BIOLOGY Volume 1 - A Guide to General Biology - 2004

4. ENZYMES

4.5. Enzyme Cofactors

4.5.3. Coenzymes (e.g., NAD, NADP, Acetyl-CoA, ATP)

Like prosthetic groups, Coenzymes are organic molecules that function as Cofactors, but unlike prosthetic groups, they bind to the enzyme only transiently during the course of the reaction. All coenzymes are vitamin derivatives.

Nicotinamide adenine dinucleotide (NAD)

(Fig. 4.17)

NAD—a derivative of the vitamin niacin ("nicotinic acid")—can exist in both oxidized and reduced forms. During catalysis, oxidized NAD acts as a hydrogen acceptor: where e₁ and e₂ are two different dehydrogenases.

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Net result: 2H are transferred from A to B. Here, the coenzyme acts as a linking agent between two different enzyme systems, e₁ and e₂.

Fig. 4.17. Structure OF THE coenzymes NAD (nicotinamide adenine dinucleotide) and NADP (NAD with an additional phosphate group). NAD and NADP are dinucleotides (see Fig. 4.16). Note that nicotinamide (niacin), which is a component of one of the NUCLEOTIDES, is a vitamin. AMP is structurally similar to ATP (which contains two additional phosphate groups in its molecule). ATP serves as the energy currency of The Cell and is generated during cellular Respiration.

4.6. List the Characteristic Properties of Enzymes.