Final answer:
The main carbon donor in fatty acid synthesis is malonyl-CoA, which is synthesized from acetyl-CoA and is essential for the chain elongation process on the fatty acid synthase complex.
Step-by-step explanation:
The main carbon donor in fatty acid synthesis is malonyl-CoA. This molecule is essential for the elongation of the fatty acid chain during synthesis. The synthesis begins with the formation of malonyl-CoA from acetyl-CoA and bicarbonate, catalyzed by the enzyme acetyl-CoA carboxylase.
Malonyl-CoA is then transferred to an acyl carrier protein (ACP) on the fatty acid synthase (FAS), where it participates in the iterative addition of two-carbon units to the growing fatty acid chain. Fatty acid synthesis continues in this fashion until a 16-carbon palmitate molecule is produced.
The fatty acid synthase complex is a large enzyme involved in this process and consists of multiple domains, including a cysteine residue on the 3-ketoacyl-ACP synthase enzyme and a phosphopantetheine group on ACP, both of which have active sulfhydryl groups crucial in the transfer and elongation processes. The growing fatty acid remains linked to ACP through the entire cycle. After synthesis of the 16-carbon chain is complete, it is released as palmitate by the enzyme hydrolase.