This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'Saturated acyl-enzyme formation causes \nSelect and explain. \nA. Freeing of PAN-SH site, B. Freeing of cyst-SH site, C. Both, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Freeing of PAN-SH site\xa0\nIn fatty acid synthesis\n\nAfter priming reactions (first two reactions)\n\n\nAcyl - malonyl - enzyme is formed\n\n\nCentral thiol (4-phosphopantetheine -SH group or Pan - SH group) of ACP is covalently linked with malonyl group\nPeripheral thiol (cysteine -SH group) of 3-ketoacyl synthase is covalently linked to the acetyl group\nDuring elongation steps\n\n\nThe acetyl group attached to peripheral thiol (cysteine -SH) is transferred to the malonyl group bound to central thiol (Pan -SH group). Simultaneously 1 CO2 is liberated from malonyl group. Thus:-\n\n\nCentral thiol (Pan -SH) contains 4 carbon atoms residue\nPeripheral thiol (cysteine -SH group) becomes free\n\n\nAfter elongation steps\nSaturated acyl-enzyme is formed\n4 carbon unit (butyryl) is transferred from central thiol (Pan -SH) to peripheral thiol (cysteine -SH group). Thus:-\n\n\nCentral thiol (Pan -SH) becomes free to accept new malonyl for next cycle\nPeripheral thiol (cysteine -SH group) contains 4 carbon unit', 'role': 'assistant'}]