Glutamate catabolic pathway

The nitrogen source in the medium is the amino add glutamate. There are several cations K Mn2, Cn2, Zn2, Mg2, Co2, Fe2, Ca2 Mo6. Phosphate (POi") is the major anionic component. Fumaric add is a TCA cycle intermediate and may improve metabolic balance through the catabolic pathways and oxidation through the TCA cyde. Peptone may improve growth through the provision of growth factors (amino acids, vitamins, nudeotides). 

FIGURE 18-26 Catabolic pathways for arginine, histidine, glutamate, glutamine, and proline. These amino acids are converted to a-ketoglutarate. The numbered steps in the histidine pathway are catalyzed by histidine ammonia lyase, urocanate hydratase, imida-zolonepropionase, and glutamate formimino transferase. 

The progression of glutamate neurotoxicity can be considered as a process of three sequential steps the overstimulation of postsynaptic glutamate receptors leading to the accumulation of Ca2+ the amplification of the detrimental signal through the additional Ca2+ influx and the release from intracellular stores the activation of catabolic pathways and the generation of free radicals (Choi, 1990). 

This chapter focuses initially on the catabolism of the amino acids. Aminotransferases can be used to catalyse the first step in the breakdown of nearly all of the amino acids. Lysine catabolism, in contrast, does not begin with an aminotrans-feraseamino acids can be catabolized via more than one pathway. Glutamate catabolism, for example, can begin by reactions catalyzed by glutamate oxaloacetate aminotransferase or glutamate dehydrogenase. 

Ammonium ions are produced by the catabolism of a number of amino acids. Glutamate dehydrogenase is the major source of ammonium ions in the body. Ammonium ions are also produced from the catabolic pathways of serine, histidine, tryptophan, glycine, glutamine, and asparagine. L-Amino acid oxidase and... 

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 Catabolic Pathways of Arginine, Proline, Histidine, Glutamine, and Glutamate. 

The illustrations here and here show the transamination reactions interconverting ot-ketoglutarate, glutamate, and glutamine (see here) and oxaloacetate, aspartate, and asparagine (see here). Notice in each case that one enzyme is primarily involved in the anabolic reactions (making an amino acid) whereas a different enzyme is involved in the catabolic pathway (breaking down an amino acid). 

A possible catabolic pathway for glutamate that includes aspartate is as follows ... 

The catabolic pathways of the carbon chains of the amino acids, alanine, glutamic, and aspartic acids, appear to be readily apparent once these amino acids lose their amino groups. When this occurs, alanine is converted to pyruvic acid, glutamic acid to a-ketoglutaric acid, and aspartic acid to either oxalacetic or fumaric acid. All of the above acids are integral members of the citric acid cycle, and the subsequent degradation of each one has been adequately explained in terms of the operation of the citric acid cycle (see the chapter. The Tricarboxylic Acid Cycle). 

Alanine. Transamination of alanine forms pyruvate. Perhaps for the reason advanced under glutamate and aspartate catabolism, there is no known metabolic defect of alanine catabolism. Cysteine. Cystine is first reduced to cysteine by cystine reductase (Figure 30-7). Two different pathways then convert cysteine to pyruvate (Figure 30-8). 

This pathway is the hub of intermediary metabolism. Four- and five-carbon end products of many catabolic processes feed into the cycle to serve as fuels. Oxaloac-etate and a-ketoglutarate, for example, are produced from aspartate and glutamate, respectively, when proteins are degraded. Under some metabolic circumstances, intermediates are drawn out of the cycle to be used as precursors in a variety of biosynthetic pathways. 

Somewhat surprisingly, within the mitochondria the ratio [NAD+]/[NADH] is 100 times lower than in the cytoplasm. Even though mitochondria are the site of oxidation of NADH to NAD+, the intense catabolic activity occurring in the (3 oxidation pathway and the citric acid cycle ensure extremely rapid production of NADH. Furthermore, the reduction state of NAD is apparently buffered by the low potential of the (3-hydroxybutyrate-acetoacetate couple (Chapter 18, Section C,2). Mitochondrial pyridine nucleotides also appear to be at equilibrium with glutamate dehydrogenase.169... 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

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