Transferase

RNA Polymerase II from S. cerevisiae. Despite the use of the term "polymerase," RNA Polymerases are classified as a form of nucleotidyl transferase.

In biochemistry, transferase is the general name for the class of enzymes that enact 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 different biochemical pathways throughout biology, and are integral to some of life’s most important processes.

Transferases are involved in a myriad of reactions in the cell. Some examples of these reactions include the activity of CoA transferase, which transfers thiol esters, the action of N-acetyltransferase, which is part of the pathway that metabolizes tryptophan, and the regulation of PDH. The regulation of PDH involves both phosphatase, which removes phosphates, and kinase, which adds phosphates. Transferases are also utilized during translation. In this case, an amino acid chain is the functional group transferred by a Peptidyl transferase. The transfer involves the removal of the growing amino acid chain from the tRNA molecule in the A-site of the ribosome and its subsequent addition to the amino acid attached to the tRNA in the P-site.

Mechanistically, an enzyme that catalyzed the following reaction would be a transferase: X–Group + Y → X + Y–Group or,

$DonorX+AcceptorY{\xrightarrow[{transferase}]{}}AcceptorX+DonorY$

In the above reaction, X would be the donor, and Y would be the acceptor. "Group" would be the functional group transferred as a result of transferase activity. The donor is often a coenzyme.

History

Some of the most important discoveries relating to transferases occurred as early as the 1930s. Earliest discoveries of transferase activity occurred in other classifications of enzymes, including Beta-galactosidase, protease, and acid/base phosphatase. Prior to the realization that individual enzymes were capable of such a task, it was believed that two or more enzymes enacted functional group transfers.

Biodegradation of dopamine via catechol-O-methyltransferase (along with other enzymes). The mechanism for dopamine degradation led to the Nobel Prize in Physiology or Medicine in 1970.

Transamination, or the transfer of an amine (or NH₂) group from an amino acid to a keto acid by an aminotransferase (also known as a "transaminase"), was first noted in 1930 by D. M. Needham, after observing the disappearance of glutamic acid added to pigeon breast muscle. This observance was later verified by the discovery of its reaction mechanism by Braunstein and Kritzmann in 1937. Their analysis showed that this reversible reaction could be applied to other tissues. This assertion was validated by Rudolf Schoenheimer's work with radioisotopes as tracers in 1937. This in turn would pave the way for the possibility that similar transfers were a primary means of producing most amino acids via amino transfer.

Another such example of early transferase research and later reclassification involved the discovery of uridyl transferase. In 1953, the enzyme UDP-glucose pyrophosphorylase was shown to be a transferase, when it was found that it could reversibly produce UTP and G1P from UDP-glucose and an organic pyrophosphate.

Another example of historical significance relating to transferase is the discovery of the mechanism of catecholamine breakdown by catechol-O-methyltransferase. This discovery was a large part of the reason for Julius Axelrod’s 1970 Nobel Prize in Physiology or Medicine (shared with Sir Bernard Katz and Ulf von Euler).

Nomenclature

Systematic names of transferases are constructed in the form of "donor:acceptor grouptransferase." For example, methylamine:L-glutamate N-methyltransferase would be the standard naming convention for the transferase methylamine-glutamate N-methyltransferase, where methylamine is the donor, L-glutamate is the acceptor, and methyltransferase is the EC category grouping. This same action by the transferase can be illustrated as follows:

methylamine + L-glutamate $\rightleftharpoons$ NH₃ + N-methyl-L-glutamate

However, other accepted names are more frequently used for transferases, and are often formed as "acceptor grouptransferase" or "donor grouptransferase." For example, a DNA methyltransferase is a transferase that catalyzes the transfer of a methyl group to a DNA acceptor. In practice, many molecules are not referred to using this terminology due to more prevalent common names. For example, RNA Polymerase is the modern common name for what was formerly known as RNA nucleotidyltransferase, a kind of nucleotidyl transferase that transfers nucleotides to the 3’ end of a growing RNA strand. In the EC system of classification, the accepted name for RNA Polymerase is DNA-directed RNA polymerase.

Classification

Described primarily based on the type of biochemical group transferred, transferases can be divided into ten categories (based on the EC Number classification). These categories comprise over 450 different unique enzymes. In the EC numbering system, transferases have been given a classification of EC2. It is important to note, that hydrogen is not considered a functional group when it comes to transferase targets; instead, hydrogen transfer is included under oxidoreductases, due to electron transfer considerations.

