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.

Variation across species
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.

Reaction
The intracellular reactions mediated by hexokinases can be typified as:
 * Hexose-CH2OH + MgATP= → Hexose-CH2O-PO3= + MgADP- + H+

where Hexose-CH2OH represents any of several hexoses (like glucose) that contain an accessible -CH2OH moiety.

Consequences of hexose phosphorylation
Phosphorylation of a hexose (such as glucose) often commits it to a limited number of intracellular metabolic processes (such as glycolysis or glycogen synthesis). This is aided by the fact that phosphorylation also makes it unable to move or be transported out of the cell.

Size of different isoforms
Most bacterial hexokinases are approximately 50kD in size. Multicellular organisms such as plants and animals often have more than one hexokinase isoform. Most are about 100kD in size, and consist of two halves (N and C terminal), which share much sequence homology. This suggests an evolutionary origin by duplication and fusion of a 50kD ancestral hexokinase similar to those of bacteria.

Types of mammalian hexokinase
There are four important mammalian hexokinase isozymes that vary somewhat in their subcellular locations, kinetic characteristics with respect to different substrates and operating conditions, and physiological function. They are designated hexokinases I, II, III, and IV or hexokinases A, B, C, and D.

Hexokinases I, II, and III
Hexokinases I, II, and III are referred to as "low-Km" isozymes because of a high affinity for glucose even at low concentrations (below 1 mM). Hexokinases I and II follow Michaelis-Menten kinetics at physiologic concentrations of substrates. All three are strongly inhibited by their product, glucose-6-phosphate. Molecular weights are around 100 kD. Each consists of two similar 50kD halves, but only in hexokinase II do both halves have functional active sites.


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


 * Hexokinase III (or C) is inhibited by excessive glucose (substrate inhibition).

Hexokinase IV ("glucokinase")
Mammalian hexokinase IV, also referred to as glucokinase, has unique characteristics and functions compared to other hexokinases.


 * The location of the phosphorylation on a subcellular level occurs when glucokinase translocates between the cytoplasm and nucleus of liver cells. Glucokinase can only phosphorylate glucose if the concentration of this substrate is high enough; its Km for glucose is 100 times higher than that of hexokinases I, II, and III.


 * It is monomeric, about 50kD, displays positive cooperativity with glucose, and is not allosterically inhibited by its product, glucose-6-phosphate.

It is present in the liver, pancreas, hypothalamus, small intestine, and perhaps certain other neuroendocrine cells, and plays an important regulatory role in carbohydrate metabolism.


 * In the beta cells of the pancreatic islets, it serves as a glucose sensor to control insulin release, and similarly controls glucagon release in the alpha cells.


 * In hepatocytes of the liver, glucokinase responds to changes of ambient glucose levels by increasing or reducing glycogen synthesis.

Hexokinase in glycolysis
The use of glucose as an energy source in cells is via the metabolic pathway known as glycolysis. The first step of this sequence of reactions is the phosphorylation of glucose by hexokinase to prepare it for later breakdown in order to provide energy.

The major reason for the immediate phosphorylation of glucose by a hexokinase is to prevent diffusion out of the cell. The phosphorylation adds a charged phosphate group so the glucose 6-phosphate cannot easily cross the cell membrane.

Association to mitochondria
Hexokinases I, II, and III can associate physically to the outer surface of the external membrane of mitochondria through specific binding to a porin (or Voltage Dependent Anion Channel). This association confers hexokinase direct access to mitochondrially-generated ATP, which is one of the two substrates of hexokinase. Mitochondrial hexokinase is highly elevated in rapidly-growing malignant tumor cells, with levels up to 200 times higher than normal tissues. Mitochondrially-bound hexokinase has been demonstrated to be the driving force for the extremely high glycolytic rates that take place aerobically in tumor cells (the so-called Warburg effect described by Otto Warburg in 1930).