Transamination unusual

A transamination route, as shown in Equation (8.4) (SiRa = SiMes, Si(Bu )Me2), °has afforded complexes with unusual trigonal, monopyramidal coordination at aluminium. 

Lysine, an exclusively ketogenic amino acid, is unusual in that nei ther of its amino groups undergoes transamination as the first step in catabolism. Lysine is ultimately converted to acetoacetyl CoA. 

Alanine, aspartate, and glutamate are synthesized by transfer of an amino group to the a-keto acids pyruvate, oxaloacetate, and a-keto-glutarate, respectively. These transamination reactions (Figure 20.12, and see p. 248) are the most direct of the biosynthetic pathways. Glutamate is unusual in that it can also be synthesized by the reverse of oxidative deamination, catalyzed by glutamate dehydrogenase (see p. 249). 

Absorption bands at 500 nm. With many PLP enzymes certain substrates and inhibitors cause the appearance of intense and unusually narrow bands at 500 nm. Such a band is observed with aspartate aminotransferases acting on eryf/zro-3-hydroxyaspartate (Fig. 14-9). This substrate undergoes transamination very slowly, and the 500-nm absorbing form which accumulates is probably an intermediate in the normal reaction sequence. A similar spectrum is produced by tryptophan indole-lyase acting on the competitive inhibitor L-alanine. Under the same conditions the... 

Returning to the major tryptophan catabolic pathway, marked by green arrows in Fig. 25-11, formate is removed hydrolytically (step c) from the product of tryptophan dioxygenase action to form kynurenine, a compound that is acted upon by a number of enzymes. Kynureninase (Eq. 14-35) cleaves the compound to anthranilate and alanine (step d), while transamination leads to the cyclic kynurenic acid (step e). Hie latter is dehydroxylated in an unusual reaction to quinaldic acid, a prominent urinary excretion product. 

The glutamate produced by transamination is oxidatively deaminated by glutamate dehydrogenase to produce ammonia. This enzyme is unusual in being able to use either NAD+ or NADP+, and is subject to allosteric regulation. GTP and ATP are allosteric inhibitors, whereas GDP and ADP are allosteric activators. 

Dialkylglycine decarboxylase (DGD) is unusual as it catalyzes both a decarboxylation and a transamination during its normal catalytic cycle. DGD uses stereoelectronic effects to control its unusual reaction specificity. The three-dimensional structure of DGD showed that the enzyme possesses two binding sites for monovalent cations (MVCs). In particular, one site, located near the active site, hinds potassium ions and controls the catalytic activity. The other is located at the carboxyl terminus of an alpha helix and prohahly has a structural role. 

Some bacteria employ an unusual pathway in which glutamine is incorporated into gln-tRNA without a glutamine-specific aminoacyl tRNA-synthetase. In this case, the synthetase for glutamate is used and glutamine is created by transamination, as follows ... 

Other unusual sugars " are formed from intermediates in Eq. 20-11. One is a 3-amino-3,4,6-trideoxyhexose in which the amino group has been provided by transamination (see also Box 20-B). 

Ornithine is an amino acid. However, it is not incorporated into proteins during the process of protein synthesis because no genetic codon exists for this amino acid. Although ornithine is normally regenerated by the urea cycle (one of the products of the arginase reaction), ornithine also can be synthesized de novo if needed. The reaction is an unusual transamination reaction catalyzed by ornithine aminotransferase under specific conditions in the intestine (Fig. 38.14). The usual direction of this reaction is the formation of glutamate semialdehyde, which is the first step of the degradation pathway for ornithine. 

Figure 23.1 Transamination. This process moves the amino group from one keto acid to another. Glutamate and cr-ketoglutarate are always one of the two pairs involved, so that glutamate serves as a clearing house for amino groups. The R in the two compounds on the left is not an unusual atom. It is the chemist s shorthand way of generalising - i.e. this could be any amino acid sidechain.

The molecular participation of the coenzyme in transamination has been partially elucidated. Transaminases are enzymes that catalyze the reversible transfer of an amino group from an amino acid to a keto acid. The most common transaminases are involved in the transfer of the a-amino group of an a-amino acid to the a-keto acid, but a number of unusual transaminases have been described. 

Dr. Davis had established that this product was not utilized by mutants blocked in the biosynthesis of phenylalanine, but that it was converted with extraordinary ease, on the slightest exposure to acidic conditions, into phenylpyruvic acid, C9H8O3 this acid was already known to be transformed into phenylalanine 3 vivo by transamination. The new metabolite was thus tentatively assumed to be an unusually sensitive derivative or conjugate of phenylpyruvic acid. 

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