Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically
transmembrane
A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...
, proteins of the
transferase
A transferase is any one of a class of enzymes that catalyse the transfer of specific functional groups (e.g. a methyl or glycosyl group) from one molecule (called the donor) to another (called the acceptor). They are involved in hundreds of ...
class of
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s that play a role in
signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular ...
across the cellular membrane. The vast majority of HKs are homodimers that exhibit
autokinase, phosphotransfer, and phosphatase activity. HKs can act as
cellular receptors
In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a recepto ...
for signaling molecules in a way analogous to
tyrosine kinase receptors
Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase ...
(RTK). Multifunctional receptor molecules such as HKs and RTKs typically have portions on the outside of the cell (
extracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
domain) that bind to hormone- or growth factor-like molecules, portions that span the cell membrane (
transmembrane domain
A transmembrane domain (TMD) is a membrane-spanning protein domain. TMDs generally adopt an alpha helix topological conformation, although some TMDs such as those in porins can adopt a different conformation. Because the interior of the lipid b ...
), and portions within the cell (
intracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
domain) that contain the enzymatic activity. In addition to
kinase
In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...
activity, the intracellular domains typically have regions that bind to a secondary effector molecule or complex of molecules that further propagate signal transduction within the cell. Distinct from other classes of
protein kinases
A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fun ...
, HKs are usually parts of a
two-component signal transduction mechanisms in which HK transfers a phosphate group from
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
to a histidine residue within the kinase, and then to an aspartate residue on the receiver domain of a
response regulator protein (or sometimes on the kinase itself). More recently, the widespread existence of protein histidine phosphorylation distinct from that of two-component histidine kinases has been recognised in human cells. In marked contrast to Ser, Thr and Tyr phosphorylation, the analysis of phosphorylated Histidine using standard biochemical and mass spectrometric approaches is much more challenging, and special procedures and separation techniques are required for their preservation alongside classical Ser, Thr and Tyr phosphorylation on proteins isolated from human cells.
In terms of
enzymology, a histidine kinase (, ''EnvZ'', ''histidine protein kinase'', ''protein histidine kinase'', ''protein kinase (histidine)'', ''
HK1
Hexokinase-1 (HK1) is an enzyme that in humans is encoded by the ''HK1'' gene on chromosome 10. Hexokinases phosphorylate glucose to produce glucose-6-phosphate (G6P), the first step in most glucose metabolism pathways. This gene encodes a ubiqu ...
'', ''HP165'', ''Sln1p'') is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
that
catalyzes
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
the
chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and break ...
:ATP + protein L-histidine
ADP + protein N-phospho-L-histidine.
Thus, the two
substrates of this enzyme are
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
and protein L-
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the ...
, whereas its two
products are
ADP
Adp or ADP may refer to:
Aviation
* Aéroports de Paris, airport authority for the Parisian region in France
* Aeropuertos del Perú, airport operator for airports in northern Peru
* SLAF Anuradhapura, an airport in Sri Lanka
* Ampara Air ...
and protein N-phospho-L-histidine.
This type of enzyme is involved in signal transduction pathways upstream of many cellular processes including various metabolic, virulence, and homeostatic pathways.
Mechanism

The mechanism for the reactions catalyzed by histidine kinase have not been completely elucidated, but current evidence suggests that the catalytic domain of one
dimeric unit may rotate in such a way that the ATP binding pocket of that unit can come into contact with a particular histidine residue on the opposite unit and a nucleophilic addition results in a phosphorylated histidine.
Structure and function
An HK is composed of several
domains starting with a short
N-terminal
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the amin ...
cytoplasmic portion connected to an extracellular sensing domain via a transmembrane
α helix
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues earli ...
. A second transmembrane α helix connects the extracellular domain to the
C-terminal
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
cytoplasmic catalytic domain. HKs are known to serve roles in many different signal transduction pathways, so it is not surprising that the extracellular sensing domain is not very well conserved in the HK family. In contrast, the cytoplasmic domain tends to have high sequence
homology and contains several well-known
motifs. These motifs include the H, N, G1, F, and G2 boxes.
The autophosphorylation H-box is contained in the N-terminal dimerization and histidine phosphotransfer (DHp) domain. In HK853-CD, crystallized from ''
Thermotoga maritima'', this domain is a helical-
hairpin
A hairpin or hair pin is a long device used to hold a person's hair in place. It may be used simply to secure long hair out of the way for convenience or as part of an elaborate hairstyle or coiffure. The earliest evidence for dressing the hai ...
and is formed by residues 232-317. The histidine phosphorylation site is located at His-260. The N, G1, F and G2 boxes are contained in the C-terminal catalytic and ATP-binding (CA) domain. This domain is formed by residues 323-489 and forms a structure known as an α/β sandwich fold. This particular fold has one layer composed of a 5-stranded
β sheet and the other layer is made of three α helices.
The dimeric unit is held together by a four-helix bundle, formed when the C-terminal segments of the α1 helices on each subunit interact in an
antiparallel manner with both α2 helices. The stability of the dimer is aided by several interactions at the interface between the DHps of each monomer. These include hydrophobic interactions between conserved
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, ...
residues as well as two
hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...
s (Thr-252
...Glu-316’ and Arg-263
...Asn-307’) and one
salt bridge (Lys-270
...Glu-303’). Further interactions are mediated via hydrogen bonds to water within a cavity inside the coiled coil and flanked by hydrophobic residues.
