Tryptophan transamination

Vitamin Ba (pyridoxine, pyridoxal, pyridoxamine) like nicotinic acid is a pyridine derivative. Its phosphorylated form is the coenzyme in enzymes that decarboxylate amino acids, e.g., tyrosine, arginine, glycine, glutamic acid, and dihydroxyphenylalanine. Vitamin B participates as coenzyme in various transaminations. It also functions in the conversion of tryptophan to nicotinic acid and amide. It is generally concerned with protein metabolism, e.g., the vitamin B8 requirement is increased in rats during increased protein intake. Vitamin B6 is also involved in the formation of unsaturated fatty acids. 

The precursors of true alkaloids and protoalkaloids are aminoacids (both their precursors and postcursors), while transamination reactions precede pseudoalkaloids (Tables 1 and 10). It is not difficult to see that from all aminoacids only a small part is known as alkaloid precursors (Table 19). Both true and proto alkaloids are synthesized mainly from the aromatic amino acids, phenylalanine, tyrosine (isoquinoline alkaloids) and tryptophan (indole alkaloids). Lysine is the... 

It is involved as a coenzyme (pyridoxal phosphate) in metabolism of tryptophan, in several metabolic transformations of amino acids including transamination, decarboxylation and racemization. 

Pyridoxal phosphate is the coenzyme for the enzymic processes of transamination, racemization and decarboxylation of amino-acids, and for several other processes, such as the dehydration of serine and the synthesis of tryptophan that involve amino-acids (Braunstein, 1960). Pyridoxal itself is one of the three active forms of vitamin B6 (Rosenberg, 1945), and its biochemistry was established by 1939, in considerable part by the work of A. E. Braunstein and coworkers in Moscow (Braunstein and Kritzmann, 1947a,b,c Konikova et al 1947). Further, the requirement for the coenzyme by many of the enzymes of amino-acid metabolism had been confirmed by 1945. In addition, at that time, E. E. Snell demonstrated a model reaction (1) for transamination between pyridoxal [1] and glutamic acid, work which certainly carried with it the implication of mechanism (Snell, 1945). 

The carbon skeletons of six amino acids are converted in whole or in part to pyruvate. The pyruvate can then be converted to either acetyl-CoA (a ketone body precursor) or oxaloacetate (a precursor for gluconeogenesis). Thus amino acids catabolized to pyruvate are both ke-togenic and glucogenic. The six are alanine, tryptophan, cysteine, serine, glycine, and threonine (Fig. 18-19). Alanine yields pyruvate directly on transamination with... 

In plants and bacteria, phenylalanine and tyrosine are synthesized from chorismate in pathways much less complex than the tryptophan pathway. The common intermediate is prephenate (Fig. 22-19). The final step in both cases is transamination with glutamate. 

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. 

Thus we designed and synthesized a bicyclic pyridoxamine derivative carrying an oriented catalytic side arm (16) [11], Rates for conversion of the ketimine Schiff base into the aldimine, formed with 26 (below) and a-ketovaleric acid, indolepyruvic acid, or pyruvic acid, were enhanced 20-30 times relative to those carried out in the presence of the corresponding pyridoxamine derivatives without the catalytic side arm. With a-ketovaleric acid, 16 underwent transamination to afford D-norvaline with 90% ee. The formation of tryptophan and alanine from indolepyruvic acid and pyruvic acid, respectively, showed a similar preference. A control compound (17), with a propylthio group at the same stereochemical position as the aminothiol side arm in 16, produced a 1.5 1 excess of L-norvaline, in contrast to the large preference for D-amino acids with 16. Therefore, extremely preferential protonation seems to take place on the si face when the catalytic side arm is present as in 16. 

The catabolism of lysine merges with that of tryptophan at the level of (3-ketoadipic acid. Both metabolic pathways are identical from this point on and lead to the formation of acetoacetyl-CoA (Figure 20.21). Lysine is thus ketogenic. It does not transaminate in the classic way. Lysine is a precursor of carnitine the initial reaction involves the methylation of e-amino groups of protein-bound lysine with SAM. The N-methylated lysine is then released proteolytically and the reaction sequence to carnitine completed. See Equation (19.6) for the structure of carnitine. 

For two transaminases the remaining unknown stereochemical parameter was determined by demonstrating an internal transfer of tritium (dialkyl amino acid transaminase) [28] or deuterium (pyridoxamine-pyruvate transaminase) [27] from the a-position of the substrate L-alanine to C-4 of the cofactor. Internal hydrogen transfer from the a-position of the substrate amino acid to C-4 of PLP has also been demonstrated for two of the abortive transamination reactions, those catalyzed by tryptophan synthase fi2 protein [32] and by aspartate-/8-decarboxylase [31]. In addition, the same phenomenon must occur in alanine transaminase, as deduced from the observation that the enzyme catalyzes exchange of the /8-hydrogens of... 

Under normal conditions, the rate-limiting enzyme of the pathway is tryptophan dioxygenase (Section 8.3.2), and there is hide accumulation of intermediates. Kynurenine transaminase, the enzyme which catalyzes the transamination and ring closure of kynurenine to kynurenic acid, and of hydroxykynurenine to xanthurenic acid, has a high relative to the normal steady-state concentrations of its substrates in the liver. Kynureninase and kynurenine hydroxylase have lower values of K, so that there is normally litde accumuladon of kynurenine or hydroxykynurenine. 

Transamination is just one of a wide range of amino acid transformations that are catalyzed by PLP enzymes. The other reactions catalyzed by PLP enzymes at the a-carbon atom of amino acids are decarboxylations, deam-inations, racemizations, and aldol cleavages (Figure 23.12). In addition, PLP enzymes catalyze elimination and replacement reactions at the P-carbon atom (e.g., tryptophan synthetase Section 24.2.11) and the y-carbon atom (e.g., cytathionine P-synthase, Section 24.2.9) of amino acid substrates. Three common features of PLP catalysis underlie these diverse reactions. 

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