Question

1. Glucokinase and hexokinase perform the same enzymatic function yet their role in carbohydrate metabolism / homeostasis is substantially different. a. Describe their cellular distribution / locations, functions, enzyme kenetics, and their regulation of each. b. Describe how each contributes to glucose homeostasis. c. Describe how insulin and glucagon may regulate these two enzymes. 2. The liver is centrally involved in the regulation of blood glucose levels. a. Describe what and how the liver contributes to glucose levels in both fed and fasted states b. Describe the molecular sources of substrates for glucose production. It is important to discuss the cellular source of these molecules as well as the physiological conditions in which they would be produced. c. Explain how and why blood glucose concentrations become dis-regulated during both fed and fasted states with type II diabetes.

Answer 1

Hexokinase is very crucial enzyme in carbohydrate metabolism; it phosphorylates 6-carbon sugar so that they can enter in to metabolic pathways. Glucokinase is most important isoform of hexokinase because it specifically act only glucose sugar, it phosphorylates glucose to produce Glucose 6-phosphate.

Hexokinase has four important isozymes and they differ in subcellular locations, kinetics with respect to different substrates and conditions, and physiological function. They are denoted as hexokinases I, II, III, and IV or hexokinases A, B, C, and D. Hexokinase I, II and III have low Km value i.e very high affinity for glucose molecules.

Hexokinase I is located in all mammalian tissues, and is also called as "housekeeping enzyme," It get less affected by physiological, hormonal, and metabolic changes.

Hexokinase II is the main regulated isoform in many cell types. Its activity found to be increased in cancerous cells. This hexokinase is located in muscle and heart. Hexokinase II is also located in the mitochondria.

Hexokinase III: We have very little information about its regulatory characteristics but we know that this isoform is inhibited by glucose at physiological concentration.

Hexokinase IV is also called as glucokinase and it is found in liver, pancrease, brain cells, hypothalamus and small intestine. Glucokinase plays very significant role in metabolism of carbohydrate.

Hexokinase other than Isozyme IV possess very high affinity for glucose and hence has low Km for glucose on the other hand Glucokinase comes into play only when blood glucose is high and possess very high Km value and low affinity for glucose. Glucokinase is not inhibited by Glucose-6-Phosphate. As stated above glucokinase mostly found in the liver; this is very much important because when sugar concentration in the blood is very high only then glucokinase in the liver should be able to convert it to glucose-6-phosphate and direct it to the path of glycogen synthesis and triglyceride synthesis and it should not do so when blood sugar levels are low.

Phosphorylation of glucose is the first step for both glycogen synthesis in liver and glycosis in liver/muscles. Other hexokinase are mainly involved in maintaining intracellular sugar level and glucokinase maintains over all blood glucose level. Hexokinases phosphorylated glucose which will enter in glycolytic pathway. But in liver, glucokinase will phosphorylates glucose only when it is in excess and allow it to be stored as glycogen or excess of glucose will be converted to triglycerides and export it to adipose tissue for storage. Glucokinase activity in the cytoplasm rises and falls with available glucose.

G6P, the product of glucokinase, is the key substrate of glycogen synthesis. Glucose-6-Phospahet is closely associated with function and regulation of glycogen synthesis. At high sugar levels, when both GK and glycogen synthase are highly active they appears to be located in the same peripheral areas of hepatocyte cytoplasm where glycogen synthesis takes place. The supply of G6P affects the rate of glycogen synthesis not only because it is a primary substrate, but it directly allosterically stimulates glycogen synthase and inhibits glycogen phosphorylase.

Insuline is produce when blood sugar levels are high. In pancreatic B cells, glucokinase acts as a glucose sensor to modify insulin secretion. Glucagon on other hand is produce by alpha cells of pancrease in response to lower blood glucose level. Glucagon mainly opposes the action of insulin. Insulin upregulates the transcription of glucokinase, phosphofructokinase, and pyruvate kinase, while glucagon downregulates their transcription. These effects take place over a period of hours to days, and generally reflect whether a person is well-fed or starving.

When carbohydrates are being digested glucose level in blood is high and it stimulates beta cell to secrete more insulin. Hepatocytes remove glucose from the blood and store it as glycogen. During the fasting period liver manufactures glucose from gluconeogenesis or from glycogen breakdown and exports it into the blood, to maintain adequate blood glucose levels.

Since Glucokinase activity rises rapidly with the glucose concentration, it functions as a central metabolic switch to shift hepatic carbohydrate metabolism between fed and fasting states.

Type II diabetes is caused because of resistance of mucles, fat and liver cells to insulin produce by pancreases. Blood Glucose level is the principal stimulus for insulin secretion. Insulin affect carbohydrate, lipids and amino acid metabolism at cellular levels like transcription and translation of require enzymes in corresponding pathways. Insulin acts at various steps in carbohydrate metabolism. Its effect on facilitated diffusion of glucose into fat and muscle cells via modulation of Glucose transporters GLUT 4. Insulin is the pivotal hormone regulating cellular energy supply and macronutrient balance, directing anabolic processes of the fed state.

Overweight people or type 2 diabetes most typically produces considerably more insulin than non-diabetic people resulting from a higher ratio of body fat to muscle. But body cannot use this insulin effectively; hence pancreas produces more and more insulin. However, the pressure of producing extra insulin exhausts the beta cells and they will start to fail.