Amino acids tetrahydrofolate

See also Glutamate as a Precursor of Other Amino Acids, Tetrahydrofolate Coenzymes... 

A subclass of lyases, involved in amino acid metabolism, utilizes pyridoxal 5-phosphate (PLP, 3-hydroxy-2-methyl-5-[(phosphonooxy)methyl]-4-pyridinecarbaldehyde) as a cofactor for imine/ enamine-type activation. These enzymes are not only an alternative to standard fermentation technology, but also offer a potential entry to nonnatural amino acids. Serine hydroxymethyl-tansferase (SHMT EC 2.1.2.1.) combines glycine as the donor with (tetrahydrofolate activated) formaldehyde to L-serine in an economic yield40, but will also accept a range of other aldehydes to provide /i-hydroxy-a-amino acids with a high degree of both absolute and relative stereochemical control in favor of the L-erythro isomers41. 

In a collaboration between the Abelson and Hecht labs [56b], a series of noncoded amino acids were introduced into dihydrofolate reductase (DHFR) to probe substrate binding and the requirement of an aspartic acid residue for catalytic competence. When aspartic acid analogs mono- or disubstituted at the )0-carbon were substituted for the active site aspartic acid residue, the mutant DHFRs were still able to catalyze the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate at 74 - 86 % of the wild-type rate. While hydride transfer from NADPH is not the rate-limiting step for the wild-type enzyme at physiological pH, a kinetic isotope experiment with NADPD indicated that hydride transfer had likely become the rate-limiting step for the mutant containing the )0,)0-dimethylaspartic acid. 

Tetrahydrofolic acid then functions as a carrier of one-carbon groups for amino acid and nucleotide metabolism. The basic ring system is able to transfer methyl, methylene, methenyl, or formyl groups, and it utilizes slightly different reagents as appropriate. These are shown here for convenience, we have left out the benzoic acid-glutamic acid portion of the structure. These compounds are all interrelated, but we are not going to delve any deeper into the actual biochemical relationships. 

Both the sulfonamides and trimethoprim interfere with bacterial folate metabolism. For purine synthesis tetrahydrofolate is required. It is also a cofactor for the methylation of various amino acids. The formation of dihydrofolate from para-aminobenzoic acid (PABA) is catalyzed by dihydropteroate synthetase. Dihydrofolate is further reduced to tetrahydrofolate by dihydrofolate reductase. Micro organisms require extracellular PABA to form folic acid. Sulfonamides are analogues of PABA. They can enter into the synthesis of folic acid and take the place of PABA. They then competitively inhibit dihydrofolate synthetase resulting in an accumulation of PABA and deficient tetrahydrofolate formation. On the other hand trimethoprim inhibits dihydrofolate... 

It acts by inhibiting dihydrofolate reductase. It inhibits conversion of dihydrofolic acid to tetrahydrofolic which is essential for purine synthesis and amino acid interconversions. It primarily affects DNA synthesis but also RNA and protein synthesis. It has cell cycle specific action and kills cells in S phase. It is readily absorbed from gastrointestinal tract but larger doses are absorbed incompletely, little drug is metabolised and it is excreted largely unchanged in urine. 

Cyanocobalamin A cofactor required for essential enzymatic reactions that form tetrahydrofolate, convert homocysteine to methionine, and metabolize l-methylmalonyl-CoA Adequate supplies are required for amino acid and fatty acid metabolism, and DNA synthesis Treatment of vitamin B12 deficiency, which manifests as megaloblastic anemia and is the basis of pernicious anemia Parenteral vitamin B12 is required for pernicious anemia and other malabsorption syndromes Toxicity No toxicity associated with excess vitamin B12... 

Tetrahydrobiopterin, another cofactor of amino acid catabolism, is similar to the pterin moiety of tetrahydrofolate, but it is not involved in one-carbon transfers instead it participates in oxidation reactions. We consider its mode of action when we discuss phenylalanine degradation (see Fig. 18-24). 

After removal of their amino groups, the carbon skeletons of amino acids undergo oxidation to compounds that can enter the citric acid cycle for oxidation to C02 and H20. The reactions of these pathways require a number of cofactors, including tetrahydrofolate and 5-adenosylmethionine in one-carbon transfer reactions and tetrahydrobiopterin in the oxidation of phenylalanine by phenylalanine hydroxylase. 

