Consider the oxidation of a single glucose molecule. How many carbons from that glucose molecule remain...

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Biology

Consider the oxidation of a single glucose molecule. How manycarbons from that glucose molecule remain to be oxidized followinga single turn of the TCA cycle?

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Oxidation of Glucose and Fatty Acids to CO2The complete aerobic oxidation of glucose is coupled to thesynthesis of as many as 36 molecules of ATPGlycolysis the initial stage of glucose metabolism takes placein the cytosol and does not involve molecular O2 It produces asmall amount of ATP and the threecarbon compoundpyruvate In aerobic cells pyruvate formed in glycolysisis transported into the mitochondria where it is oxidized by O2 toCO2 Via chemiosmotic coupling the oxidation of pyruvate in themitochondria generates the bulk of the ATP produced during theconversion of glucose to CO2 In this section we discuss thebiochemical pathways that oxidize glucose and fatty acids to CO2and H2O the fate of the released electrons is described in thenext sectionGo toCytosolic Enzymes Convert Glucose to PyruvateA set of 10 enzymes catalyze the reactions constituting theglycolytic pathway that degrade one molecule of glucosetotwo molecules of pyruvate Figure 163 All the metabolicintermediates between glucose and pyruvate are watersolublephosphorylated compounds Four molecules of ATP are formed from ADPin glycolysis reactions 6 and 9 However two ATP molecules areconsumed during earlier steps of this pathway the first by theaddition of a phosphate residue to glucose in the reactioncatalyzed by hexokinase reaction 1 and the second bythe addition of a second phosphate to fructose 6phosphate in thereaction catalyzed by phosphofructokinase1 reaction 3Thus there is a net gain of two ATP moleculesFigure 163The glycolytic pathway by which glucose is degraded to pyruvicacid Reactions in which ATP and ADP are involved are highlightedin blue the reaction involving NAD and NADH is highlighted inyellow Note that all the intermediates between glucose andmoreThe balanced chemical equation for the conversion of glucose topyruvate shows that four hydrogen atoms four protons and fourelectrons are also formedFor convenience we show pyruvate in its unionized formpyruvic acid although at physiological pH it would be largelydissociated All four electrons and two of the four protons aretransferred to two molecules of the oxidized form of the electroncarrier nicotinamide adenine dinucleotideNAD to produce the reduced form NADH Figure164The reaction that generates these hydrogen atoms and transfersthem to NAD is catalyzed by glyceraldehyde 3phosphatedehydrogenase see Figure 163 reaction 5Figure 164Structures of the electroncarrying coenzymes NAD and NADHNicotinamide adenine dinucleotide NAD and the relatednicotinamide adenine dinucleotide phosphate NADP accept onlypairs of electrons reduction to NADH or NADPH involves thetransfer moreThus the overall reaction for this first stage of glucosemetabolism isGo toSubstrateLevel Phosphorylation Generates ATP duringGlycolysisAs noted earlier the immediate energy source for ATP synthesisin chloroplasts and mitochondria is provided by the protonmotiveforce across a membrane Cells also can produce ATP by a processcalled substratelevel phosphorylation which is catalyzed bywatersoluble enzymes in the cytosol membranes and ion gradientsare not involvedSubstratelevel phosphorylation occurs twice in the glycolyticpathway The first results from the pair of reactions that convertglyceraldehyde 3phosphate to 3phosphoglycerate see Figure 163steps 5 and 6 In the first of these reactions oxidation of thealdehyde CHO group on glyceraldehyde 3phosphate by NAD iscoupled to addition of a phosphate group forming13bisphosphoglycerate with a single highenergy phosphoanhydridebond to carbon 1 In this reaction catalyzed by glyceraldehyde3phosphate dehydrogenase a highenergy thioesterenzymeintermediate is formed Figure See Answer

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