<s>[INST] context: (ALA synthase) (276-Lippencott's 3rd) (210- U.S.3rd)Biosynthesis of heme - The major sites of heme biosynthesis are the liver, which synthesizes a number of heme proteins (particularly, cytochrome P450) and the erythrocyte-producing cells of the bone marrow, which are active in hemoglobin synthesis.* ALA-synthase is the rate controlling step in hepatic porphyrin biosynthesis (276-LP)* The first committed step in heme biosynthesis catalyzed by 5-aminolevulinate (ALA) synthase is regulatory. Heme or its oxidized product hemin (Fe+++) controls this enzyme activity by three mechanisms.1. Feedback inhibition2. Repression of ALA synthatase3. Inhibition of transport of ALA synthase from cytosol to mitochondria (the site of action) (21.5-U Satyanarayan 2nd)* ALA synthase is main rate-limiting enzyme in the synthetic pathway (469-Chatterjee & Shinde 6th)* ALA dehydratase is second rate-limiting enzymeRate limiting Enzymes1. Fatty acid synthesisAcetyl Co A carboxylase2. Cholesterol synthesisHMG-Co A reductase3. Bile acid synthesis7 a-hydroxylase4. Prostaglandin synthesisPhospholipase A25. Uric acid synthesisXanthine oxidase6. Catecholamine (epinephrine)Tyrosine hydroxylase (tyrosinase) [/INST] question: The regulating enzyme in heme synthesis is | options: A: ALA synthase B: ALA dehydratase C: Ferrochelatase D: Protoporphyrin | answer: A </s>