Deamination during transamination

A very early metabolic event during imbibition, occurring in less than 15 min, appears to be the reformation of keto acids from amino acids by deamination and transamination reactions [37]. Keto acids important for respiratory pathways (e.g. a-ketoglutarate and pyruvate) may be absent from dry seeds such as wheat [70] and peanut cotyledons [138]. They are known to be chemically unstable and it has been suggested [37] that they are stored in the dry seed as the appropriate amino acid, and then reformed on rehydration. This might occur also in barley embryos, seeds of Sinapis alba, and axes and cotyledons of Phaseolus vulgaris [36, 37]. 

Since the site of modification on cytosine bases is at a hydrogen bonding position in double helix formation, the degree of bisulfite derivatization should be carefully controlled. Reaction conditions such as pH, diamine concentration, and incubation time and temperature affect the yield and type of products formed during the transamination process. At low concentrations of diamine, deamination and uracil formation dramatically exceed transamination. At high concentrations of diamine (3M), transamination can approach 100 percent yield (Draper and Gold, 1980). Ideally, only about 30-40 bases should be modified per 1,000 bases to assure hybridization ability after derivatization. 

During the degradation of most amino acids, the a-amino group is initially removed by transamination or deamination. Various mechanisms are available for this, and these are discussed in greater detail in B. The carbon skeletons that are left over after deamination undergo further degradation in various ways. 

AMINO ACID INTERCONVERSIONS Little is known of the precise amino acid requirements of cestodes. Presumably, most, if not all, their required amino acids can be provided by the host and, as discussed above, cestodes have developed numerous transport systems to acquire these compounds. However, cestodes appear to have a limited ability to catabolise amino acids. This is exemplified by H. diminuta, which has been shown (918) to generate significant 14C02 only from relabelled aspartate and, to a lesser extent, alanine during incubation in vitro with 10 different amino acids (Table 6.6). The few amino acids that are catabolised in cestodes participate in two main pathways, namely transamination and oxidative deamination. 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

The removal of the a-amino group from amino acids involves two types of biochemical reactions transamination and oxidative deamination. Both reactions have been described (Section 14.2). (Recall that transamination reactions occupy important positions in nonessential amino acid synthesis.) Because these reactions are reversible, amino groups are easily shifted from abundant amino acids and used to synthesize those that are scarce. Amino groups become available for urea synthesis when amino acids are in excess. Urea is synthesized in especially large amounts when the diet is high in protein or when there is massive breakdown of protein, for example, during starvation. 

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