Properties of the MTHFR Protein

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While the gene for MTHFR codes for four different variants, the most common form of MTHFR found in human tissues is a 656 amino acid protein .  This 77kDa polypeptide consists of a catalytic domain containing a non-covalently bound flavin adenine dinucleotide and a regulatory domain, which binds the allosteric inhibitor S-adenosylmethionine (Kutzback & Stokstad, 1971).  Although the structure of human MTHFR has not yet been determined, the structure of E.coli MTHFR has been solved.  While the bacterial form of the enzyme does not have the C-terminal regulatory domain, the N-terminal catalytic domains of E. coli and human MTHFR do show a high degree of sequence identity (Goyette et al., 1994).  Guenther & Sheppard, et. al (1999) reported a 30% sequence identity leading them to postulate that the catalytic domains of the two species will have similar structures.  The structure of E. coli MTHFR  was determined using multiwavelength anomalous diffraction.  The data revealed a tetramer of b8a8 barrels each with a bound FAD cofactor (Guenther & Sheppard et al, 1999). The b8a8 barrel is a common structural motif that, due to its prevalence in a wide variety of proteins, is thought to have developed from convergent evolution. 

Under the assumption that the structures of E.coli and human MTHFR catalytic domains will be very similar, work has begun on locating the residues with known mutations within the structure.  The common C677T mutation, leading to mild deficiency of the enzyme, is located in an area of the protein not in direct contact with the FAD cofactor.  The slight change in the backbone structure that results from this mutation causes a change in the packing of secondary, tertiary, and quarternary structure, thus leading to the decrease in FAD affinity seen in patients with this mutation.  The G482A mutation has also been examined within the E. coli MTHFR structure.  This mutation destroys the interaction between two residues which are extremely critical in binding the FAD tail to the enzyme.  Substitution for Arginine to Glutamine does not allow for this interaction, and, as a result, there are no strong forces that keep the FAD bound to the enzyme (Guenther & Sheppard et al, 1999).  Since FAD is absolutely necessary for enzyme function, this mutation results in severe deficiency of the enzyme.