The first energy-generating steps in glycolysis begin when glyceraldehyde 3-phosphate undergoes an energetically favorable reaction in which it is simultaneously oxidized and phosphorylated by the enzyme glyceraldehyde 3-phosphate dehydrogenase to

Question

form 1,3-bisphosphoglycerate, with the accompanying conversion of NAD+ to NADH. In a second energetically favorable reaction catalyzed by a second enzyme, the 1,3-bisphosphoglycerate is then converted to 3-phosphoglycerate, with the accompanying conversion of ADP to ATP. Which of the following statements is true about this reaction?
(a) The reaction glyceraldehyde 3-phosphate -> 1,3-bisphosphoglycerate should be inhibited when levels of NADH fall.
(b) The ?G° for the oxidation of the aldehyde group on glyceraldehyde 3-phosphate to form a carboxylic acid is more negative than the ?G° for ATP hydrolysis.
(c) The energy stored in the phosphate bond of glyceraldehyde 3-phosphate contributes to driving the reaction forward.
(d) The cysteine side chain on the enzyme is oxidized by NAD+.

Answer 1

Choice (b) is the correct answer. This is another way of stating that the energetically favorable oxidation of glyceraldehyde 3-phosphate provides sufficient energy to ultimately drive the energy-requiring step of ATP synthesis from ADP. Choice (a) is untrue: NADH is an end product of the reaction glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate and therefore high (not low) levels of it would inhibit the reaction. Choice (c) is untrue: the reactions do not involve the 3-phosphate group on glyceraldehyde 3-phosphate at all. Choice (d) is untrue, because the cysteine on the enzyme is important in making a covalent intermediate with the substrate and is not oxidized by NAD+.