylation/ Electron Transport Chain(ETC) Brief overview of STARCHY catabolism of FOOD glucose to α – AMYLASE ; MALTASES generate energy Glucos Glucose converted to glu-6- e PO4 Start of cycle Glycolysis Cycle : in cytosol anaerobic Aerobic 2[Pyruvate+ATP+NAD condition; in Anaerobic H] Pyruvate enters as mitochondria condition AcetylcoA - Krebs Cycle Lactic Acid fermentation in muscle. - E transport chain Only in yeast/bacteria Anaerobic respiration or Alcohol fermentation GLYCOLYSIS Show time..  GLYCOLYSIS 🞭 Glycolysis is the first stage of glucose metabolism 🞭 Glycolysis converts 1 molecule of glucose to 2 units of pyruvate (three C units) and the process involves the synthesis of ATP and reduction of NAD+ (to NADH) 🞭 The pathway has 10 steps/reactions 🞭 Glycolysis are divided into 2 stages/phases, ⮚ Phase 1=1st 5 reactions ⮚ Phase 2=2nd 5 reactions Linear pathway GLYCOLYSIS 🞭 1st stage of glucose metabolism → glycolysis 🞭 An anaerobic process, yields 2 ATP (additional energy source) 🞭 Glucose will be metabolized via gycolysis; pyruvate as the end product 🞭 The pyruvate will be converted to lactic acid (muscles → liver) 🞭 Aerobic conditions: the main purpose is to feed pyruvate into TCA cycle for further rise of ATP GLYCOLYSIS 🞭 Glycolysis are divided into 2 stages/phases, 1. Phase 1=1st 5 reactions ⮚ Energy investment – ▪ A hexose sugar (glucose) is split into 2 molecules of three-C metabolite (glyceraldehyde-3-phosphate = GAP). The process consume 2 ATP 2. Phase 2=2nd 5 reactions ⮚ Energy recovery – ▪ The two molecules of GAP are converted to 2 molecules of pyruvate with the generation of 4 ATP and 2 NADH. ❖ Overall equation – Glucose + 2 NAD+ + 2 ADP + 2Pi → Glycolysis has a net “profit” of 2 ATP per The breakdown of glucose to pyruvate as summarized: Glucose (six C atoms) → 2 pyruvate (three C atoms) 2 ATP + 4 ADP + 2 Pi → 2 ADP + 4 ATP (phosphorylation) Glucose + 2 ADP + 2 Pi → 2 Pyruvate + 2 ATP (Net reaction) Fig. 17-1, Fig. 17-2, FATES OF PYRUVATE FROM GLYCOLYSIS 🞭 Once pyruvate is formed, it has one of several fates 🞭 In aerobic metabolism- pyruvate will enter the citric acid cycle, end product in aerobic metabolism CO2 and H2O 🞭 In anaerobic metabolism- the pyruvate loses CO2  GLYCOLYSIS 🞭 Dephosphorylation of ATP By kinase enzyme at 🞭 Phosphorylation of ADP step 1, 3, 7 and 10 🞭 Oxidation of intermediates and reduction of NAD+ to NADH by dehydrogenase reactions - step 6 - glyceraldehyde-3-phosphate dehydrogenase  ATP PRODUCTION ATP is produced by phosphorylation of ADP - is through substrate-level phosphorylation Substrate-level phosphorylation – the process of forming ATP by phosphoryl Glycolysis - Step 7 group transfer from reactive and 10 intermediates to ADP 1,3-bisphosphoglycerate and phosphoenolpyruvate – “high-energy” intermediates/compounds Oxidative phosphorylation – the process of Louis Pasteur - French biologist - did research on fermentation which led to important discoveries in microbiology and chemistry HOW 6-CARBON GLUCOSE CONVERTED TO THE 3-CARBON GLYCERALDEHYDE-3- PHOSPHATE? Preparation phase Step 1 Glucose is phosphorylated to give gluc-6-phosphate p.46 Fig. 17-3, Table 17-1, p.469 Fig. 17-4, Step 2 Glucose-6-phosphate isomerize to give fructose-6- phosphate p.470 Step 3 Fructose-6-phosphate is