One-Carbon Metabolism in Health and Disease

Abstract

One-carbon (1C) metabolism, mediated by the folate cofactor, supports multiple physiological processes. These include biosynthesis (purines and thymidine), amino acid homeostasis (glycine, serine, and methionine), epigenetic maintenance, and redox defense. Both within eukaryotic cells and across organs, 1C metabolic reactions are compartmentalized. Here we review the fundamentals of mammalian 1C metabolism, including the pathways active in different compartments, cell types, and biological states. Emphasis is given to recent discoveries enabled by modern genetics, analytical chemistry, and isotope tracing. An emerging theme is the biological importance of mitochondrial 1C reactions, both for producing 1C units that are exported to the cytosol and for making additional products, including glycine and NADPH. Increased clarity regarding differential folate pathway usage in cancer, stem cells, development, and adult physiology is reviewed and highlights new opportunities for selective therapeutic intervention.

Keywords: MTHFD2; cancer metabolism; folate; mitochondria; neural tube defects; one-carbon metabolism; serine.

Figure 1. Chemical Transformations of Folates

Folic acid is reduced to THF, which can then accept a 1C unit and undergo a series of oxidative/reductive transformations. DHF, dihydrofolate; THF, tetrahydrofolate; 5,10-meTHF, 5,10-methylene-THF; 5,10-me⁺THF, 5,10-methenyl-THF.

Figure 2. Products and Compartmentalization of Folate-Mediated 1C Metabolism

Through an interlinked set of mitochondrial and cytosolic reactions, folate metabolism supports 1C anabolic reactions. All abbreviations are standard gene names. Certain descriptions utilize the common protein name for clarity. SHMT1/2, serine hydroxymethyl transferase, cytosolic(1)/mitochondrial (2); MTHFD1, methylenetetrahydrofolate dehydrogenase, cyclohydrolase, and formyltetrahydrofolate synthetase 1; MTHFD2/L, methylenetetrahydrofolate dehydrogenase 2/2-like; MTHFD1L, monofunctional tetrahydrofolate synthase, mitochondrial; MTFMT, mitochondrial methionyl-tRNA formyltransferase; TYMS, thymidylate synthetase; MTHFR, methylenetetrahydrofolate reductase; MTR, methionine synthase; DHFR, dihydrofolate reductase; GART, phosphoribosylglycinamide formyltransferase; ATIC, 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase; ALDH1L1/2, cytosolic (1)/mitochondrial (2) 10-formyltetrahydrofolate dehydrogenase.

Figure 3. [2,3,3-²H]Serine Is a Tracer for Mitochondrial Folate Pathway Activity

(A) Compartment-specific metabolism of serine can be determined by following the incorporation of ²H from [2,3,3-²H]serine into terminal products of 1C metabolism such as thymidine. If serine is metabolized in the mitochondria, one proton attached to the 3-position carbon will be lost during oxidation to formate, as represented by the blue dots. Reductive incorporation of the formate into the cytosolic folate pool will result in singly deuterated thymidine monophosphate (TMP M+1). In contrast, a serine catabolized in the cytosol by SHMT1 will transfer both protons onto 5,10-methylene-THF and generate a doubly labeled TMP M+2, as represented by the red dots. (B) Representative mass spectra of WT and SHMT2 deletion cells demonstrate the application of this tracing technique. Adapted from Ducker et al. (2016).

Figure 4. Major Sources of 1C Units in the Mammalian Cell

1C sources originate in the diet and are metabolized to generate 1C units. Dietary sources of 1C are underlined and include glucose, threonine, methionine, glycine, serine, histidine, and choline. TDH*, human TDH encodes a pseudogene without functional catalytic activity; 3PG, 3-phoshoglycerate; 3PSer, 3-phosphoserine; DMG, dimethylglycine; TDH, L-threonine dehydrogenase; GCAT, glycine C-acetyltransferase; GCS, glycine cleavage system; SARDH, sarcosine dehydrogenase; DMGDH, dimethylglycine dehydrogenase; CHDH, choline dehydrogenase; ALDH7A1, aldehyde dehydrogenase 7 family member A1; BHMT, betaine-homocysteine S-methyltransferase; GNMT, glycine N-methyltransferase; FTCD, formimidoyltransferase cyclodeaminase; PHGDH, phosphoglycerate dehydrogenase; PSAT, phosphoserine aminotransferase; PSPH, phosphoserine phosphatase.

Figure 5. 1C Metabolism across Different Cell Types

A cartoon representation of the major folate-dependent 1C metabolic pathways in three representative cell types. Variable font size and line widths represent the relative importance of each species or reaction in the indicated cell type. Reactions not drawn for an individual cell type are considered to be negligible in that context. TDH*, TDH activity is important in murine stem cells, but this enzyme is non-functional in humans.