Classification of transferases into subclasses:

EC number	Examples	Group(s) transfered
EC 2.1	methyltransferase and formyltransferase	single-carbon groups
EC 2.2	transketolase and transaldolase	aldehyde or ketone groups
EC 2.3	acyltransferase	acyl groups or groups that become alkyl groups during transfer
EC 2.4	glycosyltransferase, hexosyltransferase, and pentosyltransferase	glycosyl groups, as well as hexoses and pentoses
EC 2.5	riboflavin synthase and chlorophyll synthase	alkyl or aryl groups, other than methyl groups
EC 2.6	transaminase, and oximinotransferase	nitrogenous groups
EC 2.7	phosphotransferase, polymerase, and kinase	phosphorus-containing groups; subclasses are based on the acceptor (e.g. alcohol, carboxyl, etc.)
EC 2.8	sulfurtransferase and sulfotransferase	sulfur-containing groups
EC 2.9	selenotransferase	selenium-containing groups
EC 2.10	molybdenumtransferase and tungstentransferase	molybdenum or tungsten

Reactions

EC 2.1: single carbon transferases

EC 2.1 includes enzymes that transfer single-carbon groups. This category consists of methyl-, hydroxymethyl-, formyl-, carboxy-, carbamoyl-, and amidotransferases. Carbamoyltransferases, as an example, transfer a carbamoyl group from one molecule to another. Carbamoyl groups follow the formula NH₂CO. In ATCase such a transfer is written as Carbamyl phosphate + L-aspertate $\rightarrow$ L-carbamyl aspartate + phosphate, or graphically:

EC 2.2: aldehyde and ketone transferases

Enzymes that transfer aldehyde or ketone groups and included in EC 2.2. This category consists of various transketolases and transaldolases. Transaldolase, the namesake of aldehyde transferases, is an important part of the pentose phosphate pathway. The reaction it catalyzes consists of a transfer of a dihydroxyacetone functional group to G3P. The reaction is as follows: sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate $\rightleftharpoons$ erythrose 4-phosphate + fructose 6-phosphate.

EC 2.3: acyl transferases

Transfer of acyl groups or acyl groups that become alkyl groups during the process of being transferred are key aspects of EC 2.3. Further, this category also differentiates between amino-acyl and non-amino-acyl groups. Peptidyl transferase is a ribozyme that facilitates formation of peptide bonds during translation. As an aminoacyltransferase, it catalyzes the transfer of a peptide to an aminoacyl-tRNA, following this reaction: peptidyl-tRNA_A + aminoacyl-tRNA_B $\rightleftharpoons$ tRNA_A + peptidyl aminoacyl-tRNA_B.

EC 2.4: glycosyl, hexosyl, and pentosyl transferases

EC 2.4 includes enzymes that transfer glycosyl groups, as well as those that transfer hexose and pentose. Glycosyltransferase is a subcategory of EC 2.4 transferases that is involved in biosynthesis of disaccharides and polysaccharides through transfer of monosaccharides to other molecules. An example of a prominent glycosyltransferase is lactose synthase which is a dimer possessing two protein subunits. Its primary action is to produce lactose from glucose and UDP-glucose. This occurs via the following pathway: UDP-α-D-glucose + D-glucose $\rightleftharpoons$ UDP + lactose.

EC 2.5: alkyl and aryl transferases

EC 2.5 relates to enzymes that transfer alkyl or aryl groups, but does not include methyl groups. This is in contrast to functional groups that become alkyl groups when transferred, as those are included in EC 2.3. EC 2.5 currently only possesses one sub-class: Alkyl and aryl transferases. Cysteine synthase, for example, catalyzes the formation of acetic acids and cysteine from O₃-acetyl-L-serine and hydrogen sulfide: O₃-acetyl-L-serine + H₂S $\rightleftharpoons$ L-cysteine + acetate.

EC 2.6: nitrogenous transferases

The grouping consistent with transfer of nitrogenous groups is EC 2.6. This includes enzymes like transaminase (also known as "aminotransferase"), and a very small number of oximinotransferases and other nitrogen group transferring enzymes. EC 2.6 previously included amidinotransferase but it has since been reclassified as a subcategory of EC 2.1 (single-carbon transferring enzymes). In the case of aspartate transaminase, which can act on tyrosine, phenylalanine, and tryptophan, it reversibly transfers an amino group from one molecule to the other.

The reaction, for example, follows the following reaction: L-aspartate +2-oxoglutarate $\rightleftharpoons$ oxaloacetate + L-glutamate.

