Metabolism of C Compounds

Several processes described above use one-carbon derivatives of tetrahydrofolate (Fig. 15-19). For example, the synthesis of the purine ring (Fig. 15-16) requires N,0-formyl tetrahydrofolate. Thymidylate synthase, a key enzyme in pyrimidine synthesis, uses N5-N,0-methylene tetrahydrofolate both as a substrate and as a reducing agent. This compound, perhaps the most important in C, metabolism, is 

The pteroic acid moiety of tetrahydrofolate consists of a reduced pteridine ring and p-aminobenzoic acid. Folate from the diet is absorbed by the intestinal mucosa and in two enzymatic steps is reduced to tetrahydrofolate which is the active form of the coenzyme. Mammals cannot synthesize folate this normally does not present a problem because microorganisms of the intestinal tract readily do so. 

The two steps in the reduction of folic acid to tetrahydrofolate are catalyzed by dihydrofolate reductase. Both of these reactions require NADPH as a source of electrons. 

In order to understand the bewildering variety of reactions involving tetrahydrofolate, it is essential to realize that in biological systems, one-carbon compounds may exist in five different oxidation states. The most reduced form is methane, CH4, and the most oxidized form is C02. In between these two extremes are methanol (CH3OH), formaldehyde (CH20), and formate (HCOO ). 

Question Are all the above one-carbon compounds involved in Ci metabolism  

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