The Structure and Mechanism of Hexokinase

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The Structure and Mechanism of Glucokinase (Hexokinase D)

Glucokinase (hexokinase D) is a monomeric cytoplasmic enzyme found in the liver and pancreas but can also be found in the gut and brain. It serves to regulate glucose levels in these organs. Glucokinase uses phosphorylation to increase the metabolism of glucose. Glucokinase is a hexokinase isoenzyme All hexokinases are capable of prompting the first step of glycogen synthesis and glycolysis, the phosphorylation of glucose to glucose-6-phosphate (G6P). Glucokinase is unique from other hexokinase in kinetic properties and is coded by a different gene. The reduced affinity for glucose allows the activity of glucokinase to differ under physiological conditions according to the amount of glucose present.

Role in Organ Systems: In the liver glucokinase increases the synthesis of glycogen and is the first step in glycolysis, the main producer of ATP in the body. Glucokinase is responsible for phospohorylating the majority of glucose in the liver and pancreas. Glucokinase only binds to and phosphorylates glucose when levels are higher than normal blood glucose level, allowing it to maintain constant glucose levels^[1]. By phosphorylating glucose, glucokinase creates glucose 6-phosphate. Glucose 6-phosphate can then be used by the liver through the glycolytic pathway. Along with this process in the liver, glucokinase also facilitates glycogen synthesis. Through this the majority of the body's glucose is stored. Glucose 6-phosphate is also one of the starting materials of the TCA cycle which is responsible for the majority of ATP production in the body. In the pancreas, a rise in glucose levels increases the activity of glucokinase causing an increase in glucose 6-phosphate. This causes the triggering of the beta cells to secret insulin^[2]. Glucokinase is the first step in this reaction. Insulin then allows other cells in the body to take up glucose, actively lowering the glucose level.

General Hexokinase Structure: The tertiary structure of hexokinase includes an open alpha/beta sheet. There is a large amount of variation associated with this structure. It is composed of five beta sheets and three alpha helices. In this open alpha/beta sheet four of the beta sheets are parallel and one is in the anitparallel directions. The alpha helices and beta loops connect the beta sheets to produce this open alpha/beta sheet. The crevice indicates the ATP-binding domain of this glycolytic enzyme. The hexokinase molecule has two distinct conformations, and , and the conformational fluctuation between the two states involves relative motion of the two domains or two halves of the protein. In the open conformation, the molecule has a low affinity for both the glucose molecule and the ATP molecule. The binding of one of the molecules, say glucose, shifts the equilibrium to the closed conformation of the protein, which has a higher affinity for ATP because now the ATP binding site has the correct conformation to accommodate ATP. By the same reasoning, if ATP were to bind first, that would also shift the equilibrium to the closed conformation and hence increase the affinity for glucose. Therefore the binding of glucose and ATP are coupled and this kind of conformational coupling makes hexokinase an allosteric protein. They are categorized as actin fold proteins, sharing a common ATP binding site core surrounded by more variable sequences that determine substrate affinities and other properties.

Glucokinase vs. Other Hexokinases: The difference of glucokinase from the other hexokinases is that glucokinase has a lower affinity, thus a higher Km, for glucose. Essentially, this means that it operates only when serum glucose levels are high. High glucose is the signal to store glucose. Other tissues need to use glucose at lower serum levels and thus use the higher affinity (lower Km) hexokinase. Also, G6P inhibits hexokinase. This is simple "product inhibition". If the cell is not using up the G6P that it is making, then it should stop making it. G6P does not inhibit glucokinase. This allows it to remain active in storing as much glucose as possible in the presence of high glucose levels.

Mechanism of Glucokinase: In the first reaction of glycolysis, the gama-phosphoryl group of an ATP molecule is transferred to the oxygen at the C-6 of glucose (magnesium ion is required as the reactive form of ATP is the chelated complex with magnesium (II) ion). This step is a direct nucleophilic attack of the hydroxyl group on the terminal phosphoryl group of the ATP molecule. This produces glucose-6-phosphate and ADP. Hexokinase is the enzyme that catalyzes this phosphoryl-group-transfer. Hexokinase undergoes and induced-fit conformational change when it binds to glucose, which ultimately prevents the hydrolysis of ATP. It is also allosterically inhibited by physiological concentrations of its immediate product, glucose-6-phosphate. This is a mechanism by which the influx of substrate into the glycolytic pathway is controlled.

1. ↑ Kamata K, Mitsuya M, Nishimura T, Eiki J, Nagata Y. Structural basis for allosteric regulation of the monomeric allosteric enzyme human glucokinase. Structure. 2004 Mar;12(3):429-38. PMID:15016359 doi:10.1016/j.str.2004.02.005
2. ↑ Postic C, Shiota M, Magnuson MA. Cell-specific roles of glucokinase in glucose homeostasis. Recent Prog Horm Res. 2001;56:195-217. PMID:11237213

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