Acetyl CoA turned to Malonyl CoA

[sameeralord]

Hello everyone,

My note says in fatty acid synthesis, one acetyl unit enters the synthesis pathway directly but the remainder must be first converted to malonyl CoA.
I can understand the latter but why does one acetyl unit enter directly where did it come from? Thanks

 

[chemisttree]

It entered from or came from acetyl CoA. When acetyl CoA is carboxylated (with bicarbonate) it is known as malonyl CoA. This is catalyzed by acetyl CoA carboxylase. So you see that everything can be seen to originate with acetyl CoA which is derived from pyruvate, CoA and NAD+.

Malonyl CoA combines with acteyl CoA to start the cycle. Thereafter, the nascent fatty acid combines with more malonyl CoA since it's decarboxylation drives the energetics of the 2 carbon coupling.