FAS and obesity due to melanocortin 4 receptor deficiency: Because ACC catalyzes formation of malonyl-CoA, which along with acetyl-CoA is one of the two substrates used by FAS for fatty acid synthesis [51], our data suggest that mechanism by which obesity impairs de novo lipogenesis in the lactating mammary gland is by reducing malonyl-CoA production through posttranslational inhibition of ACC.