The product of the reaction of acetyl CoA with ACP-SH catalyzed by malonyl-CoA-acetyl-CoA-ACP transacylase (MAT) is malonyl-ACP.
Acetyl-CoA + ACP-SH + Malonyl-CoA -> Malonyl-ACP + CoA-SH
The acetyl group from acetyl-CoA is transferred to the ACP-SH, forming a thioester bond. The malonyl group from malonyl-CoA is transferred to the free sulfhydryl group of the ACP, also forming a thioester bond.
The Coenzyme A (CoA) is released as a byproduct and this reaction is the first step in fatty acid biosynthesis, where malonyl-ACP serves as a two-carbon unit for the elongation of fatty acid chains.