Unformatted text preview: Lecture 13
Overview: • glycolysis (glucose to pyruvate) • beyond pyruvate -anaerobic glycolysis & fermentation -the Cori Cycle Glucose metabolism Chapter 16: Glycolysis HW: 16.2, 16.3a, 16.6, handout questions Glucose metabolism can generate ATP for muscle contraction. When ATP needs outpace oxygen delivery, glucose is metabolized to lactate. When oxygen delivery is adequate, glucose is metabolized to carbon dioxide and water. Glucose metabolism Some fates of glucose are shown. Glycolysis • an ancient metabolic pathway • a nearly universal metabolic pathway for prokaryotes & eukaryotes -many anaerobic microorganisms rely entirely upon glycolysis -found in all mammalian cell types • conversion of glucose (6 C) to pyruvate (3 C) in 10 steps • goal is to capture free energy from the oxidation of glucose in the form of ATP (uses phosphorylated intermediates) • only a small amount of energy (ATP) is produced under anaerobic conditions (absence of oxygen) pathways of glucose • to extract the maximal amount of energy from glucose, pyruvate must be oxidized completely to CO2 under aerobic conditions • glucose for glycolysis provided by 1) bloodstream, 2) glycogen, or 3) gluconeogenesis Preview: three stages of respiration
phosphoryl transfer Basic reactions in glycolysis
O R-OH + ATP kina se R-O-P-OOOH phosphoryl shift R C CH2 O H -O O P OO R C CH2 OH H + ADP + H+ 1) glycolysis phases: preparatory (5 rxns) payoff (5 rxns) 2) citric acid cycle
O P OOmut ase H2 C OH isome rizat ion CO R isome rase HC O H C OH R dehydra tion H C H OH H C H R HC R dehydra tase R C R + H2 O 3) oxidative phosphorylation
al dol cl eavage H R C HO C C R O H OH al dolase R C HO C H O H + H O C R Glucose transport GluT1 and GluT4 Stages of glycolysis 1) Starch and glycogen in diet -> digestion by !-amylase -> cleavage of maltose units by maltase to glucose; cleavage of lactose units by lactase to glucose and galactose; cleavage of sucrose by sucrase to fructose and glucose 2) Transport into bloodstream 3) Glycolysis i) phosphorylation ii) cleavage iii) ATP generation entry of glucose from bloodstream into cells Stage 1 of glycolysis phosphorylation 1) Phosphorylation of glucose investment of ﬁrst ATP !G"° = –16.7 kJ/mol
hexokinase has broad speciﬁcity - can also phosphorylate other hexoses (e.g., fructose, mannose) induced ﬁt in hexokinase 2) Isomerization of G-6P to F-6P phosphohexose isomerase or phosphoglucose isomerase Phosphohexose isomerase mechanism !G"° = 1.7 kJ/mol 3) Phosphorylation of F-6P to F-1,6BP investment of second ATP PFK-1 plays a central role in glycolysis regulation rate-limiting step Stage 2 of glycolysis Two three-carbon fragments are produced from one sixcarbon sugar. !G"° = –14.2 kJ/mol 4) Cleavage of F-1,6BP Class I aldolase mechanism !G"° = 23.8 kJ/mol Class I aldolase mechanism TPI 5) Interconverting the triose phosphates only GAP can continue in glycolysis DHAP needs to be converted to GAP DHAP GAP 1 glucose -> 2 GAP molecules GAP DHAP !G"° = 7.5 kJ/mol Mechanism of triose phosphate isomerase Stage 3 of glycolysis ‘payoff’ phase: harvesting chemical energy this is the only oxidation step of the 10 reactions in glycolysis general acid-base catalysis kcat/KM = 2 ! 108 M-1s-1, close to diffusion control 6) Oxidation of GAP to 1,3-BPG ‘payoff’ phase: harvesting chemical energy; this is the only oxidation step of the 10 reactions in glycolysis 6) Oxidation of GAP to 1,3-BPG The sum of two reactions: Oxidation to acid powers formation of product with high phosphoryl-transfer potential !G"° = –50 kJ/mol !G"° = 6.3 kJ/mol !G"° of summed reactions = 6.3 kJ/mol 6) Oxidation of GAP to 1,3-BPG Free energy proﬁles for glyceraldehyde oxidation followed by acyl-phosphate formation: no coupling coupling GAP dehydrogenase mechanism note role of thioester intermediate 7) Phosphoryl transfer from 1,3-BPG to ADP ‘break even’ point: 2 ATPs formed (1 per each original G3P) substrate-level phosphorylation chemical energy is conserved in ATP 8) Conversion of 3PG to 2PG !G"° = –18.8 kJ/mol Phosphoglycerate mutase mechanism 9) Dehydration