Final answer:
In stage 3 fatty acid synthesis, the second round begins with the addition of a malonyl-CoA to the acyl-S-ACP produced in the first round.
Step-by-step explanation:
The second round of fatty acid synthesis in stage 3 begins with the reaction of a new malonyl-CoA with the acyl-S-ACP that was produced in the first round. In this reaction, the malonyl group is transferred to the growing fatty acid chain, and CO2 is released. This forms a new ketoacyl-ACP. Similar to the first round, the second round involves four enzymatic steps which include reduction, dehydration, and another reduction, ultimately producing a longer acyl-S-ACP. This cycle of four reactions is repeated, with each turn adding two more carbons to the chain, until the fatty acid reaches 16 carbons in length (palmitoyl-ACP).
Here's a more detailed look at the second round: The acetoacetyl group from the first cycle undergoes reduction by the enzyme 3-ketoacyl-ACP reductase using NADPH to form 3-hydroxyacyl-S-ACP. This is followed by dehydration to form trans-∆2-enoyl-S-ACP and another reduction by enoyl-ACP reductase to yield the saturated butyryl-S-ACP. The resulting product is finally elongated through subsequent additions of malonyl-CoA molecules.