Identify the main sites of glycogen warehouse in the body and the function of glycogen in this tissues.

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Outline the metabolic pathways because that synthesis and also degradation that glycogen.

Describe the mechanism whereby glycogen is mobilized in liver in response to glucagon, in muscle throughout exercise, and in both tissues in an answer to epinephrine.

Explain the beginning and after-effects of glycogen storage illness in liver and also muscle.

Describe the device for counterregulation the glycogenolysis and glycogenesis in liver.

Outline the pathway the gluconeogenesis, consisting of substrates, distinct enzymes and regulatory mechanisms.

Describe the complementary duties of glycogenolysis and gluconeogenesis in maintain of blood glucose concentration.

Introduction

The red cell and the mind have an absolute necessity for blood glucose for power metabolism. Together, lock consume around 80% that the 200 g of glucose consumed in the body per day. There space only around 10 g that glucose in the plasma and also extracellular fluid volume, so that blood glucose have to be replenished constantly. Otherwise, hypoglycemia develops and compromises mind function, causing confusion and also disorientation, and possibly life-threatening coma in ~ blood glucose concentrations below 2.5 mmol/L (45 mg/dL). We absorb glucose from ours intestines for only 2–3 h complying with a carbohydrate-containing meal, therefore there must be a device for maintain of blood glucose between meals.

Glycogen, a polysaccharide storage type of glucose, is our an initial line the defense versus declining blood glucose concentration. During and also immediately following a meal, glucose is converted right into glycogen, a process known asglycogenesis, in both liver and also muscle. The organization concentration of glycogen is greater in liver than in muscle but because of the relative masses of muscle and also liver, the majority of glycogen in the body is stored in muscle (Table 13.1).


Table 13.1


Tissue circulation of carbohydrate power reserves (70-kg adult)


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Hepatic glycogenolysis and gluconeogenesis are compelled for maintain of regular blood glucose concentration

Hepatic glycogen is progressively degraded between meals, through the pathway the glycogenolysis, releasing glucose to maintain blood glucose concentration. However, full hepatic glycogen stores room barely adequate for maintain of blood glucose concentration during a 12-h fast.

During sleep, when we room not eating, over there is a gradual change fromglycogenolysisto de novo synthesis of glucose, likewise an hepatic pathway, well-known asgluconeogenesis(Fig. 13.1). Gluconeogenesis is important for survival during fasting or starvation, as soon as glycogen stores space depleted. The liver supplies amino acids from muscle protein as the major precursor the glucose, but additionally makes use of lactate native glycolysis and also glycerol from fat catabolism. Fat acids, mobilized from adipose tissue triglyceride stores, carry out the energy for gluconeogenesis.

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FIG. 13.1Sources that blood glucose during a normal day.Between meals, blood glucose is derived primarily from hepatic glycogen. Depending on the frequency that snacking, glycogenolysis and also gluconeogenesis may be much more or less active during the day. Late in the night or in early morning, following depletion that a major portion of hepatic glycogen, gluconeogenesis i do not care the primary source of blood glucose.

Glycogen is save in muscle for usage in power metabolism

Muscle glycogen is not obtainable for maintain of blood glucose. Glucose acquired from blood and also glycogen is used specifically for energy metabolism in muscle, especially throughout bursts of physical activity. Return cardiac and skeletal muscles depend on fats as their primary source of energy, part glucose management is important for efficient fat management in these tissues.

This chapter explains the pathways of glycogenesis and also glycogenolysis in liver and muscle, and also the pathway that gluconeogenesis in liver.

Structure of glycogen

Glycogen, a highly branched glucan, is the storage type of glucose in tissues

Glycogen is a branched polysaccharide the glucose. It includes only two varieties of glycosidic linkages, chain of α1→4-linked glucose residues through α1→6 branches spaced about every 4–6 residues follow me the α1→4 chain (Fig. 13.2). Glycogen is closely related tostarch, the warehouse polysaccharide the plants, yet starch is composed of a mixture that amylose and amylopectin. The amylose component has only linear α1→4 chains; the amylopectin ingredient is much more glycogen-like in structure however with fewer α1→6 branches, about one every 12 α1→4-linked glucose residues. The gross structure of glycogen is dendritic in nature, broadening from a main point sequence bound to a tyrosine residue in the proteinglycogeninand occurring into a final structure the same, similar thing a head of cauliflower. The enzymes of glycogen metabolism space bound to the surface ar of the glycogen particle; countless terminal glucose molecule on the surface ar of the molecule administer ready access for fast release of glucose from the glycogen polymer.

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FIG. 13.2Close-up that the structure of glycogen.The figure shows α1→4 chains and also an α1→6 branch point. Glycogen is stored as granules in liver and muscle cytoplasm.