The amino acid and nucleotide biosynthetic pathways make repeated use of the biological cofactors pyridoxal phosphate, tetrahydrofolate, and A-adenosylmethionine. Pyridoxal phosphate is required for transamination reactions involving glutamate and for other amino acid transformations. One-carbon transfers require S-adenosyhnethionine and tetrahydrofolate. Glutamine amidotransferases catalyze reactions that incorporate nitrogen derived from glutamine. 

Reorganizes coverage of amino acid degradation to focus on the big picture Adds new material on the relative importance of several degradative pathways Includes a new description of the interplay of the pyridoxal phosphate and tetrahydrofolate cofactors in serine and glycine metabolism... 

Tetrahydrofolate receives one-carbon fragments from donors such as serine, glycine, and histidine and transfers them to intermediates in the synthesis of amino acids, purines, and thymine—a pyrimidine found in DNA. ... 

The atoms of a purine are contributed by amino acids (aspartic acid, glutamine, and glycine), CO2, and N10-formyl tetrahydrofolic acid. 

Figure 15-19 Drawings of the active site of E. coli dihydrofolate reductase showing the hound ligands NADP+ and tetrahydrofolate. Several key amino acid side chains are shown in the stereoscopic views on the right. The complete ribbon structures are on the left. (A) Closed form. (B) Open form into which substrates can enter and products can escape. From Sawaya and Kraut.381 Courtesy of Joseph Kraut. Molscript drawings (Kraulis, 1991).

In acute and chronic urinary tract infection, the combination of trimethoprim and sulfamethoxazole (Bactrim, Septra) exerts a truly synergistic effect on bacteria. The sulfonamide inhibits the utilization of p-amino-benzoic acid in the synthesis of folic acid (Figure 2.3), whereas trimethoprim, by inhibiting dihydrofolic acid reductase, blocks the conversion of dihydrofolic acid to tetrahydrofolic acid, which is essential to bacteria in the denovo synthesis of purines, pyrimidines, and certain amino acids. Because mammalian organisms do not synthesize folic acid and therefore need it as a vitamin in their daily diets, trimethoprim-sulfamethoxazole does not interfere with the metabolism of mammalian cells. 

Methotrexate (Amethopterin) is a folic acid antagonist that binds to dihydrofolate reductase, thus interfering with the synthesis of the active cofactor tetrahydrofolic acid, which is necessary for the synthesis of thymidylate, purine nucleotides, and the amino acids serine and methionine. 

A special initiator tRNA, tRNAme i (I stands for initiator) is used for beginning protein synthesis. In bacteria, this initiator tRNA carries the modified amino acid N-formylmethionine (fmet). The formylation reaction transfers the formyl group from formyl-tetrahydrofolate to... 

Aminopterin and amethopterin are 4-amino analogues of folic acid (Fig. 11.5) and as such are potent inhibitors of the enzyme dihydrofolate reductase (EC 1.5.1.3) (Blakley, 1969). This enzyme catalyses the reduction of folic acid and dihydrofolic acid to tetrahy-drofolic acid which is the level of reduction of the active coenzyme involved in many different aspects of single carbon transfer. As is clear from Fig. 11.6, tetrahydrofolate is involved in the metabolism of (a) the amino acids glycine and methionine (b) the carbon atoms at positions 2 and 8 of the purine ring (c) the methyl group of thymidine and (d) indirectly in the synthesis of choline and histidine. 

Vitamin B12 is required by only two enzymes in human metabolism methionine synthetase and L-methylmalonyl-CoA mutase. Methionine synthetase has an absolute requirement for methylcobalamin and catalyzes the conversion of homocysteine to methionine (Fig. 28-5). 5-Methyltetrahydrofolate is converted to tetrahydrofolate (THF) in this reaction. This vitamin B12-catalyzed reaction is the only means by which THF can be regenerated from 5-methyltetrahydrofolate in humans. Therefore, in vitamin B12 deficiency, folic acid can become trapped in the 5-methyltetrahydrofolate form, and THF is then unavailable for conversion to other coenzyme forms required for purine, pyrimidine, and amino acid synthesis (Fig. 28-6). All folate-dependent reactions are impaired in vitamin B12 deficiency, resulting in indistinguishable hematological abnormalities in both folate and vitamin B12 deficiencies. 

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