Question

Aminolevulinic acid synthase A. requires NAD for activity. B. is allosterically activated by heme. C. synthesis is inhibited by steroids. D. is synthesized in mitochondria. E. synthesis can be induced by a variety of drugs.

Short answer

Answer: Aminolevulinic acid synthase is synthesized in mitochondria, and its synthesis can be induced by a variety of drugs.

Step-by-step solution

NAD requirement for activity

Aminolevulinic acid synthase (ALAS) is the enzyme that catalyzes the first step of heme biosynthesis by condensing glycine and succinyl-CoA to form aminolevulinic acid (ALA). ALAS does not require NAD (Nicotinamide adenine dinucleotide) for its activity. Thus, statement A is false.

Allosteric activation by heme

ALAS exhibits a negative feedback mechanism, which means that the end product, heme, inhibits its own synthesis by binding to the enzyme allosterically. The enzyme is allosterically inhibited, not activated, by heme. Therefore, statement B is false.

Inhibition by steroids

The synthesis of ALAS is not directly inhibited by steroids. Steroids have no known direct effect on ALAS synthesis and activity. Consequently, statement C is false.

Synthesis in mitochondria

ALAS is synthesized in the mitochondria. The first and last three steps of heme biosynthesis occur in the mitochondria, and the remaining steps take place in the cytosol. As ALAS catalyzes the first step of the process, it is synthesized in the mitochondria. Statement D is true.

Induction by various drugs

The synthesis of ALAS can be induced by a variety of drugs, particularly those that increase the demand for heme in the body. For example, drugs that induce the cytochrome P450 enzyme system (e.g., phenobarbital, rifampicin) can increase the demand for heme and consequently upregulate ALAS. Statement E is true. In conclusion, the correct answers for this exercise are: D. Aminolevulinic acid synthase is synthesized in mitochondria. E. Aminolevulinic acid synthase synthesis can be induced by a variety of drugs.