Questions: Enzyme Cofactors and Coenzymes

Question 1

NAD⁺ functions in metabolic pathways primarily as...

• A. A structural component that holds the enzyme in its active conformation
• B. An electron carrier that shuttles electrons between oxidation and reduction reactions
• C. A metal ion that stabilizes the substrate in the active site
• D. A permanent, covalently bound prosthetic group that never leaves the enzyme

Answer: (B)

NAD⁺ accepts electrons (becoming NADH) during oxidation reactions and donates them elsewhere during reduction reactions. This shuttling role is what makes it a coenzyme rather than a structural component. It is loosely associated with enzymes and dissociates after each catalytic cycle, carrying its electron cargo to the next reaction in the pathway.

Question 2

Most coenzymes are permanently (covalently) bound to their enzymes and can seldom transfer to other enzymes.

• T. True
• F. False

Answer: False

Many coenzymes — including NAD⁺, FAD, and Coenzyme A — are loosely (non-covalently) associated with enzymes and dissociate after each reaction, allowing them to carry electrons or functional groups to other enzymes. Only prosthetic groups (a subset of cofactors) are permanently bound. The mobility of most coenzymes is essential to their function as metabolic shuttles.

Question 3

Why would a severe deficiency of B vitamins cause widespread disruption to cellular metabolism rather than affecting only one specific reaction?

Model answer: B vitamins are the dietary precursors to several essential coenzymes: niacin (B3) → NAD⁺ and NADP⁺, riboflavin (B2) → FAD and FMN, thiamine (B1) → TPP. These coenzymes participate in reactions across glycolysis, the citric acid cycle, fatty acid oxidation, and biosynthesis. Without them, dozens of enzymes across multiple pathways lose function, causing broad metabolic failure rather than a single enzyme deficiency.

The breadth of disruption reflects the fact that coenzymes are shared resources — a single molecular species (like NAD⁺) is required by many different enzymes across many pathways. A vitamin deficiency therefore has a systemic effect, unlike a single enzyme gene mutation which typically affects just one reaction. This is why nutritional deficiencies often present with complex, multi-system symptoms.