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

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Outline the metabolic pathways for synthesis and degradation that glycogen.

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

Explain the beginning and aftermath of glycogen storage conditions in liver and muscle.

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

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

Describe the complementary functions of glycogenolysis and gluconeogenesis in maintenance of blood glucose concentration.

Introduction

The red cell and the mind have one absolute need for blood glucose for power metabolism. Together, castle consume around 80% the the 200 g of glucose spend in the body every day. There space only around 10 g the glucose in the plasma and extracellular liquid volume, so the blood glucose must be replenished constantly. Otherwise, hypoglycemia develops and compromises mind function, bring about confusion and disorientation, and possibly life-threatening coma in ~ blood glucose concentrations listed below 2.5 mmol/L (45 mg/dL). Us absorb glucose from our intestines for only 2–3 h complying with a carbohydrate-containing meal, for this reason there must be a system for maintain of blood glucose in between meals.

Glycogen, a polysaccharide storage type of glucose, is our an initial line that defense against declining blood glucose concentration. During and immediately adhering to a meal, glucose is converted into glycogen, a procedure known asglycogenesis, in both liver and muscle. The tissue concentration that glycogen is higher in liver than in muscle but due to the fact that of the family member masses that muscle and liver, the bulk of glycogen in the body is save on computer in muscle (Table 13.1).


Table 13.1


Tissue distribution of carbohydrate energy reserves (70-kg adult)


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Hepatic glycogenolysis and also gluconeogenesis are forced for maintain of typical blood glucose concentration

Hepatic glycogen is progressively degraded in between meals, by the pathway of glycogenolysis, releasing glucose to preserve blood glucose concentration. However, full hepatic glycogen stores are barely adequate for maintain of blood glucose concentration throughout a 12-h fast.

During sleep, once we are not eating, there is a gradual change fromglycogenolysisto de novo synthesis of glucose, likewise an hepatic pathway, well-known asgluconeogenesis(Fig. 13.1). Gluconeogenesis is crucial for survival throughout fasting or starvation, as soon as glycogen stores room depleted. The liver supplies amino mountain from muscle protein together the primary precursor of glucose, but likewise makes usage of lactate from glycolysis and glycerol indigenous fat catabolism. Fat acids, mobilized indigenous adipose organization triglyceride stores, carry out the energy for gluconeogenesis.

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FIG. 13.1Sources that blood glucose throughout a typical day.Between meals, blood glucose is derived primarily indigenous hepatic glycogen. Depending on the frequency the snacking, glycogenolysis and gluconeogenesis might be an ext or less active during the day. Late in the night or in beforehand morning, following depletion that a major portion of hepatic glycogen, gluconeogenesis becomes the primary source of blood glucose.

Glycogen is save on computer in muscle for usage in energy metabolism

Muscle glycogen is not easily accessible for maintain of blood glucose. Glucose acquired from blood and glycogen is used specifically for energy metabolism in muscle, especially during bursts of physical activity. Back cardiac and also skeletal muscles rely on fats together their primary resource of energy, part glucose management is necessary for efficient fat metabolism in this tissues.

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

Structure that glycogen

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

Glycogen is a branched polysaccharide of glucose. It consists of only two species of glycosidic linkages, chain of α1→4-linked glucose residues v α1→6 branches spaced around every 4–6 residues follow me the α1→4 chain (Fig. 13.2). Glycogen is carefully related tostarch, the warehouse polysaccharide that plants, but starch is composed of a mixture the amylose and amylopectin. The amylose component consists of only straight α1→4 chains; the amylopectin component is more glycogen-like in structure yet with fewer α1→6 branches, about one every 12 α1→4-linked glucose residues. The gross structure of glycogen is dendritic in nature, expanding from a main point sequence bound to a tyrosine residue in the proteinglycogeninand emerging into a final structure resembling a head the cauliflower. The enzymes of glycogen metabolism are bound come the surface ar of the glycogen particle; countless terminal glucose molecule on the surface of the molecule provide ready accessibility for fast release the glucose from the glycogen polymer.

