tailieunhanh - USMLE ROAD MAP BIOCHEMISTRY – PART 6

on đường để tổng hợp của palmitate enzym tổng hợp acid béo (FAS) phức tạp. Schematic đại diện của một chu trình đơn thêm hai nguyên tử cacbon chuỗi acyl ngày càng tăng. Hình thành các thioester acetyl ban đầu có dư lượng cysteine của enzym trước | 108 USMLE Road Map Biochemistry O S C CH3 Acetyl Acyl SH Palmitate Repeat cycle six more times Figure 8-2. Pathway for synthesis of palmitate by the fatty acid synthase FAS complex. Schematic representation of a single cycle adding two carbons to the growing acyl chain. Formation of the initial acetyl thioester with a cysteine residue of the enzyme preceded the first step shown. Acyl carrier protein ACP is a component of the FAS complex that carries the malonate covalently attached to a sulfhydryl group on its phosphopantatheine coenzyme -SH in the scheme . 0 5. The ultimate product of seven cycles of these reactions is the fully saturated C16 fatty acid palmitate. D. Additions to and modifications of palmitate allow synthesis of many structurally distinct fatty acids. 1. Elongation of palmitate occurs by addition of further acetate units in the endoplasmic reticulum and mitochondria. 2. Desaturation or the creation of double bonds for synthesis of unsaturated fats is performed by mixed-function oxidases in the endoplasmic reticulum. Chapter 8 Lipid Metabolism 109 3. Storage as triacylglycerols requires activation of the fatty acid by conversion to acyl CoA with glycerol 3-phosphate as the precursor for the glycerol backbone. V. Fatty Acid Oxidation A. Mobilization of fat stores allows fats to be burned to produce energy via fatty acid oxidation. 1. The initial step to release fatty acids is triacylglycerol hydrolysis catalyzed by hormone-sensitive HS lipase. a. As its name implies the enzyme is regulated via hormonally controlled cycles of phosphorylation and dephosphorylation Figure 8-1B . b. Glucagon and epinephrine stimulate lipase activity in order to provide fatty acids and glycerol for use as fuels while insulin inhibits lipase activity as it stimulates storage of fatty acids. 2. The glycerol backbone derived from lipase-mediated triacylglycerol breakdown is released into the bloodstream and taken up by the liver. a. Glycerol is phosphorylated on its 3 .