Hexokinase

A hexokinase is an enzyme that phosphorylates hexoses (six-carbon sugars), forming hexose phosphate. In most organisms, glucose is the most important substrate for hexokinases, and glucose-6-phosphate is the most important product. Hexokinase possesses the ability to transfer an inorganic phosphate group from ATP to a substrate.

Hexokinase
Crystal structures of hexokinase 1 from Kluyveromyces lactis.[1]
Identifiers
EC number2.7.1.1
CAS number9001-51-8
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
hexokinase 1
Identifiers
SymbolHK1
NCBI gene3098
HGNC4922
OMIM142600
RefSeqNM_000188
UniProtP19367
Other data
LocusChr. 10 q22
hexokinase 2
Identifiers
SymbolHK2
NCBI gene3099
HGNC4923
OMIM601125
RefSeqNM_000189
UniProtP52789
Other data
LocusChr. 2 p13
hexokinase 3 (white cell)
Identifiers
SymbolHK3
NCBI gene3101
HGNC4925
OMIM142570
RefSeqNM_002115
UniProtP52790
Other data
LocusChr. 5 q35.2
Hexokinase_1
crystal structure of human glucokinase
Identifiers
SymbolHexokinase_1
PfamPF00349
Pfam clanCL0108
InterProIPR022672
PROSITEPDOC00370
SCOP21cza / SCOPe / SUPFAM
Hexokinase_2
rat brain hexokinase type i complex with glucose and inhibitor glucose-6-phosphate
Identifiers
SymbolHexokinase_2
PfamPF03727
Pfam clanCL0108
InterProIPR022673
PROSITEPDOC00370
SCOP21cza / SCOPe / SUPFAM

Hexokinases should not be confused with glucokinase, which is a specific isoform of hexokinase. All hexokinases are capable of phosphorylating several hexoses but glucokinase acts with a 50-fold lower substrate affinity and its main hexose substrate is glucose.

Variation

Genes that encode hexokinase have been discovered in every domain of life, and exist among a variety of species that range from bacteria, yeast, and plants to humans and other vertebrates. They are categorized as actin fold proteins, sharing a common ATP binding site core that is surrounded by more variable sequences which determine substrate affinities and other properties.

Several hexokinase isoforms or isozymes that provide different functions can occur in a single species.

Reaction

The intracellular reactions mediated by hexokinases can be typified as:

Hexose-CH2OH + MgATP2−
 → Hexose-CH2O-PO2−
3 + 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 limits it to a number of intracellular metabolic processes, such as glycolysis or glycogen synthesis. This is because phosphorylated hexoses are charged, and thus more difficult to transport out of a cell.

In patients with essential fructosuria, metabolism of fructose by hexokinase to fructose-6-phosphate is the primary method of metabolizing dietary fructose; this pathway is not significant in normal individuals.

Size of different isoforms

Most bacterial hexokinases are approximately 50 kD in size. Multicellular organisms including plants and animals often have more than one hexokinase isoform. Most are about 100 kD 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 (EC 2.7.1.1) that vary in subcellular locations and kinetics with respect to different substrates and 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 (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/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. It is the hexokinase found in muscle and heart. Hexokinase II is also located at the mitochondria outer membrane so it can have direct access to ATP.[2] The relative specific activity of hexokinase II increases with pH at least in a pH range from 6.9 to 8.5.[3]
  • Hexokinase III/C is substrate-inhibited by glucose at physiologic concentrations. Little is known about the regulatory characteristics of this isoform.

Hexokinase IV ("glucokinase")

Mammalian hexokinase IV, also referred to as glucokinase, differs from other hexokinases in kinetics and functions.

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.

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

Hexokinase IV 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.

In glycolysis

Glucose is unique in that it can be used to produce ATP by all cells in both the presence and absence of molecular oxygen (O2). The first step in glycolysis is the phosphorylation of glucose by hexokinase.

D-Glucose Hexokinase α-D-Glucose-6-phosphate
 
ATP ADP
 
 

Compound C00031 at KEGG Pathway Database. Enzyme 2.7.1.1 at KEGG Pathway Database. Compound C00668 at KEGG Pathway Database. Reaction R01786 at KEGG Pathway Database.

By catalyzing the phosphorylation of glucose to yield glucose 6-phosphate, hexokinases maintain the downhill concentration gradient that favors the facilitated transport of glucose into cells. This reaction also initiates all physiologically relevant pathways of glucose utilization, including glycolysis and the pentose phosphate pathway.[4] The addition of a charged phosphate group at the 6-position of hexoses also ensures 'trapping' of glucose and 2-deoxyhexose glucose analogs (e.g. 2-deoxyglucose, and 2-fluoro-2-deoxyglucose) within cells, as charged hexose phosphates cannot easily cross the cell membrane.

Association with mitochondria

Hexokinases I and II 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 ATP generated by mitochondria, 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[5] for the extremely high glycolytic rates that take place aerobically in tumor cells (the so-called Warburg effect described by Otto Heinrich Warburg in 1930).

Hydropathy plot
Hydropathy plot of hexokinase

The potential transmembrane portions of a protein can be detected by hydropathy analysis. A hydropathy analysis uses an algorithm that quantifies the hydrophobic character at each position along the polypeptide chain. One of the accepted hydropathy scales is that of Kyte and Doolittle which relies on the generation of hydropathy plots. In these plots, the negative numbers represent hydrophilic regions and the positive numbers represent hydrophobic regions on the y-axis. A potential transmembrane domain is about 20 amino acids long on the x-axis.

A hydropathy analysis of hexokinase in yeast has been created by these standards. It appears as if hexokinase possesses a single potential transmembrane domain located around amino acid 400. Therefore, hexokinase is most likely not an integral membrane protein in yeast.[6]

Deficiency

Hexokinase deficiency is a genetic autosomal recessive disease that causes chronic haemolytic anaemia. Chronic haemolytic anaemia is caused by a mutation in the HK gene, which codes for the HK enzyme. The mutation causes a reduction of the HK activity, which causes hexokinase deficiency.[7]

See also

References
  1. PDB: 3O08​; Kuettner EB, Kettner K, Keim A, Svergun DI, Volke D (2010). "Crystal structure of dimeric KlHxk1 in crystal form I". doi:10.2210/pdb3o08/pdb. Cite journal requires |journal= (help)
  2. "Hexokinase data on Uniprot". uniprot.org.
  3. Šimčíková D, Heneberg P (August 2019). "Identification of alkaline pH optimum of human glucokinase because of ATP-mediated bias correction in outcomes of enzyme assays". Scientific Reports. 9 (1): 11422. Bibcode:2019NatSR...911422S. doi:10.1038/s41598-019-47883-1. PMC 6684659. PMID 31388064.
  4. Robey, RB; Hay, N (2006). "Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt". Oncogene. 25 (34): 4683–96. doi:10.1038/sj.onc.1209595. PMID 16892082.
  5. Bustamante E, Pedersen P (1977). "High aerobic glycolysis of rat hepatoma cells in culture: role of mitochondrial hexokinase". Proc Natl Acad Sci USA. 74 (9): 3735–9. Bibcode:1977PNAS...74.3735B. doi:10.1073/pnas.74.9.3735. PMC 431708. PMID 198801.
  6. Bowen, R. A. Molecular Toolkit: Protein Hydrophobicity Plots. Colorado State University, 1998. Web. 15 November 2010. <http://www.vivo.colostate.edu/molkit/index.html Archived 25 June 2010 at the Wayback Machine>
  7. "Hexokinase deficiency". Enerca. Enerca. Retrieved 6 April 2017.
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