It does not take place in the reverse direction of synthesis, but by phosphorolysis (a reaction where the phosphate ion plays a role similar to that of water in hydrolysis). The pathway of the synthesis of oligo- and polysaccharides most commonly followed by the cells is the one which uses the nucleotides-monosaccharides, a partial list of which was given above. 0000005013 00000 n
CARBOHYDRATE METABOLISM Glucose is the major form of sugar moiety present in blood and other body fluids. Metabolism of Carbohydrates | Biochemistry. H�lTMo1��W̑T�Y��}
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���6F�$������W�P�P��C9O�U�;�a�f"�}�9H ���x��z�&���ZՓ��nAWj4�+Un��?6P�,��L��{uި�5��W�;|M.�Dٱ Ԭk�f This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. 2) Synthesis of a Glycan of the N-Acetyl-Lactosaminic Type: It was long believed that the residues of monosaccharides were conjugated one after the other by direct transfer from glycosylnucleotides as in the case of O-glycosylproteins. The pathways of the synthesis and degradation of glycogen can be summarized in a simple diagram (see fig. Biochemistry Topics, Carbohydrate Metabolism. 0000021573 00000 n
But the following synthesis of sucrose has been described, catalyzed by sucrose-phosphorylase. <]>>
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Carbohydrates serve several biochemical functions: Monosaccharides serve as fuel for cellular metabolism. 0000007190 00000 n
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The glycosyl-trans- ferases involved in the biosynthesis of glycans of glycoproteins are enzymes integrated in the luminal face of membranes of the endoplasmic reticulum. It is MCQ on Carbohydrates Page. N- acetylglucosaminyl transferase I then forms glycan D. Next, α-mannosidases lead to the compound E on which N-acetylglucosaminyl transferase II conjugates a second residue of N-acetylglucosamine (compound F). Phosphorylases can catalyze in vitro, the formation (or lengthening) of polysaccharide chains from glucose-1-℗ and thus enable the synthesis of glucosans in which the glucose residues are joined by α-1, 4 linkages. This is a rather common phenomenon which we will observe again especially in connection with the synthesis and degradation of fatty acids, and which presents the very great advantage of enabling a better regulation of cellular metabolism. The concentration of a product on one of the sides of the membrane is a process which requires energy; for this reason it is spoken of as active transport. On the contrary, the non- phosphorylated glycoproteins evolve towards the N-acetyl-lactosaminic type by the cascade of transformations described in figure 4-47. It is however known that glucose and galactose cross the membrane of the intestinal epithelial cell with the help of a Na+ dependent carrier. One of the most important possibilities of control lies in the regulation of the activity of enzymes, which can be either activated or inhibited. Some organisms, especially yeast, possess a hexokinase capable of phosphorylating glucose, fructose and man- nose to the corresponding hexose-6-℗. The ratio of hydrogen and oxygen in the majority of carbohydrates will be in 2:1 as in water. We have seen that glycogen molecules are synthesized and degraded by different processes. 0000009581 00000 n
As a matter of fact, only the glycoproteins which carry the mannose-6-phosphate signal will be firmly attached to a receptor of the Golgi membrane and their maturation towards a N-acetyl-lactosaminic structure will be blocked. These 3 reactions are represented diagrammatically in figure 4-22. ���?¢
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