phosphorylated producing fructose-1,6-bisphosphate p.470 Fig. 17-6, Step 4 Fructose-1,6-bisphosphate split into two 3-carbon fragments p.471 Step 5 Dihydroxyacetone phosphate is converted to glyceraldehyde-3-phosphate p.471 HOW IS GLYCERALDEHYDE-3-PHOSPHATE CONVERTED TO PYRUVATE Payoff phase Step 6 Glyceraldehyde-3-phosphate is oxidized to 1,3-bisphosphoglycerate p.47 Fig. 17-7, p.474a Fig. 17-8, Step 7 Production of ATP by phosphorylation of ADP p.47 Step 8 Phosphate group is transferred from C-3 to C-2 p.477 Step 9 Dehydration reaction of 2-phosphoglycerate to phosphoenolpyruvate p.477 Step 10 Phosphoenolpyruvate transfers its phosphate group to ADP → ATP and pyruvate p.47 Control points in glycolysis Fig. 17-10,  ANAEROBIC METABOLISM OF PYRUVATE 🞭 Under anaerobic conditions, the most important pathway for the regeneration of NAD+ is reduction of pyruvate to lactate 🞭 Lactate dehydrogenase (LDH) is a tetrameric In muscle, during vigorous isoenzyme consisting of H and M subunits; H exercise – demand of 4ATP ↑ but predominates in heart muscle, O isand M4 supply in short 2 in skeletal ∴ is largely synthesized via anaerobic muscle glycolysis which rapidly generates ATP rather than through slower oxidative phosphorylation HOW IS PYRUVATE METABOLIZED ANAEROBICALLY? Conversion of pyruvate to lactate in muscle p.47 Fig. 17-11b,  ALCOHOLIC FERMENTATION In anaerobic 🞭 Two reactions lead to the production of ethanol: bacteria 🞤 Decarboxylation of pyruvate to acetaldehyde 🞤 Reduction of acetaldehyde to ethanol Pyruvate decarboxylase is the enzyme that catalyzes the first reaction This enzyme require Mg2+ and the cofactor, thiamine pyrophosphate (TPP) Alcohol dehydrogenase catalyzes the conversion of acetaldehyde to ethanol Pyruvate decarboxylase Fig. 17-11a, Fig. 17-12, Acetaldehyde + NADH → Ethanol + NAD+ Glucose + 2 ADP + 2 Pi + 2 H+ → 2 Ethanol + 2 ATP + 2 CO2 + 2 H2O p.48  Chapter 3 (cont.) Carbohydrate metabolism  Gluconeogenesis ⚫ Conversion of pyruvate to glucose ⚫ Biosynthesis and the degradation of many important biomolecules follow different pathways ⚫ There are three irreversible steps in glycolysis and the differences between glycolysis and gluconeogenesis are found in these reactions ⚫ Different pathway, reactions and enzyme ST EP 1 p.49 ⚫ is the biosynthesis of new glucose from non-CHO precursors. ⚫ this glucose is as a fuel source by the brain, testes, erythrocytes and kidney medulla ⚫ comprises of 9 steps and occurs in liver and kidney ⚫ the process occurs when quantity of glycogen have been depleted - Used to maintain blood glucose levels. ⚫ Designed to make sure blood glucose levels are high enough to meet the demands of brain and muscle (cannot do gluconeogenesis). ⚫ promotes by low blood glucose level and high ATP ⚫ inhibits by low ATP ⚫ occurs when [glu] is low or during periods of fasting/ starvation or intense exercise ⚫ pathway is highly endergonic *endergonic is energy consuming STEP 2 ⚫ The oxalocetate formed in the mitochondria have two fates: - continue to form PEP - turned into malate by malate dehydrogenase and leave the mitochondria, have a reaction reverse by cytosolic malate dehydrogenase ⚫ Reason? Controlling glucose metabolism found in Cori cycle shows the cycling of glucose due to gycolysis in