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You are watching: In the body, the major storage sites for glycogen are the

Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. New York: W H Freeman; 2002.


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Glycogen is a readily mobilized storage kind of glucose. The is a very large, branched polymer of glucose residual water (Figure 21.1) that can be damaged down to productivity glucose molecule when energy is needed. Many of the glucose residues 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, vice versa, β linkages produce almost straight strands that form structural fibrils, together in cellulose (Section 11.2.3).


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Figure 21.1

Glycogen Structure. In this framework of two external branches the a glycogen molecule, the residual water at the nonreducing ends are presented in red and residue that starts a branch is shown in green. The remainder of the glycogen molecule is stood for by R.


Glycogen is not as diminished as fat acids are and consequently not as energy rich. Why do animals store any type of energy together glycogen? Why not transform all excess fuel right into fatty acids? Glycogen is vital fuel reserve for number of reasons. The controlled break down of glycogen and release the glucose increase the amount of glucose the is accessible between meals. Hence, glycogen serves together a buffer to preserve blood-glucose levels. Glycogen"s role in maintaining blood-glucose levels is especially important since glucose is virtually the just fuel supplied by the brain, except during an extensive starvation. Moreover, the glucose native glycogen is easily mobilized and is thus a good source of power for sudden, strenuous activity. Unlike fat acids, the exit glucose can carry out energy in the lack of oxygen and also can for this reason supply energy for anaerobic activity.

The two major sites the glycogen storage room the liver and skeletal muscle. The concentration the glycogen is higher in the liver 보다 in muscle (10% matches 2% by weight), but more glycogen is save on computer in skeleton muscle in its entirety because that its much better mass. Glycogen is existing in the cytosol in the kind of granules ranging in diameter indigenous 10 come 40 nm (Figure 21.2). In the liver, glycogen synthesis and degradation room regulated to preserve blood-glucose level as forced to satisfy the demands of the organism together a whole. In contrast, in muscle, these procedures are regulation to fulfill the power needs of the muscle itself.


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Figure 21.2

Electron Micrograph of a Liver Cell. The thick particles in the cytoplasm are glycogen granules.


21.0.1. An introduction of Glycogen Metabolism:

Glycogen degradation and also synthesis are fairly simple biochemical processes. Glycogen degradation consists of three steps: (1) the relax of glucose 1-phosphate native glycogen, (2) the remodeling of the glycogen substrate come permit further degradation, and also (3) the counter of glucose 1-phosphate right into glucose 6-phosphate for more metabolism. The glucose 6-phosphate derived from the breakdown of glycogen has three fates (Figure 21.3): (1) the is the early substrate for glycolysis, (2) it deserve to be processed by the pentose phosphate pathway to yield NADPH and also ribose derivatives; and (3) it deserve to be converted into free glucose for release right into the bloodstream. This conversion takes place greatly in the liver and also to a lesser level in the intestines and also kidneys.


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Figure 21.3

Fates the Glucose 6-Phosphate. Glucose 6-phosphate derived from glycogen have the right to (1) be provided as a fuel because that anaerobic or aerobic metabolism as in, because that instance, muscle; (2) be convert into free glucose in the liver and also subsequently released right into the blood; (more...)


Glycogen synthesis calls for an activated kind of glucose, uridine diphosphate glucose (UDP-glucose), i m sorry is developed by the reaction that UTP and also glucose 1-phosphate. UDP-glucose is added to the nonreducing end of glycogen molecules. As is the case for glycogen degradation, the glycogen molecule should be remodeled for ongoing synthesis.

The regulation that these processes is fairly complex. Several enzymes taking component in glycogen metabolism allosterically respond come metabolites the signal the energy needs that the cell. These allosteric responses permit the convey of enzyme task to meet the needs of the cell in which the enzymes are expressed. Glycogen line is additionally regulated through hormonally created cascades that bring about the reversible phosphorylation of enzymes, which changes their kinetic properties. Regulation through hormones allows glygogen management to change to the demands of the whole organism. By both this mechanisms, glycogen destruction is incorporated with glycogen synthesis. We will an initial examine the metabolism, complied with by enzyme regulation and then the sophisticated integration of regulate mechanisms.

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Figure

Signal cascades result in the mobilization of glycogen to develop glucose, an energy resource for runners. <(Left) Mike Powell/Allsport.>


21.1. Glycogen failure Requires the Interplay of numerous Enzymes21.2. Phosphorylase Is regulation 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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