The
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecul ...
/
ATP
ATP may refer to:
Companies and organizations
* Association of Tennis Professionals, men's professional tennis governing body
* American Technical Publishers, employee-owned publishing company
* ', a Danish pension
* Armenia Tree Project, non ...
binding pocket is contained within the CA domain and the structural similarity of this pocket is high between most HKs. The cavity of CheA, also crystallized from T. maritima, is first formed by β sheet P4 in the rear and the sides of the cavity are formed by the 4 motifs mentioned earlier, the N, G1, F, and G2 boxes.
The majority of the residues coming from the β sheet are hydrophobic with Asp449 being the exception. This residue is invariant and forms a hydrogen bond along with a water molecule to the
adenine
Adenine () (symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivativ ...
amine group. Three other water molecules form direct hydrogen bonds with the adenine base. A Mg
2+ ion forms a bridge between all three phosphates and an invariant Asn residue. Finally, two more water molecules complete octahedral coordination with Mg
2+ and are linked to Arg-408 and His-405. When the γ phosphate of ATP is destabilized, the Mg
2+ is no longer observed due to its inability to octahedrally coordinate. Marina et al. argue that similar coordination of Mg
2+ occurs in HK853 but that it is unobserved due to the usage of the ATP
analog
Analog or analogue may refer to:
Computing and electronics
* Analog signal, in which information is encoded in a continuous variable
** Analog device, an apparatus that operates on analog signals
*** Analog electronics, circuits which use analo ...
AMPPNP in the crystal structure.
During crystallization, the analog was hydrolyzed into a product similar to ADP.
The final side of the ATP binding pocket is conveniently named the “ATP lid.” The stability of this structure is mediated by the presence of the γ phosphate and thus the Mg
2+ ion in the binding site. Also the presence of the nucleotide base has proved to play a significant role in stabilization of the lid in a closed
conformation. The ATP lid is connected via hydrophobic residues to the rest of the protein. The γ phosphate of ATP is somewhat exposed allowing for
dephosphorylation
In biochemistry, dephosphorylation is the removal of a phosphate (PO43−) group from an organic compound by hydrolysis. It is a reversible post-translational modification. Dephosphorylation and its counterpart, phosphorylation, activate and deact ...
.
Upon ATP binding in this pocket, it is believed that a conformational change occurs allowing the rotation of the CA domain to come into contact with the DHp of the other monomer and thus allowing the conserved His-260 to rest near the γ phosphate. The Nε of His-260 then attacks the γ phosphate of ATP in a
nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions ...
and bumps off
ADP
Adp or ADP may refer to:
Aviation
* Aéroports de Paris, airport authority for the Parisian region in France
* Aeropuertos del Perú, airport operator for airports in northern Peru
* SLAF Anuradhapura, an airport in Sri Lanka
* Ampara Air ...
as its leaving group.
Role in fungal infections
A
two-component system
In the field of molecular biology, a two-component regulatory system serves as a basic stimulus-response coupling mechanism to allow organisms to sense and respond to changes in many different environmental conditions. Two-component systems t ...
(TCSs), involving histidine kinase and a variable
response regulator protein, may be critical to the virulence of some fungal strains such as ''
Candida albicans
''Candida albicans'' is an opportunistic pathogenic yeast that is a common member of the human gut flora. It can also survive outside the human body. It is detected in the gastrointestinal tract and mouth in 40–60% of healthy adults. It is usua ...
'', which is often responsible for causing
candidiasis
Candidiasis is a fungal infection due to any type of '' Candida'' (a type of yeast). When it affects the mouth, in some countries it is commonly called thrush. Signs and symptoms include white patches on the tongue or other areas of the mouth ...
in
immunocompromised
Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...
persons.
''C. albicans'' with a deletion of CHK1, the two-component histidine kinase gene, show defects in
morphogenesis
Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...
and a drastic decrease in the cell’s ability to resist elimination by human
neutrophils
Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying i ...
. As humans lack this two-component system, it may be a good target for
anti-microbial agents in order to treat
candidiasis
Candidiasis is a fungal infection due to any type of '' Candida'' (a type of yeast). When it affects the mouth, in some countries it is commonly called thrush. Signs and symptoms include white patches on the tongue or other areas of the mouth ...
.
Role in bacteria infections
Similar to fungus, Two component systems can also be found in several persistent bacteria infections. For example, ''Staphylococcus aureus'' was reported to use SrrAB TCSs consisting of a sensor HKs (SrrB), which would transfer phosphate group to an effector response regulator (SrrA), leading to the modification of SrrA activity including gene regulation. This TCSs has been used by ''S. aureus'' in order to sense changes of environmental condition and transmit the signal to an appropriate responding system, for example, ''ica'' genes is induced by SrrAB to mediate cell assembly and biofilm formation to survive under anaerobic condition.
References
Further reading
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{{DEFAULTSORT:Histidine Kinase
EC 2.7.13
Enzymes of known structure