The Structure and Mechanism of Hexokinase

A hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose 6-phosphate the most important product. Hexokinases have been found in every organism checked, ranging from bacteria, yeast, and plants, to humans and other vertebrates. 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. Several hexokinase isoforms or isozymes providing different functions can occur in a single species.

-Hexokinase I/A is found in all mammalian tissues, and is considered a "housekeeping enzyme," unaffected by most physiological, hormonal, and metabolic changes.

-Hexokinase II/B constitutes the principal regulated isoform in many cell types and is increased in many cancers.

-Hexokinase III/C is substrate-inhibited by glucose at physiologic concentrations. Little is known about the regulatory characteristics of this isoform.

-Hexokinase IV/D is also known as glucokinase and is described below.

Hexokinase Structure: The tertiary structure of hexokinase includes an . There is a large amount of variation associated with this structure. The ATP-binding domain is composed of five beta sheets and three alpha helices. In this open alph/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 molecular weights of hexokinases are around 100 kD. Each consists of two similar 50kD halves, but only in hexokinase II do both halves have functional active sites.

Mechanism of Hexokinase:

In the first reaction of glycolysis, the gamma-phosphoryl group of an ATP molecule is transferred to the oxygen at the C-6 of glucose. The magnesium ion is required as the reactive form of ATP is the 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 ^[1]. 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.

Active Sites The active site residues for Hexokinase are Asp205, Lys169, Asn204, Glu256,and Thr168. ^[2] . These residues are located in the deep cleft at the interface between the two lobes. This active site is capable of bonding two ligands, glucose, and glucose-6-phosphate. Hexokinase undergoes an induced fit conformational change when glucose binds. This conformational change prevents the hydrolysis of ATP, and is allosterically inhibited by physiological concentrations of glucose-6-phosphate the product. Hexokinase has two conformational states. The open state occurs prior to glucose binding. ATP is bound to the large lobe, but is far away from the glucose binding site, and in a different position than it assumes in the active site. When the glucose binds to Hexokinase a large conformational change occurs. This change closes the two lobes around the glucose substrate. This conformational state is referred to as the closed state. ^[3].

Glucokinase, an Isoenzyme of Hexokinase

Glucokinase (hexokinase D) is a monomeric cytoplasmic enzyme found in the liver and pancreas that serves to regulate glucose levels in these organs. Glucokinase is used in the first step of the metabolism of glucose, during this step the phosphorylation of glucose by ATP generates glucose-6-phosphate and ADP. Glucokinase is a hexokinase isoenzyme. Glucokinase's role in metabolism of glucose can be can be inhibited due to the allosteric properties of glucokinase.It also plays an important role in the regulation of glucose metabolism and because of this regulation glucokinase is a target for drug development in type 2 diabetes. ^[4]

Glucokinase vs. Other Hexokinases: Glucokinase is unique from other hexokinase in kinetic properties and is coded by a different gene. The difference of glucokinase from the other hexokinases is that glucokinase has a lower affinity, thus a higher Km, for glucose. The reduced affinity for glucose allows the activity of glucokinase to differ under physiological conditions according to the amount of glucose present. Essentially, this means that it operates only when serum glucose levels are high. Other tissues need to use glucose at lower serum levels and thus use the higher affinity (lower Km) hexokinase. Glucose-6-phosphate inhibits hexokinase and, if the cell is not using up the G6P that it is making, then it should stop making it, thus making the process a product inhibition. In contrast, G6P does not inhibit glucokinase, and this allows it to remain active in storing as much glucose as possible in the presence of high glucose levels.

Glucokinase Structure: Glucokinase also contains and conformation. Glucokinase consists of one chain or subunit of 448 amino acids forming a monomeric molecule consisting of 13 alpha helices and 5 beta sheets that can phosporylate glucose and other hexoses. The chain is folded into two distinct regions, a small and large domain. Glucokinase has one active binding site for glucose and one for ATP, which is the energy source for phosphorylation. This active binding site is located between the small and large domains. The carboxyl terminus is part of the alpha 13 helix, which codes for the region that forms half of the binding site for glucose. Glucokinase can be modulated to form an inactive and active complex. The inactive conformation forms when the alpha 13 helix has been modulated away from the rest of the molecule forming a large space. This space is too large to bind glucose so it is said to be in the inactive form. The alternative is when the alpha 13 helix is modulated to form a smaller space thus activating the protein^[4].

Glucokinase includes the where glucose forms hydrogen bonds at the bottom of the deep crevice between the large domain and the small domain. E256, E290 (shown in green) of the large domain, T168, K169 (shown in red) of the small domain, and N204, D205 (shown in yellow) of a connecting region form hydrogen bonds with glucose. The shows a different conformation. At the , ATP forms hydrogen bonds with R63 and Y215 (shown in orange) and hydrophobically interacts with M210, Y214 (shown in blue) of the α5 helix and V452, V455 (shown in green) of the α13 helix. The again shows structural differences. The differences in these two conformations allows glucokinase to function properly in different levels of glucose concentration.

Proposed Mechanism for Glucokinase: As described above, glucokinase has a distinct conformation change from the active and inactive form. Experiments have also shown an intermediate open form based on analysis of the movement between the active and inactive form. The switch in conformations between the active form and the intermediate is a kinetically faster step than the change between the intermediate and the inactive form. The inactive form of gluckokinase is the thermodynamically favored unless there is glucose present. Glucokinase does not change conformation until the glucose molecule binds. The conformation change may be triggered by the interaction between Asp 205 and the glucose molecule. Once glucokinase is in the active form, the enzymatic reaction is carried out with the presence of ATP. All in all, glucose binds to glucokinase and then is phosphorylated by ATP to produce glucose-6-phosphate and ADP.^[4]

Regulation of Hepatic Glucokinase by Glucokinase Regulatory Protein: Recent studies have shown glucokinase regulatory protein (GKRP)regulate the activity of hepatic glucokinase. The experiments suggest that glucokinase is found in hepatocyte nuclei and are found inactive at low plasma glucose levels, but found active when higher glucose levels are present. GKRP would then would likely be an allosteric inhibitor of glucokinase that specifically binds to the inactive form of glucokinase.The crystal structures of the glucokinase-GKRP complex are being determined to clearly identify the interactions between glucokinase and glucokinase regulatory protein.^[4]

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^[4]. 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^[5]. 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