Transamination and deamination

Amino nitrogen accumulates during protein degradation. In contrast to carbon, amino nitrogen is not suitable for oxidative energy production. If they are not being reused for biosynthesis, the amino groups of amino acids are therefore incorporated into urea (see p. 182) and excreted in this form. 

The ketimine (3) is hydrolyzed to yield the 2-oxoacid and pyridoxamine phosphate (4). 

In the second part of the reaction (see A, lb), these steps take place in the opposite direction pyridoxamine phosphate and the second 2-oxoacid form a ketimine, which is isomerized into aldimine. Finally, the second amino acid is cleaved and the coenzyme is regenerated. 

Among the NH2 transfer reactions, transaminations (1) are particularly important. They are catalyzed by transaminases, and occur in both catabolic and anabolic amino acid metabolism. During transamination, the amino group of an amino acid (amino acid 1) is transferred to a 2-oxoacid (oxoacid 2). From the amino acid, this produces a 2-oxo-acid (a), while from the original oxoacid, an amino acid is formed (b). The NH2 group is temporarily taken over by enzyme-bound pyridoxal phosphate (PLP see p. 106), which thus becomes pyridoxamine phosphate. 

In the absence of substrates, the aldehyde group of pyridoxal phosphate is covalently bound to a lysine residue of the transaminase (1). This type of compound is known as an aldimine or Schiffs base. During the reaction, amino acid 1 (A, la) displaces the lysine residue, and a new aldimine is formed (2). The double bond is then shifted by isomerization. 

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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 ... 

The citric acid cycle is not only a pathway for oxidation of two-carbon units—it is also a major pathway for interconversion of metabolites arising from transamination and deamination of amino acids. It also provides the substtates for amino acid synthesis by transamination, as well as for gluconeogenesis and fatty acid synthesis. Because it fimctions in both oxidative and synthetic processes, it is amphibolic (Figure 16—4). 

Try to acetyl-CoA via ring oxidation and cleavage to ketoadipate Lys to acetyl-CoA via transamination and deamination to ketoadipate... 

Glycogenolysis and glycogen synthesis P-oxidation of fatty acids transamination and deamination of amino acids Cori cycle and glucose-alanine cycle, which recycles substrates between muscle and liver. 

The physiological relevance together with chnical importance of transamination and deamination is wide-ranging. As an aid to understanding the somewhat complex nature of amino acid metabolism, it can be considered (or imagined) as a metabolic box (represented in Figure 8.13). Some pathways feed oxoacids into the box whereas others remove oxoacids and the ammonia that is released is removed to form urea. The box illustrates the role of transdeamination as central to a considerable amount of the overall metabolism in the liver cell (i.e. protein, carbohydrate and fat metabohsm, see below). 

Know in detail how amino acids can lose their nitrogen by transamination and deamination reactions and combination of the two know in detail how nitrogen is disposed of in the organism the alanine, glutamine, and urea cycles be able to recite the names of all enzymes, cofactors, and intermediates, and be familiar with their regulatory mechanisms be able to recognize and draw structures of all intermediates involved in these reactions. 

I. Which of the structures in the figure beiow is invoived in amino transferase (transamination and deamination) and decarboxyiation reactions of amino acids ... 

Pyridoxal (B ) Growth retardation Pyridoxal phosphate is a cofactor for transaminations and deaminations... 

Write equations for transamination and deamination reactions. (Section 14.9)... 

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