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


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We candlestick now think about the biochemical responses come a series of physiological conditions. Our an initial example is the starved-fed cycle, which we all suffer in the hours after an night meal and through the night"s fast. This nightly starved-fed cycle has three stages: the postabsorptive state after a meal, the early on fasting during the night, and the refed state ~ breakfast. A major goal the the plenty of biochemical alterations in this period is to maintain glucose homeostasis—that is, a constant blood-glucose level.

1.

The well-fed, or postabsorptive, state. After we consume and also digest an night meal, glucose and amino acids space transported native the intestine come the blood. The dietary lipids room packaged into chylomicrons and also transported to the blood by the lymphatic system. This fed problem leads come the cheap of insulin, which is among the two most crucial regulators the fuel metabolism, the various other regulator being glucagon. The secretion of the hormone insulin by the β cell of the kidneys is engendered by glucose and also the parasympathetic nervous system (Figure 30.15). In essence, insulin signal the fed state—it stimulates the storage of fuels and the synthetic of protein in a selection of ways. Because that instance, insulin initiates protein kinase cascades—it stimulates glycogen synthesis in both muscle and also the liver and suppresses gluconeogenesis by the liver. Insulin also accelerates glycolysis in the liver, which in turn increases the synthesis of fatty acids.

The liver help to limit the amount of glucose in the blood during times of lot of by storing it together glycogen so as to be may be to relax glucose in times of scarcity. How is the overfill blood glucose existing after a enjoy the meal removed? Insulin accelerates the uptake of blood glucose into the liver by GLUT2. The level of glucose 6-phosphate in the liver rises since only then perform the catalytic website of glucokinase end up being filled with glucose. Recall the glucokinase is active only when blood-glucose levels room high. Consequently, the liver forms glucose 6-phosphate more rapidly together the blood-glucose level rises. The boost in glucose 6-phosphate coupled with insulin action leads to a buildup of glycogen stores. The hormonal results on glycogen synthesis and storage space reinforced through a direct action of glucose itself. Phosphorylase a is a glucose sensor in addition to being the enzyme the cleaves glycogen. When the glucose level is high, the binding of glucose come phosphorylase a provides the enzyme prone to the action of a phosphatase the converts it into phosphorylase b, which go not readily degrade glycogen. Thus, glucose allosterically shifts the glycogen mechanism from a degradative come a man-made mode.

The high insulin level in the fed state likewise promotes the entry of glucose into muscle and also adipose tissue. Insulin stimulates the synthesis of glycogen by muscle and also by the liver. The entry of glucose into adipose tissue offers glycerol 3-phosphate because that the synthesis of triacylglycerols. The activity of insulin likewise extends to amino acid and protein metabolism. Insulin promotes the uptake of branched-chain amino acids (valine, leucine, and isoleucine) by muscle. Indeed, insulin has a basic stimulating effect on protein synthesis, i m sorry favors a structure up that muscle protein. In addition, the inhibits the intracellular deterioration of proteins.

2.

The early on fasting state. The blood-glucose level begins to autumn several hrs after a meal, causing a decrease in insulin secretion and a increase in glucagon secretion; glucagon is secreted through the α cell of the kidneys in response to a short blood-sugar level in the fasting state. Just as insulin signal the fed state, glucagon signals the starved state. It offer to mobilize glycogen stores as soon as there is no diet intake of glucose. The key target body organ of glucagon is the liver. Glucagon stimulates glycogen breakdown and inhibits glycogen synthetic by triggering the cyclic AMP cascade bring about the phosphorylation and activation the phosphorylase and also the inhibition of glycogen synthase (Section 21.5). Glucagon also inhibits fatty acid synthesis by diminishing the production of pyruvate and by lowering the task of acetyl CoA carboxylase by maintaining it in one unphosphorylated state. In addition, glucagon stimulates gluconeogenesis in the liver and also blocks glycolysis by lowering the level of F-2,6-BP.

All known actions the glucagon space mediated by protein kinases that space activated by cyclic AMP. The activation of the cyclic AMP cascade outcomes in a greater level of phosphorylase a activity and a reduced level of glycogen synthase a activity. Glucagon"s effect on this cascade is reinforced by the lessened binding of glucose to phosphorylase a, which makes the enzyme much less susceptible come the hydrolytic activity of the phosphatase. Instead, the phosphatase continues to be bound to phosphorylase a, and so the synthase remains in the in-active phosphorylated form. Consequently, there is a quick mobilization of glycogen.

The huge amount the glucose developed by the hydrolysis of glucose 6-phosphate obtained from glycogen is climate released indigenous the liver into the blood. The entrance of glucose into muscle and adipose organization decreases in response to a short insulin level. The lessened utilization the glucose by muscle and adipose tissue likewise contributes to the maintenance of the bloodglucose level. The net result of these actions that glucagon is to markedly boost the release of glucose through the liver.

Both muscle and liver use fatty acids together fuel once the blood-glucose level drops. Thus, the blood-glucose level is preserved at or over 80 mg/dl by three significant factors: (1) the mobilization that glycogen and the release of glucose through the liver, (2) the release of fatty acids by adipose tissue, and (3) the shift in the fuel offered from glucose to fatty acids by muscle and also the liver.

What is the an outcome of depletion of the liver"s glycogen stores? Gluconeogenesis from lactate and also alanine continues, yet this procedure merely replace instead instead glucose the had currently been converted right into lactate and also alanine through the peripheral tissues. Moreover, the brain oxidizes glucose totally to CO2 and H2O. Thus, for the net synthesis of glucose to occur, another resource of carbons is required. Glycerol released from adipose organization on lipolysis gives some that the carbons, through the continuing to be carbons coming from the hydrolysis the muscle proteins.

3.

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The refed state. What space the biochemical responses come a hearty breakfast? Fat is processed specifically as the is handle in the common fed state. However, this is not the instance for glucose. The liver does not initially absorb glucose indigenous the blood, however rather pipeline it because that the peripheral tissues. Moreover, the liver remains in a gluconeogenic mode. Now, however, the newly synthesized glucose is provided to replenish the liver"s glycogen stores. Together the blood-glucose levels proceed to rise, the liver completes the replenishment that its glycogen stores and begins to procedure the remaining excess glucose for fatty mountain synthesis.