Final answer:
ALA synthase is the enzyme that catalyzes the committed step in heme synthesis, acting as a key regulatory point and is negatively regulated by heme itself. The correct option is a.
Step-by-step explanation:
The compound that catalyzes the committed step in heme synthesis is ALA synthase. This enzyme serves as the key regulatory enzyme for heme synthesis and operates at the first step, where glycine and succinyl-CoA are converted into 5-aminolevulinic acid (ALA).
Regulation of heme synthesis highlights that ALA synthase activity is inhibited by the end product heme, showcasing its role as a negative feedback mechanism.
Heme functions as an aporepressor molecule and is a negative regulator for the synthesis of ALA synthase-I, meaning that the increased concentration of heme inhibits the gene encoding ALA synthase. The correct option is a.