Histidine carbon catabolism

The histidine catabolic pathway is discussed under Folate in Chapter 9. The material reveals that histidine is catabolized to produce glutamate. Glutamate in turn, can be converted to a-ketoglutarate and completely oxidized to CO in the Krebs cycle. In the study depicted in Figure 8,26, the dietary histidine was spiked with I Cjhistidine, The term "spiked" means that only a very small proportion of the histidine contained carbon-14. The metabolic behavior of the radioactive histidine, which can be followed, mirrors the metabolic fate of nonradioactive histidine in the diet. All of the CQz exhaled by the rats can be easily collected, The " COj present in the rat s breath can be measured by use of a liquid scintillation counter. The amount of CO2 produced directly mirrors the proportion of histidine, absorbed from the diet that was degraded the rat s body. 

The system has been reviewed by Erkama and Virtanen. A similar reaction (discussed in the chapter. Carbon Catabolism of Amino Acids) is carried out by histidine deaminase to yield urocanic acid. 

Enzymatic mechanisms dealing with the 5-amino group of ornithine, -amino group of lysine, indole-nitrogen of tryptophan, and the imidazole-nitrogen of histidine have not as yet been sufficiently well studied. Some reference to the fate of these nitrogen moieties will be made in the discussion of amino acid interconversions (see the chapters. Carbon Catabolism of Amino Acids, and Synthetic Processes InvolAung Amino Acids). 

Evidence is slowly accumulating on the mechanism of the biosynthesis of histidine. It has been established that glutamate and formate are products of the catabolism of histidine (see the chapter. Carbon Catabolism of Amino Acids). This offered a clue to the possible approach to the problem, as it is conceivable that the reversal of at least some of the reactions of degradation may be involved in the synthesis. 

Arginine and histidine contain five adjacent carbons and a sixth carbon attached through a nitrogen atom. The catabolic conversion of these amino acids to glutamate is therefore slightly more complex than the path from proline or glutamine (Fig. 18-26). Arginine is... 

The histidine load test is not used in the clinical setting and is only sometimes used by researchers however, a description of this test provides a clear-cut example of how folates behave in the mediation of 1-carbon metabolism. Histidine catabolism takes place in the liver according to the pathway sho vn (Figures 9.16 and 9.17). The intermediates, formiminoglulamic acid and j-forrnirnino-H folate, bear the formimino group —CH hJH. 

In considering amino acid catabolism, one must distinguish the catabolism of the carbon chain from that of the nitrogen moiety. The breakdown of the carbon chain of the amino acids yields carbon units that can be used in carbohydrate metabolism, acetate metabolism, or the metabolism of single carbon units. The fate of the carbon units of the individual amino acids has been discussed in other sections of this book, and only a synopsis of the results will be presented here. The carbon skeletons of isoleucine, phenylalanine, threonine, tryptophan, valine, histidine, alanine, arginine, aspartic acid, glycine, proline, glutamic acid, and hydroxyproline are ultimately converted to pyruvic acid. 

Oxidation reduction Cell wall organization Regulation of cell shape Peptidyl-histidine phosphorylation Chemotaxis Cell division Cell cycle Regulation oftranscription Glycyl-tRNA aminoacylation Proteolysis Carbon utilization RNA processing Two-componet signal transduction rRNA catabolic process Fatty acid biosynthetic process... 

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. 

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