In eukaryotes, folate metabolism is compartmentalized between the cytoplasm and organelles. The folate pathways of mitochondria are adapted to serve the metabolism of the organism. In yeast, mitochondria support cytoplasmic purine synthesis through the generation of formate. This pathway is important but not essential for survival, consistent with the flexibility of yeast metabolism. In plants, the mitochondrial pathways support photorespiration by generating serine from glycine. This pathway is essential under photosynthetic conditions and the enzyme expression varies with photosynthetic activity. In mammals, the expression of the mitochondrial enzymes varies in tissues and during development. In embryos, mitochondria supply formate and glycine for purine synthesis, a process essential for survival; in adult tissues, flux through mitochondria can favor serine production. The differences in the folate pathways of mitochondria depending on species, tissues and developmental stages, profoundly alter the nature of their metabolic contribution.

Enzymes involved in tetrahydrofolate metabolism are of particular pharmaceutical interest, as their function is crucial for amino acid and DNA biosynthesis. The crystal structure of the human cytosolic methylenetetrahydrofolate dehydrogenase/cyclohydrolase (DC301) domain of a trifunctional enzyme has been determined previously with a bound NADP cofactor. While the substrate binding site was identified to be localized in a deep and rather hydrophobic cleft at the interface between two protein domains, the unambiguous assignment of catalytic residues was not possible. We succeeded in determining the crystal structures of three ternary DC301/NADP/inhibitor complexes. Investigation of these structures followed by site-directed mutagenesis studies allowed identification of the amino acids involved in catalysis by both enzyme activities. The inhibitors bind close to Lys56 and Tyr52, residues of a strictly conserved motif for active sites in dehydrogenases. While Lys56 is in a good position for chemical interaction with the substrate analogue, Tyr52 was found stacking against the inhibitors' aromatic rings and hence seems to be more important for proper positioning of the ligand than for catalysis. Also, Ser49 and/or Cys147 were found to possibly act as an activator for water in the cyclohydrolase step. These and the other residues (Gln100 and Asp125), with which contacts are made, are strictly conserved in THF dehydrogenases. On the basis of structural and mutagenesis data, we propose a reaction mechanism for both activities, the dehydrogenase and the cyclohydrolase.

Methylenetetrahydrofolate dehydrogenase)methenyltetrahydrofolate cyclohydrolase)formyltetrahydrofolate synthetase (MTHFD1) is a trifunctional enzyme that interconverts tetrahydrofolate (THF) derivatives for nucleotide synthesis. A common variant in MTHFD1, p.Arg653Gln (c.1958G>A), may increase the risk for neural tube defects (NTD). To examine the biological impact of this variant on MTHFD1 function, we measured enzyme activity and stability in vitro and assessed substrate flux in transfected mammalian cells. The purified Arg653Gln enzyme has normal substrate affinity but a 36% reduction in half)life at 42 degrees C. Thermolability is reduced by magnesium adenosine triphosphate and eliminated by the substrate analog folate pentaglutamate, suggesting that folate status may modulate impact of the variant. The mutation reduces the metabolic activity of MTHFD1 within cells: formate incorporation into DNA in murine Mthfd1 knockout cells transfected with Arg653Gln is reduced by 26%+/-7.7% (P<0.05), compared to cells transfected with wild)type protein, indicating a disruption of de novo purine synthesis. We assessed the impact of the variant on risk for congenital heart defects (CHD) in a cohort of Quebec children (158 cases, 110 controls) and mothers of children with heart defects (199 cases, 105 controls). The 653QQ genotype in children is associated with increased risk for heart defects (odds ratio [OR], 2.11; 95% confidence interval [CI], 1.01-4.42), particularly Tetralogy of Fallot (OR, 3.60; 95% CI, 1.38-9.42) and aortic stenosis (OR, 3.13; 95% CI, 1.13-8.66). There was no effect of maternal genotype. Our results indicate that the Arg653Gln polymorphism decreases enzyme stability and increases risk for CHD. Further evaluation of this polymorphism in folate)related disorders and its potential interaction with folate status is warranted.