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Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. New York: W H Freeman; 2002.
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Glycogen is a easily mobilized storage form of glucose. That is a very large, branched polymer that glucose residual water (Figure 21.1) that deserve to be broken down to yield glucose molecules when power is needed. Many of the glucose residual water in glycogen are attached by α-1,4-glycosidic bonds. Branches at about every tenth residue are developed by α-1,6-glycosidic bonds. Recall the α-glycosidic linkages form open helical polymers, whereas β linkages produce practically straight strands that type structural fibrils, as in cellulose (Section 11.2.3).
Glycogen Structure. In this framework of two external branches that a glycogen molecule, the residues at the nonreducing end are presented in red and residue that 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 energy rich. Why do animals store any type of energy together glycogen? Why not transform all excess fuel into fatty acids? Glycogen is critical fuel make reservation for numerous reasons. The controlled failure of glycogen and release the glucose rise the amount of glucose that is easily accessible between meals. Hence, glycogen serves as a buffer to keep blood-glucose levels. Glycogen"s function in keeping blood-glucose level is especially important due to the fact that glucose is practically the just fuel used by the brain, other than during an extensive starvation. Moreover, the glucose from glycogen is easily mobilized and is because of this a good source of energy for sudden, strenuous activity. Unlike fatty acids, the exit glucose can carry out energy in the absence of oxygen and can hence supply energy for anaerobic activity.
The two significant sites that glycogen storage are the liver and also skeletal muscle. The concentration that glycogen is higher in the liver than in muscle (10% versus 2% through weight), but an ext glycogen is stored in skeletal muscle all at once because that its much higher mass. Glycogen is existing in the cytosol in the type of granules varying in diameter indigenous 10 to 40 nm (Figure 21.2). In the liver, glycogen synthesis and degradation room regulated to maintain blood-glucose level as forced to meet the needs of the organism as a whole. In contrast, in muscle, these procedures are regulation to meet the energy needs that the muscle itself.
Electron Micrograph the a Liver Cell. The dense particles in the cytoplasm room glycogen granules.
21.0.1. Summary of Glycogen Metabolism:
Glycogen degradation and also synthesis are relatively simple biochemical processes. Glycogen degradation is composed of three steps: (1) the relax of glucose 1-phosphate native glycogen, (2) the remodeling that the glycogen substrate come permit additional degradation, and also (3) the conversion of glucose 1-phosphate into glucose 6-phosphate for additional metabolism. The glucose 6-phosphate derived from the failure of glycogen has actually three fates (Figure 21.3): (1) that is the early stage substrate because that glycolysis, (2) it can be handle by the pentose phosphate pathway to yield NADPH and also ribose derivatives; and (3) it deserve to be convert into cost-free glucose because that release right into the bloodstream. This conversion takes place largely in the liver and to a lesser level in the intestines and also kidneys.
Fates that Glucose 6-Phosphate. Glucose 6-phosphate derived from glycogen can (1) be used as a fuel because that anaerobic or aerobic metabolism as in, for instance, muscle; (2) be converted into free 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), which is created by the reaction of UTP and glucose 1-phosphate. UDP-glucose is included to the nonreducing end of glycogen molecules. Together is the situation for glycogen degradation, the glycogen molecule should be remodeled for continued synthesis.
The regulation that these procedures is quite complex. Number of enzymes taking part in glycogen metabolism allosterically respond to metabolites the signal the energy needs that the cell. This allosteric responses permit the mediate of enzyme activity to satisfy the demands of the cell in i beg your pardon the enzymes space expressed. Glycogen line is also regulated by hormonally stimulated cascades that cause the reversible phosphorylation that enzymes, which changes their kinetic properties. Regulation by hormones permits glygogen line to adjust to the requirements of the entire organism. Through both this mechanisms, glycogen degradation is combined with glycogen synthesis. Us will first examine the metabolism, complied with by enzyme regulation and then the sophisticated integration of manage mechanisms.
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Signal cascades cause the mobilization of glycogen to create glucose, an energy resource for runners. <(Left) Mike Powell/Allsport.>
21.1. Glycogen malfunction Requires the Interplay of several Enzymes21.2. Phosphorylase Is regulation by Allosteric Interactions and Reversible Phosphorylation21.3. Epinephrine and Glucagon Signal the require for Glycogen Breakdown21.4. Glycogen Is Synthesized and Degraded by different Pathways21.5. Glycogen malfunction and Synthesis are Reciprocally RegulatedSummaryProblemsSelected Readings
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