Which enzyme is identified as the key rate-limiting step in heme synthesis?

The key rate-limiting step in heme synthesis is catalyzed by aminolevulinate synthase, which converts succinyl-CoA and glycine into aminolevulinic acid (ALA). This reaction is critical as it initiates the entire heme biosynthetic pathway. The activity of aminolevulinate synthase is tightly regulated by feedback mechanisms, primarily influenced by heme levels; when heme concentrations are high, the enzyme's activity is inhibited.

This regulatory role ensures that heme synthesis matches the physiological needs of the cell, reflecting the importance of this enzyme in maintaining proper cellular function and homeostasis. The other enzymes listed do not play a role in the rate-limiting step of heme synthesis; carbonic anhydrase is involved in carbon dioxide transport and bicarbonate buffer systems, protoporphyrin IX reductase is involved in later stages of heme synthesis, and glucose 6-phosphate dehydrogenase plays a role in the pentose phosphate pathway, which is unrelated to heme production. Thus, aminolevulinate synthase is recognized for its critical regulatory function in this pathway.