Pathway of glycogenesis indigenous blood glucose in liver

Glycogenesis is caused in liver and also muscle complying with a meal

The liver is wealthy in the high-capacity, low-affinity (Km>10 mmol/L) glucose transporterGLUT-2, make it easily permeable to glucose delivered at high concentration in portal blood during and also following a enjoy the meal (seeTable 8.2). The liver is also rich inglucokinase, an enzyme the is certain for glucose and converts it right into glucose 6-phosphate (Glc-6-P). Glucokinase (GK) is inducible by continued intake of a high-carbohydrate diet. It has a highKm, about 5–7 mmol/L, so that it is poised to rise in task as portal glucose increases above the normal 5 mmol/L (100 mg/dL) blood glucose concentration. Unequal hexokinase, GK is no inhibited through Glc-6-P, so that the concentration the Glc-6-P rises rapidly in liver adhering to a carbohydrate-rich meal, forcing glucose into all the significant pathways of glucose metabolism: glycolysis, the pentose phosphate pathway, and glycogenesis (seeFig. 12.2). Glucose is channeled right into glycogen, giving a carbohydrate reserve for maintain of blood glucose during the postabsorptive state. Overabundance Glc-6-P in liver, past that essential to replenish glycogen reserves, is climate funneled into glycolysis, in part for power production however primarily for conversion right into fatty acids and also triglycerides, which space exported for storage in adipose tissue. Glucose the passes through the liver causes boost in peripheral blood glucose concentration complying with carbohydrate-rich meals. This glucose is supplied in muscle because that synthesis and also storage that glycogen and in adipose tissue as a source of glycerol for triglyceride biosynthesis.

The pathway of glycogenesis native glucose (Fig. 13.3A) requires four steps:

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FIG. 13.3Pathways that glycogenesis (A) and also glycogenolysis (B).

Conversion that Glc-6-P right into glucose-1-phosphate (Glc-1-P) through phosphoglucomutase.

Activation of Glc-1-P to the sugar nucleotide uridine diphosphate (UDP)-glucose by the enzyme UDP-glucose pyrophosphorylase.

Transfer that glucose indigenous UDP-Glc to glycogen in α1→4 linkage by glycogen synthase, a member the the class of enzymes well-known as glycosyl transferases.

When the α1→4 chain exceeds eight residues in length, glycogen branching enzyme, a transglycosylase, transfers few of the α1→4-linked street to one α1→6 branch, setting the stage for continued elongation the both α1→4 chains till they, in turn, come to be long sufficient for carry by branching enzyme.

Glycogen synthaseis the regulatory enzyme for glycogenesis, fairly than UDP-glucose pyrophosphorylase, due to the fact that UDP-glucose is likewise used because that synthesis of various other sugars, and as a glycosyl donor because that synthesis that glycoproteins, glycolipids and also proteoglycans (Chapters 27–29). Pyrophosphate (PPi), the other product the the pyrophosphorylase reaction, is a high power phosphate anhydride. The is rapidly hydrolyzed to not natural phosphate by pyrophosphatase, giving the thermodynamic driving force for biosynthesis the glycogen.

Pathway the glycogenolysis in liver

Hepatic glycogen phosphorylase provides for fast release that glucose into blood throughout the postabsorptive state

As with many metabolic pathways, different enzymes, periodically in separate subcellular compartments, are required for the forward and also reverse pathways. The pathway that glycogenolysis (Fig. 13.3B) starts with removal of the abundant, exterior α1→4-linked glucose residues in glycogen. This is completed not through a hydrolase however byglycogen phosphorylase, one enzyme that provides cytosolic phosphate and releases glucose from glycogen in the kind of Glc-1-P. The Glc-1-P is isomerized by phosphoglucomutase to Glc-6-P, place it at the top of the glycolytic pathway; the phosphorylase reaction, in effect, bypasses the need for ATP in the hexokinase or glucokinase reactions. In liver, the glucose is released from Glc-6-P by glucose-6-phosphatase (Glc-6-Pase), and the glucose exits via the GLUT-2 transporter into blood. The rate-limiting, regulatory step in glycogenolysis is catalyzed by phosphorylase, the first enzyme in the pathway.

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Phosphorylase is certain for α1→4 glycosidic linkages; it cannot cleave α1→6 linkages. Further, this large enzyme cannot method the branching glucose residues efficiently. Thus, as displayed inFigure 13.3B, phosphorylase cleaves the exterior glucose residues till the branches space three or four residues long, thendebranching enzyme, which has both transglycosylase and glucosidase activity, move a brief segment that glucose residues bound come the α1→6 branch to the finish of an adjacent α1→4 chain, leave a solitary glucose residue at the branch point. This glucose is then removed by the exo-1,6-glucosidase activity of debranching enzyme, enabling glycogen phosphorylase to continue with destruction of the prolonged α1→4 chain until one more branch suggest is approached, setup the phase for a repeat of the transglycosylase and glucosidase reactions. Around 90% of the glucose is exit from glycogen as Glc-1-P, and the remainder, obtained from the α1→6 branching residues, as complimentary glucose.