Generalities of Amino Acid Catabolism

If a vitamin or cofactor is involved in amino acid metabolism, it s most likely pyridoxal phosphate (Bg), unless it involves serine, and then it s Bg and folic acid. 

Nitrogen is dumped into the urea cycle by transamination to make Asp or Glu or by deamination to make ammonia. 

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Nonessential Amino Acid Synthesis Essential Amino Acids Amino Acid Degradation Generalities of Amino Acid Catabolism Products of Amino Acid Degradation... 

The initial step of amino acid catabolism frequently involves the loss of the amine nitrogen to yield a carbon skeleton in the form of a keto acid, either as a result o transamination or by deamination. In general, energy is derived from amino acids bj the oxidation of the carbon skeletons after entry into the intermediary metabolic pathways. 

Some catabolic reactions of amino acid carbon chains are easy transformations to and from TCA cycle intermediates—for example, the transamination of alanine to pyruvate. Reactions involving 1-carbon units, branched-chain, and aromatic amino acids are more complicated. This chapter starts with 1-carbon metabolism and then considers the catabolic and biosynthetic reactions of a few of the longer side chains. Amino acid metabolic pathways can present a bewildering amount of material to memorize. Perhaps fortunately, most of the more complicated pathways lie beyond the scope of an introductory course or a review such as this. Instead of a detailed listing of pathways, this chapter concentrates on general principles of amino acid metabolism, especially those that occur in more than one pathway. 

Curtin, A. C., McSweeney, P.L.H. 2004. Catabolism of amino acids in cheese during ripening. In Cheese Chemistry, Physics and Microbiology, Vol. 1, General Aspects, 3rd edn (P.F. Fox, P.LH. McSweeney, T.M. Cogan, T.P. Guinee, eds.), pp. 435-454, Elsevier Academic Press, Amsterdam. 

These are the energy producers within the cell. They generate energy in the form of Adenosine Tri-Phosphate (ATP). Generally, the more energy a cell needs, the more mitochondria it contains. Site for Kreb s Citric Acid Cycle Electron transport system and Oxidative Phosphorylation Fatty acid oxidation Amino acid catabolism Interconversion of carbon skeletons. 

Despite the complexity of amino acid degradative pathways, the following general statements can be made. The catabolism of the amino acids usually begins by removing the amino group. Amino groups can then be disposed of in urea... 

Katta Bolic was in a severe stage of negative nitrogen balance on admission, which was caused by both her malnourished state and her intra-abdominal infection complicated by sepsis. The physiologic response to her advanced catabolic status includes a degradation of muscle protein with the release of amino acids into the blood. This release is coupled with an increased uptake of amino acids for "acute phase" protein synthesis by the liver (systemic response) and other cells involved in the immune response to general and severe infection. 

The concentration of amino acids in the blood of patients with liver disease is often elevated. This change is, in part, attributable to a significantly increased rate of protein turnover (general catabolic effect seen in severely ill patients) as well as to impaired amino acid uptake by the diseased liver. It is unlikely that the increased levels are due to degradation of liver protein and the subsequent release of amino acids from the failing hepatocyte into the blood. This is true because the total protein content of the liver is only approximately 300 g. To account for the elevated amino acid levels in the blood, the entire protein content of the liver would have to be degraded within 6 to 8 hours to account for the increased protein turnover rates found. Because 18 to 20 times more protein is present in skeletal muscle (greater mass), the muscle is probably the major source of the elevated plasma levels of amino acids seen in catabolic states such as cirrhosis of the liver. 

We can also make some generalizations about amino acid metabolism in terms of the relationship of the carbon skeleton to the citric acid cycle and the related reactions of pyruvate and acetyl-GoA (Figure 23.7). The citric acid cycle is amphibolic it has a part in both catabohsm and anabolism. The anabolic aspect of the citric acid cycle is of interest in amino acid biosynthesis. The catabolic aspect is apparent in the breakdown of amino acids, leading to their eventual excretion, which takes place in reactions related to the citric acid cycle. 

The catabolism of amino acids is complex there are too many differences between the amino acids for any useful generalization to be made about the processes except for the turnover of the amino group. 

In general, silages of this type are characterised by having high pH values, usually within the range 5.0-7.0. The main fermentation acid present is either acetic or butyric acid. Lactic acid and residual water-soluble carbohydrates are present in low concentrations or are absent. The ammonia nitrogen levels are usually above 200 g/kg TN. This ammonia, which is derived from the catabolism of amino acids, is accompanied by other degradation products such as amines and various keto acids and fatty acids (see Table 19.1). 

The catabolism of nucleotides is generally more complex than that of amino acids, carbohydrates, or fatty acids because the structures of the nucleotides themselves are more complex. As a result, we ll treat the subject lightly and... 

In general, increases in habitual protein intake lead to increases in amino acid catabolism and deamination as well as an increase in the cycling protein gains and losses. " Interestingly, the amount of nitrogen loss, as urea or laek of absorption. 

Figure 1.3. Diagram of the proteolytic systems of lactic acid bacteria, (a) Extracellular components PrtP, cell-envelope proteinase PrtM, proteinase maduration protein Opp, oligopetide permease DtpT, the ion linked trasnsporter for di-and tripeptides and Opt, the ABC transporter for peptides, (b) Intracelullar components pool of about 20-25 peptidases, including general (PepN, PepC) and specific (PepX, PepQ) peptidases, and amino acid catabolic enzymes (carboxylases, aminotransferases, etc.).

The catabolism of proteins is much more complex than that of fats and carbohydrates because each of the 20 amino acids is degraded through its own unique pathway. The general idea, however, is that the amino nitrogen atom is removed and the substance that remains is converted into a compound that enters the citric acid cycle. 

In Saccharomyces cerevisiae, as in most eukaryotic cells, the plasma membrane is not freely permeable to nitrogenous compounds such as amino acids. Therefore, the first step in their utilization is their catalyzed transport across the plasma membrane. Most of the transported amino acids are accumulated inside the yeast cells against a concentration gradient. When amino acids are to be used as a general source of nitrogen, this concentration is crucial because most enzymes which catalyze the first step of catabolic pathways have a low affinity for their substrates. 

The regulation of NCR-sensitive amino acid transporters in Saccharomyces cerevisiae has many points in common with that of catabolic enzymes. Amino acid permeases, as well as some other transporters of nitrogenous nutrients, are integrated into the regulatory circuits, both general and specific, which control catabolic processes. 

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