This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
Rate limiting step in porphyrin synthesis is 
Select and explain. 
A. ALA dehydratase, B. ALA synthase, C. UPG decarboxylase, D. Ferrochelatase
B: ALA synthase
Answer: B: ALA synthase 
Explanation: The synthesis stas with the condensation of succinyl CoA and glycine in the presence of pyridoxal phosphate to form delta amino levulinic acid (ALA).The enzyme ALA synthase is located in the mitochondria and is the rate limiting ezyme of pathway. Next few steps occurs in cytoplasm.2 molecules of ALA are condensed to form porphobilinogen (PBG) Condensation of 4 molecules of PBG results in the formation of the first porphyrin of the pathway,namely urobilinogen. REFERENCE:DM VASUDEVAN Textbook SEVENTH EDITION, Page no:272 and 273