Galactokinase is an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

(phosphotransferase) that facilitates the

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...

of α-D-galactose to

galactose 1-phosphate D-Galactose-1-phosphate is an intermediate in the intraconversion of glucose and uridine diphosphate galactose. It is formed from galactose by galactokinase.The improper metabolism of galactose-1-phosphate is a characteristic of galactosemia. The ...

at the expense of one

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

of ATP. Galactokinase catalyzes the second step of the

Leloir pathway The Leloir pathway is a metabolic pathway for the catabolism of D-galactose. It is named after Luis Federico Leloir, who first described it. In the first step, galactose mutarotase facilitates the conversion of β-D-galactose to α-D-galactos ...

, a

metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...

found in most

organisms In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fungi; ...

for the

catabolism Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...

of α-D-galactose to

glucose 1-phosphate Glucose 1-phosphate (also called cori ester) is a glucose molecule with a phosphate group on the 1'-carbon. It can exist in either the α- or β- anomeric form. Reactions of α-glucose 1-phosphate Catabolic In glycogenolysis, it is the direct pr ...

. First isolated from

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

ian

liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...

, galactokinase has been studied extensively in

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebac ...

plants Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...

, and

humans Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

Structure

Galactokinase is composed of two domains separated by a large cleft. The two regions are known as the N- and C-terminal domains, and 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 derivati ...

ring of ATP binds in a hydrophobic pocket located at their interface. The N-terminal domain is marked by five strands of mixed

beta-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a g ...

and five

alpha-helices 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 ear ...

, and the C-terminal domain is characterized by two layers of anti-parallel beta-sheets and six alpha-helices. Galactokinase does not belong to the

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

kinase In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...

family, but rather to a class of ATP-dependent enzymes known as the GHMP superfamily. GHMP is an abbreviation referring to its original members: galactokinase,

homoserine kinase In enzymology, a homoserine kinase () is an enzyme that catalyzes the chemical reaction :ATP + L-homoserine \rightleftharpoons ADP + O-phospho-L-homoserine Thus, the two substrates of this enzyme are ATP and L-homoserine, whereas its two pro ...

mevalonate kinase Mevalonate kinase is an enzyme (specifically a kinase) that in humans is encoded by the ''MVK'' gene. Mevalonate kinases are found in a wide variety of organisms from bacteria to mammals. This enzyme catalyzes the following reaction: . Funct ...

, and phosphomevalonate kinase. Members of the GHMP superfamily have great three-dimensional similarity despite only ten to 20% sequence identity. These enzymes contain three well-conserved motifs (I, II, and III), the second of which is involved in nucleotide binding and has the sequence

Pro Pro is an abbreviation meaning "professional". Pro, PRO or variants thereof may also refer to: People * Miguel Pro (1891–1927), Mexican priest * Pro Hart (1928–2006), Australian painter * Mlungisi Mdluli (born 1980), South African retired f ...

-X-X-X-

Gly Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...

- Leu-X-

Ser Ser or SER may refer to: Places * Ser, a village in Bogdand Commune, Satu Mare County, Romania * Serpens (Ser), an astronomical constellation of the northern hemisphere * Serres, known as Ser in Serbian, a city in Macedonia, Greece Organization ...

-Ser-

Ala Ala, ALA, Alaa or Alae may refer to: Places * Ala, Hiiu County, Estonia, a village * Ala, Valga County, Estonia, a village * Ala, Alappuzha, Kerala, India, a village * Ala, Iran, a village in Semnan Province * Ala, Gotland, Sweden * Alad, S ...

Sugar specificity

Galactokinases across different species display a great diversity of substrate specificities. ''

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

'' galactokinase can also phosphorylate 2-deoxy-D-galactose, 2-amino-deoxy-D-galactose, 3-deoxy-D-galactose and D-fucose. The enzyme cannot tolerate any C-4 modifications, but changes at the C-2 position of D-galactose do not interfere with enzyme function. Both human and rat galactokinases are also able to successfully phosphorylate 2-deoxy-D-galactose. Galactokinase from ''

S. cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have bee ...

'', on the other hand, is highly specific for D-galactose and cannot phosphorylate

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

mannose Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...

arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally ...

, fucose,

lactose Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from ' (gen. '), the Latin word for milk, plus the suffix '' - ...

galactitol Galactitol (dulcitol) is a sugar alcohol, the reduction product of galactose. It has a slightly sweet taste. In people with galactokinase deficiency, a form of galactosemia, excess dulcitol forms in the lens of the eye leading to cataracts. Gala ...

