The body's largest stores of glycogen are found in the liver and skeletal muscles.  Individual glucosyl residues (ie, glucose monomers) in muscle and liver glycogen particles are linked by α(1→4) and α(1→6) glycosidic bonds and are released in the reactions of glycogenolysis.  Glycogen phosphorylase catalyzes the removal of most of the glucosyl residues, although debranching enzyme also releases the single glucose molecule that remains after it repositions three residues onto an adjacent linear chain.Binding of the hormone epinephrine to its receptor on the surface of muscle cells stimulates muscle glycogenolysis through activation of a signaling cascade (Figure 1).Figure 1  Stimulation of skeletal muscle glycogenolysis by epinephrine.The relative signal amplification (SA) at each step (I–IV in Figure 1) can be determined by comparing the number of active signaling molecules before (Mbefore) and after (Mafter) the step:SA=MafterMbeforeSA=𝑀after𝑀before Question 36The skeletal muscles collectively contain more than twice as much glucose stored in glycogen as is found in the liver.  Compared with the liver, the amount of glucose released into the blood from muscles is likely:A.greater from glycogenolysis, but less from gluconeogenesis.B.the same from glycogenolysis, but more from gluconeogenesis.C.less from both glycogenolysis and gluconeogenesis.D.the same from both glycogenolysis and gluconeogenesis.

Glycogen molecules are stored in liver and --------------

La glycogénolyse est activée par : 1. Déphosphorylation de la glycogène phosphorylase; 2. Activation de la glycogène phosphorylase kinase 3. L'insuline; 4. Le glucagon; 5. Le glucose et l'ATPA1,3B4,5C2,4D1,4E2,5

Dietary carbohydrates are used to synthesize liver glycogen, which exists as particles of various sizes.  During the synthesis of a new glycogen particle, a glucose molecule is linked to the protein glycogenin, after which additional glucose molecules are linked to the first glucose and to each other to form regions that are successively farther from glycogenin.  Glycogen synthesis is catalyzed by the enzyme glycogen synthase, whose activity is inhibited by phosphorylation of three carboxy-terminal serine residues.  During the postabsorptive state (ie, after carbohydrate absorption from a meal is finished), hormone-stimulated release of glucose residues from liver glycogen stores help maintain adequate glucose levels in the blood.Although the important role of liver glycogen in whole-body glucose metabolism has long been appreciated, much remains unknown about the regulation of liver glycogen synthesis and breakdown.  Based on previous studies suggesting a relationship between glycogen particle size and the rate of glycogenolysis, a team of researchers measured the average sizes of liver α and β glycogen particles in wild-type (WT) mice (Table 1).Table 1  Dimensions of Liver Glycogen ParticlesTo better understand the role of the protein glycogenin in glycogen synthesis, the researchers also mated male and female mice that were heterozygous for glycogenin expression (GN +/−) to produce heterozygous offspring as well as mice with WT and knockout (GN −/−) genotypes.  Relative glycogenin gene expression (ie, mRNA levels) was measured in liver and heart samples from the mice (Table 2).  Glycogenin was undetectable in western blots of tissue samples from knockout mice.Table 2  Gene Expression (mRNA Levels Relative to WT) in Liver and Heart Samples.The researchers also measured glycogen concentrations in liver and heart samples from the mice.  The results are presented in Figure 1.Figure 1  Liver and heart glycogen concentrations Question 12The glucagon-induced entry of glucose derived from liver glycogen into the bloodstream for transport to other tissues involves close interaction among which enzymes?A.Glycogen phosphorylase, debranching enzyme, glycogen synthaseB.Glycogen phosphorylase, debranching enzyme, phosphoglucomutase, glucose 6-phosphataseC.Debranching enzyme, phosphoglucomutase, glycogen synthaseD.Debranching enzyme, phosphoglucomutase, phosphofructokinase, glucose 6-phosphatase

_____________ cleaves individual glucose molecules from glycogen.*1 pointPhosphoglucomutaseGlycogen phosphorylase

Contrairement au foie, le muscle ne contribue pas directement a la régulation de la glycémie carADépourvu de glycogène phosphorylaseBDépourvu de glucokinaseCDépourvu de glucose-6-phosphataseDDépourvu d'enzyme débranchanteEPauvre en glycogène