Final answer:
In stage 3 of fatty acid synthesis, the first step involves the transfer of acetyl-CoA and malonyl-CoA to the acyl carrier protein (ACP).
Step-by-step explanation:
In stage 3 of fatty acid synthesis, the first step is the transfer of acetyl-CoA and malonyl-CoA from coenzyme-A to the acyl carrier protein (ACP). Acetyl-CoA is transferred to the -SH group of cysteine, forming acetyl-S-Cys, and malonyl-CoA is transferred to the -SH group of ACP, forming malonyl-S-ACP.
The next step involves the condensation of acetyl-S-Cys and malonyl-S-ACP, which is catalyzed by the enzyme 3-ketoacyl synthase. This forms acetoacetyl-S-ACP.
This process continues, with the fatty acid chain being built two carbons at a time until palmitoyl-ACP, an acyl-ACP with 16 carbon atoms, is formed.
During fatty acid synthesis, NADPH is used as a reducing agent, provided by the HMP pathway. The ACP bond is subsequently hydrolyzed, releasing the free fatty acid, which reacts with coenzyme-A to form an acyl-CoA in the cell.
This is followed by the condensation of acetyl-S-Cys and malonyl-S-ACP, resulting in the formation of acetoacetyl-S-ACP. The process continues until palmitoyl-ACP, an acyl-ACP with 16 carbon atoms, is formed.