Biochemistry

11.3 INTERMEDIARY CARBOHYDRATE METABOLISM In the cells of the tissues of heterotrophic organisms, carbohydrates are synthesized from glucose and non-carbohydrate compounds. Excess of glucose in the blood is used for glycogen biosynthesis in the liver and muscles. Glycogen accumulates in the form of granules, which also contain enzymes of its synthesis, breakage and regulation of these processes. Glycogen biosynthesis occurs under aerobic conditions. At the beginning of this process, glucose is phosphorylated under the influence of hexokinase (glucokinase) using ATP: Hexokinase Glucose + ATP * Glucose-6-phosphate + ADP Then, glucose-6-phosphate, is isomerized to glucose- 1 -phosphate, with involvement of phosphoglucomutase-. Phosphoglucomutase Glucose-6-phosphate ------------ ► Glucose -1 -phosphate Glucose -1-phosphate reacts -with UTP (uridine triphosphate) and forms UDP-glucose, which is an activated form of glucose. This reaction is catalyzed by glucose-1-phosphate-uridylyl -transferase (UDPG-pyrophosphorylase) UDPG-pyrophosphorylase Glucose-1-phosphate+UTP « ------ — UDP-glucose+ H4P2O7 Pyrophosphate That pyrophosphate undergoes hydrolysis, and the released energy "starts" the synthesis of UDP-glucose. At the next stage, the activated glycosidic component of UDP-glucose is transferred to the hydroxyl group C-4 of the terminal glucose residue of the chain, forming a 1,4-glycosidic bond. For the synthesis of glycogen, it is necessary to have a small, "seed ” quantity containing at least four glucose residues. When this happens, the transfer of glucose residue from UDP-glucose to glycogen occurs and its chain is extended by forming a 1,4 bond: Glycogensynthase UDP-glucose + (СбНі 0 О5) п -------------- ► UDP + (СбНюС^п+і Glycogen"source" Glycogen "extended" 216