In glycolysis, is NADH a substrate (reactant) or a product, and at which step is it formed?

• A. Product; formed when phosphoenolpyruvate is converted to pyruvate by pyruvate kinase
• B. Substrate; consumed when pyruvate is reduced to lactate by lactate dehydrogenase during anaerobic glycolysis
• C. Substrate; consumed when glucose is phosphorylated to glucose-6-phosphate by hexokinase
• D. Product; formed when glyceraldehyde-3-phosphate is oxidized to 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase

Answer: (D)

Verified step by step guidance

1. Recall that NADH is the reduced form of NAD+, which acts as an electron carrier in metabolic pathways like glycolysis.
2. Identify the step in glycolysis where NAD+ is reduced to NADH, meaning NAD+ gains electrons (and a proton) to become NADH.
3. Focus on the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, which is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase.
4. Understand that during this step, glyceraldehyde-3-phosphate loses electrons, which are transferred to NAD+, reducing it to NADH, making NADH a product of this reaction.
5. Confirm that NADH is not a substrate in glycolysis but a product formed specifically at the glyceraldehyde-3-phosphate dehydrogenase step.