The 1‐carbon transformations require two cofactors especially: folic acid and vitamin B 12.
Several compounds that interfere with folic acid metabolism are used in clinical medicine as inhibitors of cancer cells or bacterial growth.
Folic acid participates in the activation of single carbons and in the oxidation and reduction of single carbons. Folate‐dependent single‐carbon reactions are important in amino acid metabolism and in biosynthetic pathways leading to DNA, RNA, membrane lipids, and neurotransmitters.
Folic acid is a composite molecule, being made up of three parts: a pteridine ring system (6‐methylpterin), para‐aminobenzoic acid, and glutamic acid. The glutamic acid doesn't participate in the coenzyme functions of folic acid. Instead, folic acid in the interior of the cell may contain a “chain” of three to eight (1–6) glutamic acids, which serves as a negatively charged “handle” to keep the coenzyme inside cells and/or bound to the appropriate enzymes. The pteridine portion of the coenzyme and the p‐aminobenzoic acid portion participate directly in the metabolic reactions of folate.
To carry out the transfer of 1‐carbon units, NADPH must reduce folic acid two times in the cell. The “rightmost” pyrazine ring of the 6‐methylpterin is reduced at each of the two N‐C double bonds. See Figure 1.
Figure 1
The resulting 5,6,7,8‐tetrahydrofolate is the acceptor of 1‐carbon groups.
Tetrahydrofolate accepts methyl groups, usually from serine. The product, N 5,N 10‐methylene‐tetrahydrofolate, is the central compound in 1‐carbon metabolism. Tetrahydrofolate can also accept a methyl group from the complete breakdown of glycine. In Figure 2, only the N 5, C 6, and N 10 atoms of the pteroic acid are shown for clarity.
Figure 2
The N5,N10‐methylene‐tetrahydrofolate can either donate its single‐carbon group directly, be oxidized by NADP to the methenyl form, or be reduced by NADH to the methyl form. Depending on the biosynthetic pathway involved, any of these species can donate the 1‐carbon group to an acceptor. The methylene form donates its methyl group during the biosynthesis of thymidine nucleotides for DNA synthesis, the methenyl form donates its group as a formyl group during purine biosynthesis, and the methyl form is the donor of the methyl group to sulfur during methionine formation.
Sulfanilamide is the simplest of the sulfa drugs, used as antibacterial agents. Note the similarity of sulfanilamide to p‐aminobenzoic acid as shown in Figure 3 . Because its shape is similar to that of p‐aminobenzoic acid, sulfanilamide inhibits the growth of bacteria by interfering with their ability to use p‐aminobenzoic acid to synthesize folic acid. Sulfa drugs were the first antimetabolites to be used in the treatment of infectious disease. Because humans don't make folic acid, sulfanilamide is not toxic to humans in the doses that inhibit bacteria. This ability to inhibit bacteria while sparing humans made them useful in preventing or treating various infections.
Figure 3
In humans, vitamin B 12 participates in two reactions only, but they are essential to life. Humans who cannot absorb vitamin B 12 die of pernicious anemia if untreated (now by injection of the vitamin; formerly by eating large amounts of raw liver). Vitamin B 12 contains a cobalt metal ion bound to a porphyrin ring. Cobalt normally forms six coordinate bonds. Besides the four bonds to the nitrogens of the porphyrin, one bond is to a ring nitrogen of dimethylbenzamidine. The final bond is to a cyanide ion in the vitamin, or to the 5′ carbon of adenosine in the active coenzyme.
Vitamin B 12 is essential for the methylmalonyl‐CoA mutase reaction. Methylmalonyl‐CoA mutase is required during the degradation of odd‐chain fatty acids and of branched‐chain amino acids. Odd‐chained fatty acids lead to propionyl‐CoA as the last step of β‐oxidation. Methylmalonyl‐CoA can be derived from propionyl‐CoA by a carboxylase reaction similar to that of fatty acid biosynthesis. The cofactor for this carboxylation reaction is biotin, just as for acetyl‐CoA carboxylase. The reaction of methylmalonyl‐CoA mutase uses a free radical intermediate to insert the methyl group into the dicarboxylic acid chain. The product is succinyl‐CoA, a Krebs cycle intermediate. The catabolisms of branched‐chain lipids and of the branched‐chain amino acids also require the methylmalonyl‐CoA mutase, because these pathways also generate propionyl‐CoA.
Vitamin B 12 activates methyl groups for methionine biosynthesis by binding them to the Co ion at the sixth position. The methyl group donor to B 12 is 5‐methyl tetrahydrofolate. The methyl‐B 12 donates its methyl group to homocysteine, forming methionine.
Figure 4
Besides being incorporated into proteins, methionine is the source of methyl groups for several important reactions, including the modification of cellular RNAs and the biosynthesis of lipids.
Most methyltransferase reactions use methionine as the source of methyl groups. The actual methyl donor is S‐adenosyl methionine, abbreviated S‐AdoMet, or more colloquially, SAM. S‐Adenosylmethionine is made from methionine and ATP. Note how the reaction consumes all three “high‐energy” phosphate bonds of the ATP as shown in Figure 5 .
Figure 5
Methyl transfer from S‐AdoMet is highly favored chemically and metabolically. First, transfer of the methyl group relieves a positive charge on the Sulfur of S‐AdoMet. Secondly, the bond between the Sulfur and the 5′ carbon of the adenosine is rapidly hydrolyzed, leaving homocysteine and free adenosine. This last step is important, because the product remaining when S‐AdoMet gives up its methyl group, S‐adenosyl‐homocysteine, is a potent inhibitor of methyltransferases. Cleavage of this product removes the inhibitor from the reaction.
Homocysteine itself is converted to methionine by the transfer of a methyl group from N5‐methyl‐tetrahydrofolate to homocysteine, regenerating methionine. The methyl group of N5‐methyl‐tetrahydrofolate is derived from serine, originally, so the net effect of this pathway is to move methyl groups from serine to a variety of acceptors, including homocysteine, nucleic acid bases, membrane lipids, and protein side chains. Serine itself is easily made from 3‐phosphoglycerate by an amino transfer reaction, followed by cleavage of the phosphate.
Homocysteine isn't harmless. Evidence from population studies indicates that high levels of homocysteine in the blood are correlated with heart disease. Folic acid supplements may prevent this problem by ensuring that the homocysteine is rapidly converted to methionine. Similarly, pregnant women generally take folic acid supplements to prevent their babies from being born with neural tube defects. The mechanism for its action isn't known, but the folic acid may help decrease the level of homocysteine in this case as well.