• •  The input to fatty acid synthesis is acetyl-CoA, which is carboxylated to malonyl-CoA.

The ATP-dependent carboxylation provides energy input. The CO₂ is lost later during condensation with the growing fatty acid. The spontaneous decarboxylation drives the condensation.

• •  Acetyl-CoA Carboxylase catalyzes the 2-step reaction by which acetyl-CoA is carboxylated to form malonyl-CoA. As with other carboxylation reactions (e.g., Pyruvate Carboxylase), the enzyme prosthetic group is biotin.

ATP-dependent carboxylation of the biotin, carried out at one active site (1), is followed by transfer of the carboxyl group to acetyl-CoA at a second active site (2).

The overall reaction, which is is spontaneous, may be summarized as:

...................................Fig. 20. The Acyl Carrier protein (ACP)