ACC1 a subunit of acetyl-CoA carboxylase (ACC), a multifunctional enzyme system. Catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the rate-limiting step in fatty acid synthesis. Acetyl-CoA carboxylase (ACC) catalyzes the pivotal step of the fatty acid synthesis pathway. The 265 kDa ACCα (ACC1) is the predominant isoform found in liver, adipocytes, and mammary gland, while the 280 kDa ACCβ (ACC2) is the major isoform in skeletal muscle and heart.