muscle and gluconeogenesis in liver This two metabolic pathways are not active simultaneously. As energy store for when the cell needs next exercise ATP, glycolysis is more active When there is little need for ATP, gluconeogenesis is more active Fig. 18-12, Cori cycle requires the net hydrolysis of two ATP and two GTP. Fig. 18-13,  The Citric Acid cycle ⚫ Cycle where 30 to 32 molecules of ATP can be produced from glucose in complete aerobic oxidation ⚫ Amphibolic – play roles in both catabolism and anabolism ⚫ The other name of citric acid cycle: Krebs cycle and tricarboxylic acid cycle (TCA)  TCA Circular pathway Two-carbon unit needed at the start of the citric acid cycle The two-carbon unit is acetyl-CoA Involves 8 reactions The overall reaction from 1 acetyl-CoA produce 3 NADH, 1 FADH2, 2 CO2 and 1 GTP (equivalent to 1 ATP) Fig. 19-2, ⚫5 enzymes make up the pyruvate dehydrogenase complex: ✔ pyruvate dehydrogenase (PDH) Conversion of pyruvate ✔ Dihydrolipoyl transacetylase to acetyl-CoA ✔ Dihydrolipoyl dehydrogenase ✔ Pyruvate dehydrogenase kinase ✔ Pyruvate dehydrogenase phosphatase Steps 3,4,6 and 8 – oxidation reactions Fig. 19-3b, Step 1 Formation of citrate p.51 Step 2 Isomerization Table 19-1, p.518 cis-Aconitate as an intermediate in the conversion of citrate to isocitrate Fig. 19-6,  Fluoroacetate is a potent poison for mitochondrial oxidative metabolism. The LD50 for most target species is ≤ 1 mg/kg. Dogs are exquisitely sensitive, with an oral LD50 of 0.07 mg/kg/day. Fluoroacetate enters the tricarboxylic acid (Krebs) cycle in place of acetate and is converted to fluorocitrate, which competitively inhibits aconitase and thereby prevents the conversion of citrate to isocitrate. This leads to citrate accumulation, reducing glucose metabolism, energy stores, and cellular respiration. Citrate and fluorocitrate are also calcium chelators; thus, hypocalcemia is an important contributor to fluoroacetate poisoning. These metabolic disruptions have their greatest effect on the CNS, heart, kidneys, and other vital organs. Metabolism and excretion of sublethal doses is generally complete within 4 days.  Step 3 Formation of α- ketoglutarate and CO2 – first oxidation Fig. 19-7, Step 4 Formation of succinyl-CoA and CO2 – 2nd oxidation p.52 Step 5 Formation of succinate p.52  Step 6 Formation of fumarate – FAD-linked oxidation p.523 Step 7 Formation of L-malate p.524 Step 8 Regeneration of oxaloacetate – final oxidation step p.524 Krebs cycle produced: 6 CO2 2 ATP 6 NADH 2 FADH2 Fig. 19-8,  Citric acid cycle - amphibolic ⚫ Amphibolic (both Replenish TCA- catabolism catabolic & anabolic) of amino a. and fatty a. ⚫ Serves 2 purposes: Anabolic pathway 1. Oxidize Acetyl-CoA to CO2 to produce energy (ATP & reducing power of NADH & FADH2)- involved in the aerobic catabolism of carbohydrates, lipids and amino acids 2. Supply precursors for biosynthesis (anabolism) of carbohydrates, lipids, amino acids, nucleotides and porphyrins Require aerobic condition Table 19-3, p.527 Fig. 19-10, Fig. 19-11, Fig. 19-12, Fig. 19-15, Overall production from glycolysis, oxidative decarboxylation and TCA: Oxidative Glycolysis TCA cycle decarboxylatio n - 2 ATP 2 ATP 2 NADH 2 NADH 6 NADH , 2 FADH2 2 CO2 2 Pyruvate 4 CO2 Electron transportation system