, or 2-deoxy-D-galactose. Moreover, the kinetic properties of galactokinase also differ across species. The sugar specificity of galactokinases from different sources has been dramatically expanded through

directed evolution Directed evolution (DE) is a method used in protein engineering that mimics the process of natural selection to steer proteins or nucleic acids toward a user-defined goal. It consists of subjecting a gene to iterative rounds of mutagenesis (cre ...

and structure-based

protein engineering Protein engineering is the process of developing useful or valuable proteins. It is a young discipline, with much research taking place into the understanding of protein folding and recognition for protein design principles. It has been used to imp ...

. The corresponding broadly permissive sugar anomeric kinases serve as a cornerstone for ''in vitro'' and ''in vivo''

glycorandomization Glycorandomization, is a drug discovery and drug development technology platform to enable the rapid diversification of bioactive small molecules, drug leads and/or approved drugs through the attachment of sugars. Initially developed as a facile me ...

Mechanism

Recently, the roles of

active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...

residues in human galactokinase have become understood. Asp-186 abstracts a

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

from C1-OH of α-D-galactose, and the resulting

alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

attacks the γ-

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

of ATP. A

phosphate group In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosphor ...

is transferred to the sugar, and Asp-186 may be

deprotonated Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju.edu ...

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

. Nearby

Arg Arg or ARG may refer to: Places *''Arg'' () means "citadel" in Persian, and may refer to: **Arg, Iran, a village in Fars Province, Iran **Arg (Kabul), presidential palace in Kabul, Afghanistan **Arg, South Khorasan, a village in South Khorasan P ...

-37 stabilizes Asp-186 in its

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ic form and has also been proven to be essential to galactokinase function in

point mutation A point mutation is a genetic mutation where a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome. Point mutations have a variety of effects on the downstream protein product—consequences ...

experiments. Both the aspartic acid and arginine active site residues are highly conserved among galactokinases. Galactokinase crystal structure

Biological function

The Leloir pathway catalyzes the conversion of galactose to glucose. Galactose is found in

dairy products Dairy products or milk products, also known as lacticinia, are Food product, food products made from (or containing) milk. The most common dairy animals are cow, water buffalo, dairy goat, nanny goat, and Sheep, ewe. Dairy products include commo ...

, as well as in

fruits In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particula ...

and

vegetables Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still commonly used and is applied to plants collectively to refer to all edible plant matter, including the flowers, fruits, stems, ...

, and can be produced endogenously in the breakdown of

glycoproteins Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycos ...

and

glycolipids Glycolipids are lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connec ...

. Three enzymes are required in the Leloir pathway: galactokinase,

galactose-1-phosphate uridylyltransferase Galactose-1-phosphate uridylyltransferase (or GALT, G1PUT) is an enzyme () responsible for converting ingested galactose to glucose. Galactose-1-phosphate uridylyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose ...

, and UDP-galactose 4-epimerase. Galactokinase catalyzes the first committed step of galactose catabolism, forming galactose 1-phosphate.

Disease relevance

Galactosemia Galactosemia (British galactosaemia, from Greek γαλακτόζη + αίμα, meaning galactose + blood, accumulation of galactose in blood) is a rare genetic metabolic disorder that affects an individual's ability to metabolize the sugar galacto ...

, a rare

metabolic disorder A metabolic disorder is a disorder that negatively alters the body's processing and distribution of macronutrients, such as proteins, fats, and carbohydrates. Metabolic disorders can happen when abnormal chemical reactions in the body alter the ...

characterized by decreased ability to metabolize galactose, can be caused by a mutation in any of the three enzymes in the Leloir pathway.

Galactokinase deficiency Galactokinase deficiency is an autosomal recessive metabolic disorder marked by an accumulation of galactose and galactitol secondary to the decreased conversion of galactose to galactose-1-phosphate by galactokinase. The disorder is caused by mu ...

, also known as galactosemia type II, is a

recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

metabolic disorder caused by a

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

in human galactokinase. About 20 mutations have been identified that cause galactosemia type II, the main

symptom Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign for example may be a higher or lower temperature than normal, raised or lowered blood pressure or an abnormality showin ...

of which is early onset

cataracts A cataract is a cloudy area in the lens of the eye that leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble w ...

. In

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

of the human eye,

aldose reductase In enzymology, aldose reductase (or aldehyde reductase) () is a cytosolic NADPH-dependent oxidoreductase that catalyzes the reduction of a variety of aldehydes and carbonyls, including monosaccharides. It is primarily known for catalyzing the re ...

converts galactose to galactitol. As galactose is not being catabolized to glucose due to a galactokinase mutation, galactitol accumulates. This galactitol gradient across the lens cell membrane triggers the

osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...

uptake of water, and the swelling and eventual

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

of lens cells ensues.

References

External links

* {{Portal bar, Biology, border=no EC 2.7.1 Moonlighting proteins