Q.17 The free energy required to synthesize a mixed anhydride bond of 1,3-bisphosphoglycerate is
generated by the oxidation of ________.
(A) an aldehyde to acid
(B) an alcohol to acid
(C) an alcohol to aldehyde
(D) NADH to NAD +
The free energy required to synthesize the mixed anhydride bond in 1,3-bisphosphoglycerate during glycolysis comes from the oxidation of an aldehyde group to a carboxylic acid (acyl phosphate) in glyceraldehyde-3-phosphate (G3P). This couples an exergonic oxidation reaction to drive the endergonic formation of the high-energy bond. The correct answer is (A) an aldehyde to acid.
Option Analysis
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(A) an aldehyde to acid: G3P features an aldehyde (-CHO) at carbon 1, which glyceraldehyde-3-phosphate dehydrogenase (GAPDH) oxidizes to a carboxylic acid, forming a thioester intermediate that reacts with phosphate (Pi) to create the mixed anhydride (acyl phosphate) in 1,3-BPG. The oxidation releases ~50 kJ/mol free energy (ΔG°’ = -50 kJ/mol), coupling to the endergonic phosphorylation (+50 kJ/mol) for a net slightly endergonic reaction (+6.3 kJ/mol).
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(B) an alcohol to acid: Incorrect; no alcohol oxidation occurs here. The C3 alcohol in G3P remains unchanged, later dehydrated in later steps.
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(C) an alcohol to aldehyde: Incorrect; this reverse oxidation does not happen in glycolysis, and G3P already has an aldehyde, not an alcohol at C1.
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(D) NADH to NAD+: Incorrect; the reaction reduces NAD+ to NADH (opposite direction), using the hydride from aldehyde oxidation; NADH oxidation occurs later in respiration.
Reaction Mechanism
GAPDH catalyzes G3P + NAD+ + Pi → 1,3-BPG + NADH + H+. First, the aldehyde forms a hemithioacetal with enzyme cysteine, oxidizing to a thioester (NAD+ reduced). Pi then attacks, yielding the mixed anhydride. This high-energy bond (~ -49 kJ/mol hydrolysis ΔG) transfers to ADP via phosphoglycerate kinase, forming ATP (substrate-level phosphorylation).
In glycolysis, the free energy required to synthesize a mixed anhydride bond of 1,3-bisphosphoglycerate powers ATP production at substrate level. This high-energy acyl phosphate forms when glyceraldehyde-3-phosphate dehydrogenase oxidizes the aldehyde group of G3P to a carboxylic acid, coupling energy release to phosphorylation.
Key Glycolysis Step
Step 6 of glycolysis converts G3P to 1,3-BPG, the sole oxidation yielding NADH per glucose triose. The exergonic aldehyde oxidation (ΔG°’ = -50 kJ/mol) drives endergonic Pi addition, creating the C1 mixed anhydride with ~ -49 kJ/mol hydrolysis potential. This conserves energy wasted as heat.
Why Aldehyde to Acid?
G3P’s aldehyde (-CHO) at C1 oxidizes via thiohemiacetal intermediate to acyl phosphate. No alcohol involvement; NAD+ accepts electrons, not vice versa. This ensures efficient energy capture for the payoff phase.
Exam Relevance
For CSIR NET Life Sciences, recognize this as coupled oxidation-phosphorylation. Arsenate uncouples by forming unstable 1-arseno-3-PG, blocking ATP. Master for biochemistry units on glycolysis energetics.
