Which of the following statements is true of ethanal reductase?

• A. It represents a hydrolase that acts upon the ethanol substrate to oxidize it. 
• B. It represents an oxidoreductase that acts upon the ethanal substrate to oxidize it.
• C. It represents a lyase that acts upon the ethanal enzyme to reduce it. 
• D. It represents an oxidoreductase that acts upon the ethanal substrate to reduce it. 

Answer: (D)

Verified step by step guidance

1. Identify the enzyme class: Ethanal reductase is an enzyme, and enzymes are classified based on the type of reaction they catalyze. The main classes include oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
2. Understand the function of oxidoreductases: Oxidoreductases are enzymes that catalyze oxidation-reduction reactions, where the transfer of electrons occurs between molecules. They often involve the reduction of one substrate and the oxidation of another.
3. Analyze the substrate: In this case, the substrate is ethanal (acetaldehyde). The enzyme ethanal reductase would act on ethanal to facilitate a reduction reaction.
4. Determine the reaction type: Since ethanal reductase is involved, the reaction likely involves the reduction of ethanal to ethanol. This means ethanal gains electrons (is reduced).
5. Evaluate the statements: The correct statement should reflect that ethanal reductase is an oxidoreductase that reduces ethanal. Therefore, the statement 'It represents an oxidoreductase that acts upon the ethanal substrate to reduce it.' is true.