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)