tailieunhanh - PDQ BIOCHEMISTRY - PART 8

Một khi trong bào tương, một loại enzyme được gọi là lyase citrate sử dụng citrate, ATP, và CoA tái sinh acetyl-CoA và oxaloacetate. Vì vậy, việc chuyển đổi ban đầu của acetyl-CoA thành citrate trong ma trận ty thể chỉ đơn giản là tạo điều kiện cho thế hệ của acetyl-CoA bên ngoài ty thể. | 244 PDQ BIOCHEMISTRY in the inner membrane Figure 7-24 . Once in the cytosol an enzyme called citrate lyase uses citrate ATP and CoA to regenerate acetyl-CoA and oxaloacetate. Thus the initial conversion of acetyl-CoA into citrate within the mitochondrial matrix simply facilitates the generation of acetyl-CoA outside the mitochondria. In the cytosol acetyl-CoA is carboxylated by an important enzyme known appropriately enough as acetyl-CoA carboxylase sometimes abbreviated as ACC but be careful as three-letter abbreviations for enzymes can often show redundancy within biochemistry . ACC is a complex enzyme that has biotin as a prosthetic group and the ability to both transfer carbon dioxide to biotin and then transfer that bound carbon dioxide to acetyl-CoA. Thus ACC can both carry and transfer carbon dioxide. The overall reaction is noted below CH3-C-S-CoA ATP HCO3- -OOC-CH2-C-S-CoA ADP Pi O O Acetyl-CoA Malonyl-CoA Here bicarbonate HCO3- serves as the source of carbon dioxide and ATP provides energy for the carboxylation. The product with the extra carboxyl group is known as malonyl-CoA. Acetyl-CoA carboxylase is important as malonyl-CoA provides most of the carbons for fatty acid synthesis. You will soon see that the extra carboxyl group is added to acetyl-CoA so that the decarboxylation of malonyl-CoA provides an energy kick that helps to drive the assembly of two-carbon units in fatty acid synthesis. ACC is the major point of regulation for fatty acid synthesis. Thus when ACC is phos- Citrate Lyase Figure 7-24 Citrate and the production of acetyl-CoA in cytosol. To supply acetyl-CoA for the cytosolic acetyl-CoA carboxylase and for fatty acid synthase acetyl-CoA is converted into citrate by the action of mitochondrial citrate synthase an enzyme of the Krebs cycle. Citrate can be transported fromthe mitochondrial matrix to cytosol where it is converted to acetyl-CoA and the molecule oxaloacetate by the action of citrate lyase. Chapter 7 Metabolism 245 phorylated