Transamination reactions nitrogen removal

Overall, in a transamination reaction, an amino group from one amino acid becomes the amino group of a second amino acid. Because these reactions are readily reversible, they can be used to remove nitrogen from amino acids or to transfer nitrogen to a-keto acids to form amino acids. Thus, they are involved both in amino acid degradation and in amino acid synthesis. 

The biological importance of this enzyme has already been discussed (section II). Its role in producing glutamate, as the first organic amino compound, in bacteria and plants seems reasonably well established. In animals, which do not have the ability to produce all amino compounds from a simple nitrogen source, the enzyme seems to be concerned with the removal of excess amino compounds (equations 2-5) as well as with the production of glutamate for conversion to the acid amide (section V.B) or to take part in transamination reactions to form the non-essential amino compounds. Cellular control of the direction in which reaction occurs may well lie in the ratio of the concentrations of the oxidised and reduced forms of the cofactor. This ratio is not a fixed quantity but depends on the metabolic activity of the cell (NAD and NADP are cofactors for many oxidation-reduction reactions) as well as on the availability of molecular oxygen for the terminal step in respiration, by which the reduced cofactor is reoxidised by the cytochrome system (section IV.A.1). 

Proposed mechanism for the Pd/C-catalyzed oxidative synthesis of 2-substituted benzimidazoles was depicted in Scheme 14.11. The initial step seems to be the insertion of palladium into a C-H adjacent to the nitrogen, leading to a highly active intermediate complex of an iminium ion. The proposed catalytic cycle involves the removal of a hydride from tertiary amine by a metal catalyst to form (putative) oxidized iminium cation species, which undergoes a transamination reaction with o-phenylenediamine to generate intermediate imine, with generation of Imol equivalent of H2. Subsequent formation of... 

Figure 11.3 is a flow model representing in extremely simple form the main relevant features of nitrogen metabolism. It is not difficult to propose a sufficient explanation why Bprot is isotopically heavier than the diet. We might expect that the net effect of transamination and deamination of amino acids is to remove isotopically lighter N (Macko et al. 1987). That is to say, we may expect that the equilibrium constant for the reaction ... 

Removal of a-amino nitrogen by transamination (see Figure 28-3) is the first catabolic reaction of amino acids except in the case of proline, hydroxyproline, threonine, and lysine. The residual hydrocarbon skeleton is then degraded to amphibolic intermediates as outhned in Figure 30-1. 

Transamination is the most common initial reaction of amino acid catabohsm. Subsequent reactions remove any additional nitrogen and restmcmre the hydrocarbon skeleton for conversion to oxaloacetate, a-ketoglutarate, pyruvate, and acetyl-CoA. 

Glutamate can then participate in the formation of other amino acids via the process called transamination. Transamination is the exchange of the amino group from an amino acid to a keto acid, and provides the most common process for the introduction of nitrogen into amino acids, and for the removal of nitrogen from them. The reaction is catalysed by a transaminase enzyme, and the coenzyme pyridoxal phosphate (PLP) is required. 

The dominant reactions involved in removing amino acid nitrogen from the body are known as transaminations. This class of reactions funnels nitrogen from all free amino acids into a small number of compounds then, either they are oxidatively deaminated, producing ammonia, or their amine groups are converted to urea by the urea cycle. 

The first step in the catabolism of most amino acids is the transfer of the o-amino group from the amino acid to a-ketoglutarate (tx-KG). This process is catalyzed by transaminase (aminotransferase) enzymes that require pyridoxal phosphate as a cofactor. The products of this reaction are glutamate (Glu) and the a-ketoacid analog of the amino acid destined for catabolic breakdown. For example, aspartate is converted to its a-keto analog, oxalo-acetate, by the action of aspartate transaminase (AST), which also produces Glu from a-KG. The transamination process is freely reversible, and the direction in which the reaction proceeds is dependent on the concentrations of the reactants and products. These reactions do not effect a net removal of amino nitrogen the amino group is only transferred from one amino acid to another. 

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