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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Brand-new York: W H Freeman; 2002.
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Glycogen is a conveniently mobilized storage form of glucose. The is a an extremely large, branched polymer of glucose residues (Figure 21.1) that can be damaged down to yield glucose molecule when power is needed. Most of the glucose residual water in glycogen are attached by α-1,4-glycosidic bonds. Branches at around every tenth residue are created by α-1,6-glycosidic bonds. Recall the α-glycosidic linkages type open helical polymers, conversely, β linkages produce virtually straight strands that form structural fibrils, together in cellulose (Section 11.2.3).
Glycogen Structure. In this framework of two external branches that a glycogen molecule, the residual water at the nonreducing end are shown in red and also residue the starts a branch is displayed in green. The remainder of the glycogen molecule is represented by R.
Glycogen is not as decreased as fat acids are and also consequently no as power rich. Why do pets store any type of energy together glycogen? Why not convert all overabundance fuel into fatty acids? Glycogen is vital fuel reserve for number of reasons. The controlled malfunction of glycogen and also release the glucose increase the lot of glucose the is accessible between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen"s function in maintaining blood-glucose levels is especially important since glucose is essentially the just fuel offered by the brain, except during prolonged starvation. Moreover, the glucose indigenous glycogen is readily mobilized and is as such a an excellent source of energy for sudden, strenuous activity. Unlike fat acids, the released glucose can provide energy in the absence of oxygen and also can thus supply power for anaerobic activity.
The two significant sites the glycogen storage are the liver and skeletal muscle. The concentration of glycogen is higher in the liver than in muscle (10% versus 2% through weight), but an ext glycogen is save in skeleton muscle overall because that its much better mass. Glycogen is current in the cytosol in the type of granules ranging in diameter native 10 come 40 nm (Figure 21.2). In the liver, glycogen synthesis and degradation space regulated to keep blood-glucose levels as forced to fulfill the needs of the organism together a whole. In contrast, in muscle, these processes are regulation to fulfill the power needs of the muscle itself.
Electron Micrograph of a Liver Cell. The thick particles in the cytoplasm space glycogen granules.
21.0.1. Summary of Glycogen Metabolism:
Glycogen degradation and also synthesis are relatively simple biochemical processes. Glycogen degradation consists of three steps: (1) the release of glucose 1-phosphate native glycogen, (2) the remodeling that the glycogen substrate come permit further degradation, and also (3) the conversion of glucose 1-phosphate right into glucose 6-phosphate for further metabolism. The glucose 6-phosphate derived from the failure of glycogen has actually three fates (Figure 21.3): (1) that is the early substrate because that glycolysis, (2) it have the right to be handle by the pentose phosphate pathway to yield NADPH and also ribose derivatives; and also (3) it have the right to be convert into complimentary glucose for release right into the bloodstream. This conversion takes place mainly in the liver and to a lesser extent in the intestines and also kidneys.
Fates of Glucose 6-Phosphate. Glucose 6-phosphate acquired from glycogen deserve to (1) be offered as a fuel for anaerobic or aerobic metabolism together in, for instance, muscle; (2) be converted into complimentary glucose in the liver and subsequently released into the blood; (more...)
Glycogen synthesis calls for an activated form of glucose, uridine diphosphate glucose (UDP-glucose), i m sorry is created by the reaction of UTP and glucose 1-phosphate. UDP-glucose is included to the nonreducing finish of glycogen molecules. Together is the situation for glycogen degradation, the glycogen molecule must be remodeled for continued synthesis.
The regulation that these processes is fairly complex. Number of enzymes taking component in glycogen metabolism allosterically respond come metabolites the signal the energy needs of the cell. This allosteric responses enable the mediate of enzyme activity to accomplish the demands of the cell in which the enzymes space expressed. Glycogen line is additionally regulated through hormonally created cascades that cause the reversible phosphorylation of enzymes, which alters their kinetic properties. Regulation by hormones permits glygogen management to readjust to the requirements of the whole organism. By both this mechanisms, glycogen degradation is combined with glycogen synthesis. We will first examine the metabolism, complied with by enzyme regulation and also then the intricate integration of regulate mechanisms.
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Signal cascades lead to the mobilization of glycogen to develop glucose, one energy resource for runners. <(Left) Mike Powell/Allsport.>
21.1. Glycogen malfunction Requires the Interplay of several Enzymes21.2. Phosphorylase Is regulated by Allosteric Interactions and Reversible Phosphorylation21.3. Epinephrine and also Glucagon Signal the require for Glycogen Breakdown21.4. Glycogen Is Synthesized and Degraded by different Pathways21.5. Glycogen failure and Synthesis room Reciprocally RegulatedSummaryProblemsSelected Readings
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