Publisher Summary At the oxidation states of formate, formaldehyde, and methanol, the enzyme reactions occur as steps in the metabolic production or breaking down of purines, pyrimidines, and amino acids. These transfer reactions are mediated by the coenzyme form of folic acid, tetrahydrofolate, whose structural features allow it to form adducts with the one-carbon groups by covalent linkages at N-5 and N-10. This property provides the basis for the ability of the coenzyme to accept and donate one-carbon groups from metabolites. This chapter describes all of the enzymes responsible for these reactions, with the exception of the B12-dependent 5-methyltetrahydrofolate-homocysteine transmethylase and methylenetetrahydrofolate dehydro-genase and methylenetetrahydrofolate reductase, both of which catalyze reactions involving oxidoreduction rather than the transfer of the one-carbon group. The chapter discusses the transfer of fomate and its congeners, including 10-formyltetrahyderofolate synthetase, 10-formyltetrahydrofolate decyclase, 5-formyltetrahydrofolate cyclodehydrase, 5,10-methenyltetrahydrofolate cyclohydrolase, glycinamide ribonucleotide transformylase, formiminoglycine formiminotransferase, formiminoglutamate formiminotransferase, N-formylglutamate transformylase, and methionyl-transfer RNA transformylase. The deacylation of 10-formyltetrahydrofolate proceeds through a formyl intermediate that is oxidized before being released as CO2. These results also help to explain why CO2 production from formate is depressed when animals are maintained on a folate-deficient diet. ATP-supplemented pigeon liver extracts utilizes 5-formyltetrahydrofolate (folinic acid) in the conversion of the purine precursor, 5-amino-4-imidazole carboxamide-5′-phosphoribotide, to inosine-5’-phosphate. Both the transformylase and inosinicase have been partially purified from chicken liver. The transformylase, but not inosinicase, is markedly stimulated by K+. The existence of two separate enzymes is suggested by the K+ dependence of only the transformylase. The degradation of histidine in Pseudomonas fluorescens, Aerobacter aerogenes, and mammalian liver leads to the formation of N-formimino-L-glutamate. In liver, the formimino group is transferred to tetrahydrofolate.