A and B transferases and the histo-blood group

The A and B transferases are the foundation of the human ABO blood group system. Both A and B transferases are glycosyltransferases, meaning they transfer a sugar molecule onto an H-antigen. This allows H-antigen to synthesize the glycoprotein and glycolipid conjugates that are known as the A/B antigens. It is possible for Homo sapiens to have any of four different blood types: Type A (express A antigens), Type B (express B antigens), Type AB (express both A and B antigens) and Type O (express neither A nor B antigens) The gene for A and B transferases is located on chromosome nine. The gene contains seven exons and six introns and the gene itself is over 18kb long. The alleles for A and B transferases are extremely similar. The resulting enzymes only differ in 4 amino acid residues. The differing residues are located at positions 176, 235, 266, and 268 in the enzymes.

A transferase

The full name of A transferase is alpha 1-3-N-acetylgalactosaminyltransferase. Its function in the cell is to add N-acetylgalactosamine to H-antigen, creating A-antigen.

B transferase

The full name of B transferase is alpha 1-3-galactosyltransferase. Its function in the cell is to add a galactose molecule to H-antigen, creating B-antigen.

Uses in Biotechnology

Terminal transferases

Terminal transferases are transferases that can be used to label DNA or to produce plasmid vectors. It accomplishes both of these tasks by adding deoxynucleotides in the form of a template to the downstream end or 3' end of an existing DNA molecule. Terminal transferase is one of the few DNA polymerases that can function without an RNA primer.

Glutathione transferases

The family of glutathione transferases (GST) is extremely diverse, and therefore can be used for a number of biotechnological purposes. Plants use glutathione transferases as a means to segregate toxic metals from the rest of the cell. These glutathione transferases can be used to create biosensors to detect contaminants such as herbicides and insecticides. Glutathione transferases are also used in transgenic plants to increase resistance to both biotic and abiotic stress. Glutathione transferases are currently being explored as targets for anti-cancer medications due to their role in drug resistance. Further, glutathione transferase genes have been investigated due to their ability to prevent oxidative damage and have shown improved resistance in transgenic cultigens.

Rubber transferases

Currently the only available commercial source of natural rubber is the Hevea plant (Hevea brasiliensis). Natural rubber is superior to synthetic rubber in a number of commercial uses. Efforts are being made to produce transgenic plants capable of synthesizing natural rubber, including tobacco and sunflower. These efforts are focused on sequencing the subunits of the rubber transferase enzyme complex in order to transfect these genes into other plants.

Transferase deficiencies

A deficiency of this transferase, E. coli galactose-1-phosphate uridyltransferase is a known cause of galactosemia

Transferase deficiencies are at the root of many common illnesses. The most common result of a transferase deficiency is a buildup of a cellular product.

SCOT deficiency

Succinyl-CoA:3-ketoacid CoA transferase deficiency (or SCOT deficiency) leads to a buildup of ketones. Ketones are created upon the breakdown of fats in the body and are an important energy source. Inability to utilize ketones leads to intermittent ketoacidosis, which usually first manifests during infancy. Disease sufferers experience nausea, vomiting, inability to feed, and breathing difficulties. In extreme cases, ketoacidosis can lead to coma and death. The deficiency is caused by mutation in the gene OXTC1. Treatments mostly rely on controlling the diet of the patient.

CPT-II deficiency

Carnitine palmitoyltransferase II deficiency (also known as CPT-II deficiency) leads to an excess long chain fatty acids, as the body lacks the ability to transport fatty acids into the mitochondria to be processed as a fuel source. The disease is caused by a defect in the gene CPT2. This deficiency will present in patients in one of three ways: lethal neonatal, severe infantile hepatocardiomuscular, and myopathic form. The myopathic is the least severe form of the deficiency and can manifest at any point in the lifespan of the patient. The other two forms appear in infancy. Common symptoms of the lethal neonatal form and the severe infantile forms are liver failure, heart problems, seizures and death. The myopathic form is characterized by muscle pain and weakness following vigorous exercise. Treatment generally includes dietary modifications and carnitine supplements.

Galactosemia

Galactosemia results from an inability to process galactose, a simple sugar. This deficiency occurs when the gene for galactose-1-phosphate uridylyltransferase (GALT) has any number of mutations, leading to a deficiency in the amount of GALT produced. There are two forms of Galactosemia: classic and Duarte. Duarte galactosemia is generally less severe than classic galactosemia and is caused by a deficiency of galactokinase. Galactosemia renders infants unable to process the sugars in breast milk, which leads to vomiting and anorexia within days of birth. Most symptoms of the disease are caused by a buildup of galactose-1-phosphate in the body. Common symptoms include jaundice, ketonuria, bacterial infection, failure to grow, and mental impairment, among others. Buildup of a second toxic substance, galacitol, occurs in the lenses of the eyes, causing cataracts. Currently, the only available treatment is adherence to a diet devoid of lactose, and antibiotics for infections that may develop.

See also

• EC 2 Introduction from the Department of Chemistry at Queen Mary, University of London