of 2PG to PEP 2PG PEP either could be used to phosphorylate ADP to ATP, but PEP has a greater !Ghydrolysis than 2PG dehydration redistributes energy in PEP 10) Phosphoryl transfer from PEP to ADP payoff step: 2 ATPs formed (1 per original GAP) Why does phosphoenolpyruvate have such a high phosphoryltransfer potential? The phosphoryl modiﬁcation traps the less stable enol form; conversion to pyruvate generates the more stable ketone. know this structure! !G"° = –62 kJ/mol Glycolysis summary overall reaction: glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O Free energy changes during glycolysis next stage - what happens to pyruvate and NADH? Entry of glycogen, starch, disaccharides, hexoses into glycolysis pathway overall reaction: glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O !G"° = –96 kJ/mol
Even more energy can be generated from NADH - to be covered in later chapters. GAP Three catabolic fates of pyruvate regenerate NAD+ CAC + 2 CO2 4 CO2 + 4 H2O ‘electrons’ in the form of reduced coenzymes (NADH, FADH2) continue on to the oxidative phosphorylation pathway and yield large quantities of ATP Fates of pyruvate pyruvate is the terminal electron acceptor in lactic acid fermentation NAD+ used to continue glycolysis (recall step 6) Cori Cycle: metabolic cooperation between skeletal muscle & liver
muscle: ATP produced by glycolysis for contraction Cori cycle: glucose -> lactate -> glucose after muscle activity ends, the ‘recovery phase’ is marked by increased O2 consumption: this excess of oxygen equals the amount needed to supply extra ATP (via oxidative phosphorylation) to replace the glycogen used through gluconeogenesis lactate glycogen ATP (LDH) fermentation to lactate: produce ATP during intense activity LDH is abundant in muscle Lactic acid fermentation: build-up of lactic acid leads to muscle soreness after extreme exertion blood lactate blood glucose lactate ATP glucose Glucose + 2 Pi + 2 ADP -> 2 lactate + 2 ATP + 2 H2O liver: ATP used in synthesis of glucose (gluconeogenesis) during recovery Alcohol fermentation What is the role of TPP??
glycolysis Alcohol dehydrogenase mechanism yeast and other microorganisms ferment glucose to ethanol and CO2 • cereal grains germinate to yield amylases/maltases • grains enzymatically degraded to simpler saccharides • yeast added; grow quickly under aerobic condition •when O2 is consumed, yeast begin to ferment pyruvate to yield ethanol Alcohol dehydrogenase active site Maintaining redox balance: NADH produced by glyceraldehyde 3-phosphate dehydrogenase reaction must be reoxidized to NAD+ for the glycolytic pathway to continue. In alcoholic fermentation, alcohol dehydrogenase oxidizes NADH and generates ethanol. In lactic acid fermentation, lactate dehydrogenase oxidizes NADH while generating lactic acid. Summary of alcoholic fermentation: Glucose + 2 Pi + 2 ADP + 2 H+ -> 2 ethanol + 2 CO2 + 2 ATP + 2 H2O Note NADH and NAD+ do not appear in equation, but are crucial for overall process. Only a fraction of energy of glucose is released in anaerobic conversion into ethanol or lactate. Much more energy can be extracted aerobically by means of the citric acid cycle and electrontransport chain. The entry point is acetyl coenzyme A (acetyl CoA). Pyruvate + NAD+ + CoA –> acetyl CoA + CO2 + NADH (Ch. 17) NAD+-binding site in dehydrogenases Rossmann fold Entry of fructose and galactose into glycolytic pathway Entry of fructose and galactose into glycolytic pathway Entry of fructose and galactose into glycolytic pathway Galactose is converted to glucose 6-phosphate in four steps. Step 1: Fructose is metabolized by the liver using the fructose 1-phosphate pathway. Phosphorylation is carried out by fructokinase. F-1,6-BP is then split into GAP and DHAP by fructose 1-phosphate aldolase. Galactose is converted to glucose 6phosphate in four steps. Steps 2 –4 are shown. An important reaction for conversion of milk lactose to monosaccharides Overall: Galactose + ATP –> Glucose 1-phosphate + ADP + H+ role of Lactobacillus in fermentation ...
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