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.
The overall reaction, which is is spontaneous, may be summarized as:
HCO3- + ATP + acetyl-CoA à ADP + Pi + malonyl-CoA
Biotin is linked to the enzyme by an amide bond between the terminal carboxyl of the biotin side chain and the e-amino group of a lysine residue
NADPH as electron donor in the two reactions involving substrate reduction. The NADPH is produced mainly by the Pentose Phosphate Pathway
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