Methylenetetrahydrofolate reductase

methylenetetrahydrofolate reductase [NAD(P)H] Ribbon diagram of the active site of E. coli MTHFR. The flavin cofactor (top) is shown interacting with the bound substrate NADH. Image drawn from PDB 1ZPT Identifiers EC number 1.5.1.20 CAS number 9028-69-7 IntEnz IntEnz view BRENDA BRENDA entry ExPASy NiceZyme view KEGG KEGG entry MetaCyc metabolic pathway PRIAM profile PDB structures Gene Ontology AmiGO / EGO Search PMC articles PubMed articles

edit 5,10-methylenetetrahydrofolate reductase (NADPH) Identifiers Symbols MTHFR; External IDs OMIM: 607093 MGI: 106639 HomoloGene: 4349 GeneCards: MTHFR Gene EC number 1.5.1.20 Gene ontology Molecular function • methylenetetrahydrofolate reductase (NADPH) activity • protein binding • oxidoreductase activity Biological process • amino acid metabolic process • methionine metabolic process • circulation • methionine biosynthetic process RNA expression pattern More reference expression data Orthologs Species Human Mouse Entrez 4524 17769 Ensembl ENSG00000177000 ENSMUSG00000029009 UniProt P42898 Q3V399 RefSeq (mRNA) NM_005957 NM_010840 RefSeq (protein) NP_005948 NP_034970 Location (UCSC) Chr 1: 11.77 - 11.79 Mb Chr 4: 146.88 - 146.9 Mb PubMed search [1] [2]

Methylenetetrahydrofolate reductase (MTHFR) is an enzyme that in humans is encoded by the MTHFR gene.^[1]

Biochemistry

MTHFR irreversibly reduces 5,10-methylenetetrahydrofolate (substrate) to 5-methyltetrahydrofolate (product).

• 5,10-methylenetetrahydrofolate is used to convert dUMP to dTMP for de novo thymidine synthesis.
• 5-Methyltetrahydrofolate is used to convert homocysteine (a potentially-toxic amino acid) to methionine by the enzyme methionine synthase. (Note that homocysteine can also be converted to methionine by the folate-independent enzyme, betaine-homocysteine methyltransferase (BHMT))

MTHFR contains a bound flavin cofactor and uses NAD(P)H as the reducing agent.

Structure

Mammalian MTHFR is composed of an N-terminal catalytic domain and a C-terminal regulatory domain. MTHFR has at least two promoters and two isoforms (70 kDa and 77 kDa).^[2]

Regulation

MTHFR activity may be inhibited by binding of dihydrofolate (DHF)^[3] and S-adenosylmethionine (SAM, or AdoMet).^[4] MTHFR can also be phosphorylated - this decreases its activity by ~20% and allows it to be more easily inhibited by SAM.^[5]

Genetics

The enzyme is coded by the gene with the symbol MTHFR on chromosome 1 location p36.3 in humans.^[6] There are DNA sequence variants (genetic polymorphisms) associated with this gene. In 2000 a report brought the number of polymorphisms up to 24.^[7] Two of the most investigated are C677T (rs1801133) and A1298C (rs1801131) single nucleotide polymorphisms (SNP).

C677T SNP (Ala²²²Val)

The MTHFR nucleotide at position 677 in the gene has two possibilities: C (cytosine) or T (thymine). C at position 677 (leading to an alanine at amino acid 222) is the normal allele. The 677T allele (leading to a valine substitution at amino acid 222) encodes a thermolabile enzyme with reduced activity.

Individual with two copies of 677C (677CC) have the "normal" or "wildtype" genotype. 677TT individuals (homozygous) are said to have mild MTHFR deficiency. 677CT individuals (heterozygotes) are almost the same as normal individuals because the normal MTHFR can make up for the thermolabile MTHFR. About ten percent of the North American population are T-homozygous for this polymorphism. There is ethnic variability in the frequency of the T allele - frequency in Mediterranean/Hispanics > Caucasians > Africans/African-Americans).^[8]

The degree of enzyme thermolability (assessed as residual activity after heat inactivation) is much greater in 677TT individuals (18-22%) compared with 677CT (56%) and 677CC (66-67%).^[9] Individuals of 677TT are predisposed to mild hyperhomocysteinemia (high blood homocysteine levels), because they have less active MTHFR available to produce 5-methyltetrahydrofolate (which is used to decrease homocysteine. Low dietary intake of the vitamin folic acid can also cause mild hyperhomocysteinemia.

Low folate intake affects individuals with the 677TT genotype to a greater extent than those with the 677CC/CT genotypes. 677TT (but not 677CC/CT) individuals with lower plasma folate levels are at risk for elevated plasma homocysteine levels.^[10] In studies of human recombinant MTHFR, the protein encoded by 677T loses its FAD cofactor three times faster than the wild-type protein.^[11] 5-Methyl-THF slows the rate of FAD release in both the wild-type and mutant enzymes, although it is to a much greater extent in the mutant enzyme.^[12] 677TT individuals are at a decreased risk for certain leukemias^[13] and colon cancer.^[14]

Mutations in the MTHFR gene could be one of the factors leading to increased risk of developing schizophrenia.^[15] Schizophrenic patients having the risk allele (T\T) show more deficiencies in executive function tasks.^[16]

A1298C SNP (Glu⁴²⁹Ala)

At nucleotide 1298 of the MTHFR, there are two possibilities: A or C. 1298A (leading to a Glu at amino acid 429) is the most common while 1298C (leading to an Ala substitution at amino acid 429) is less common. 1298AA is the "normal" homozygous, 1298AC the heterozygous, and 1298CC the homozygous for the "variant". In studies of human recombinant MTHFR, the protein encoded by 1298C cannot be distinguished from 1298A in terms of activity, thermolability, FAD release, or the protective effect of 5-methyl-THF.^[11] The C mutation does not appear to affect the MTHFR protein. It does not result in thermolabile MTHFR and does not appear to affect homocysteine levels.

Compound Heterozygotes

Mutations at 677 and 1298 are both in the same gene, MTHFR. They are at different locations in the same gene. Some studies have shown that the MTHFR protein in people with the genotype 677CT 1298AC does its job a bit less well than the normal MTHFR

Severe MTHFR deficiency

Severe MTHFR deficiency is rare (about 50 cases worldwide) and caused by mutations resulting in 0-20% residual enzyme activity.^[7] Patients exhibit developmental delay, motor and gait dysfunction, seizures, and neurological impairment and have extremely high levels of homocysteine in their plasma and urine as well as low to normal plasma methionine levels.

Reaction schematic and folate pathway

Schematic diagram of the reductive carbon-nitrogen bond cleavage (represented by wavy line) catalyzed by methylenetetrahydrofolate reductase.		Folate pathway.
MTHFR = methylenetetrahydrofolate reductase		DHF = dihydrofolate	THF = tetrahydrofolate
5,10-methylene-THF = 5,10-methylenetetrahydrofolate	5-methyl-THF = 5-methyltetrahydrofolate	MTR = methionine synthase	SAH = S-adenosylhomocysteine
NADPH = reduced form of Nicotinamide adenine dinucleotide phosphate	NADP+ = oxidized form of Nicotinamide adenine dinucleotide phosphate	SAM = S-Adenosyl methionine	TS = thymidylate synthase

References

Further reading

External links

• Smith DO (2007-02-08). "MTHFR and homocysteine". Ask Dr. Stephan Moll / Factor V Leiden / Thrombophilia Support Page. Thrombophilia Awareness Project. http://www.fvleiden.org/ask/51.html. Retrieved 2008-08-02.