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

Pathway of glycogenesis from blood glucose in liver

Glycogenesis is caused in liver and also muscle adhering to a meal

The liver is rich in the high-capacity, low-affinity (Km>10 mmol/L) glucose transporterGLUT-2, do it easily permeable to glucose yielded at high concentration in portal blood during and following a meal (seeTable 8.2). The liver is likewise rich inglucokinase, one enzyme the is details for glucose and converts it right into glucose 6-phosphate (Glc-6-P). Glucokinase (GK) is inducible through continued usage of a high-carbohydrate diet. It has a highKm, around 5–7 mmol/L, so that it is poised to boost in task as portal glucose increases above the typical 5 mmol/L (100 mg/dL) blood glucose concentration. Unequal hexokinase, GK is no inhibited by Glc-6-P, so the the concentration the Glc-6-P boosts rapidly in liver following a carbohydrate-rich meal, forcing glucose right into all the major pathways the glucose metabolism: glycolysis, the pentose phosphate pathway, and glycogenesis (seeFig. 12.2). Glucose is channeled right into glycogen, providing a carbohydrate make reservation for maintain of blood glucose throughout the postabsorptive state. Excess Glc-6-P in liver, past that essential to replenish glycogen reserves, is then funneled into glycolysis, in component for power production however primarily because that conversion into fatty acids and triglycerides, which room exported for storage in adipose tissue. Glucose that passes through the liver causes an increase in peripheral blood glucose concentration following carbohydrate-rich meals. This glucose is offered in muscle because that synthesis and also storage that glycogen and also in adipose tissue as a resource of glycerol because that triglyceride biosynthesis.

The pathway the glycogenesis native glucose (Fig. 13.3A) involves four steps:

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

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

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

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

When the α1→4 chain above eight residues in length, glycogen branching enzyme, a transglycosylase, transfers some of the α1→4-linked street to one α1→6 branch, setup the phase for ongoing elongation the both α1→4 chains till they, in turn, end up being long sufficient for transfer by branching enzyme.

Glycogen synthaseis the regulatory enzyme because that glycogenesis, quite than UDP-glucose pyrophosphorylase, since UDP-glucose is also used because that synthesis of other sugars, and also as a glycosyl donor for synthesis that glycoproteins, glycolipids and proteoglycans (Chapters 27–29). Pyrophosphate (PPi), the other product that the pyrophosphorylase reaction, is a high energy phosphate anhydride. It is rapidly hydrolyzed to inorganic phosphate by pyrophosphatase, providing the thermodynamic driving pressure for biosynthesis that glycogen.

Pathway the glycogenolysis in liver

Hepatic glycogen phosphorylase offers for rapid release the glucose right into blood throughout the postabsorptive state

As with most metabolic pathways, separate enzymes, occasionally in different subcellular compartments, are required for the forward and also reverse pathways. The pathway the glycogenolysis (Fig. 13.3B) starts with removal of the abundant, outside α1→4-linked glucose residual water in glycogen. This is achieved not by a hydrolase yet byglycogen phosphorylase, an enzyme that provides cytosolic phosphate and also releases glucose native glycogen in the type of Glc-1-P. The Glc-1-P is isomerized by phosphoglucomutase come Glc-6-P, put it at the optimal of the glycolytic pathway; the phosphorylase reaction, in effect, bypasses the necessity for ATP in the hexokinase or glucokinase reactions. In liver, the glucose is exit from Glc-6-P through glucose-6-phosphatase (Glc-6-Pase), and also the glucose exit via the GLUT-2 transporter into blood. The rate-limiting, regulatory step in glycogenolysis is catalytic analysis by phosphorylase, the very first enzyme in the pathway.

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Phosphorylase is certain for α1→4 glycosidic linkages; it can not cleave α1→6 linkages. Further, this large enzyme cannot approach the branching glucose residual water efficiently. Thus, as presented inFigure 13.3B, phosphorylase cleaves the external glucose residues until the branches space three or four residues long, thendebranching enzyme, which has both transglycosylase and glucosidase activity, move a short segment the glucose residues bound come the α1→6 branch to the finish of an adjacent α1→4 chain, leave a single glucose residue in ~ the branch point. This glucose is then removed by the exo-1,6-glucosidase task of debranching enzyme, allowing glycogen phosphorylase to continue with deterioration of the expanded α1→4 chain until an additional branch suggest is approached, setup the phase for a repeat the the transglycosylase and also glucosidase reactions. About 90% the the glucose is released from glycogen together Glc-1-P, and also the remainder, acquired from the α1→6 branching residues, as totally free glucose.