Glutamic acid interconversion

Amino acids. Glutamate (along with aspartate) is a key substrate and product in transamination (aminotransferase) reactions for amino acid interconversions. Aminotransferases carry out the general reaction ... 

Fig. 11.6. Interconversions of tetrahydrofolate derivatives. FH2 = dihydrofolic acid FH4 = tetrahydrofolic acid AICAR -= 5 aminoimidazole 4-carboxamide ribonucleotide FAICAR = formyl AICAR GAR = glycinamide ribonucleotide FGAR = formyl GAR Glu = glutamic acid FIGLU = formimino glutamic acid. (Modified from Mudd and Cantoni, 1964.)...

Cyclization reactions of glutamic acid analogous to those of aspartic acid have not been found in sequence studies with peptides, but an a — 7 interconversion in model peptides of glutamic acid has been observed (Kornguth el al., 1963). 

In the experiments of Haurowitz et al. (1957) proteins and peptides were in an acetic anhydride/acetic acid medium during thiohydantoin formation. Their results with a-peptides and poly-a-glutamic acid indicated that very little a —> 7 interconversion took place, but they accepted that it could occur if glutamic acid residues were adjacent to certain other amino acid residues. These authors also suggested that possible y a conversion... 

Ingested protein is digested in a stepwise fashion in the stomach, small intestinal lumen, and small intestinal mucosal cells (Chapter 12). Peptides formed in the intestinal lumen are absorbed into the mucosal cells and degraded to free amino acids. The outflow of amino acids to the portal vein does not reflect the amino acid composition of the ingested protein. Thus, alanine levels increase two-to fourfold, and glutamine, glutamate, and aspartate are absent. These changes arise from amino acid interconversions within the intestinal cell. 

L-Glutamate dehydrogenases (EC 1.4.1.2-4) catalyze the interconversion of a-ketoglutarate and L-glutamic acid ... 

Glucose-phosphate isomerase is one of the best studied enzymes catalyzing the interconversion of aldo- and ketohexose phosphates. An active site carboxyl group is a possible candidate for the base catalyzing the intramolecular proton transfer reaction. The affinity label 1,2-anhydro-D-mannitol 6-phosphate (8) inactivates the enzyme by forming an ester linkage between C-l of the affinity label and an active site carboxyl of a glutamic acid residue (98). 

Figure 10 Amino acid residues. The obvious structural similarities between tyrosine and phenylalanine make it is easy to imagine the possible interconversion (mutation) between these two residues. The dissimilar structural features between tryptophan and glutamic acid illustrate the improbability of mutation.

The thermodynamic stability, the aggregation state, and the relative orientation of the helical strands in such peptides were shown to depend largely on the identity of the amino acid residues at positions a, d, e, and g. The use of leucine and valine at the hydrophobic core (positions a and glutamic acid and lysine at the e and g positions were shown to control equilibrium between dimeric and trimeric coiled-coil aggregation states." Peptide sequences that allow interconversion between dimeric and trimeric parallel coiled coils were found as productive templates to enhance substrate selectivity, catalytic efficiency, and turnover and were commonly used in peptide-replicating systems. 

Nutritional and isotopic studies have shown that the 5-carbon amino acids, glutamic acid, proline, and ornithine, are capable of interconversion and that in different groups of organisms these amino acids do indeed replace each other. In animals the reactions of these compounds... 

The existence of enzymes in microorganisms which catalyze the interconversion of D- and L-amino acids is of considerable interest, since the intramolecular transfer of an amino group is apparently involved. The term racemase has been proposed for such enzymes. Two racemases have been reported. Alanine racemase has been shown to be present in a large number of microorganisms and has been partially purified from extracts of S. faecalis. Glutamic acid racemase has been demonstrated in acetone powders of Lactobacillus arabinosus. Both enzymes catalyze the interconversion of the n- and l- forms of their respective substrates. Alanine racemase requires pyridoxal phosphate as coenzyme. Pyri-doxamine phosphate under the conditions employed was not active. Glutamic acid racemase was found not to be affected by the addition of pyridoxal phosphate. However, further studies with purified preparations are necessary before pyridoxal phosphate can be excluded as cofactor for the glutamic acid racemase. Examination of animal tissues under conditions favorable for the demonstration of bacterial alanine racemase failed to reveal any activity. 

The structural similarities of the 5-carbon amino acids, glutamic acid, ornithine, proline, and hydroxyproline suggested interrelations in their metabolism very early (110). Interconversions of these amino acids were first demonstrated by isotopic studies on the intact rat (111, 112). Later work with mutant strains of microorganisms and with enzyme preparations has revealed the normal biosynthetic pathways from glutamic acid to the... 

The key compound in the interconversions of the amino acids, glutamic acid, ornithine, and proline is glutamic semialdehyde. This cyclizes spontaneously in aqueous solution to A -p3Troline-5-carboxylic acid and the two compounds are in equilibrium with each other. 

In summary, the biochemical function of folate coenzymes is to transfer and use these one-carbon units in a variety of essential reactions (Figure 2), including de novo purine biosynthesis (formylation of glycinamide ribonucleotide and 5-amino-4-imidazole carboxamide ribonucleotide), pyrimidine nucleotide biosynthesis (methylation of deoxyuridylic acid to thy-midylic acid), amino-acid interconversions (the interconversion of serine to glycine, catabolism of histidine to glutamic acid, and conversion of homocysteine to methionine (which also requires vitamin B12)), and the generation and use of formate. 

---