Transamination during

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. 

Fig. 12. Hypothetical stereochemical mechanism for abortive transamination during the glutamate decarboxylase reaction (261). The same active site base (BH ) is envisioned to be involved in both the normal decarboxylation reaction and abortive transamination.

PLP dependent enzymes belong to five enzyme classes transferases, hydrolases, lyases, isomerases and oxidoreductases. Nearly half of PLP-dependent enzymes are involved in important steps of amino acid metabolism (decarboxylation, a,p-elimination reactions, racemization and transamination). During the reaction of amino acids (and other amino compounds), the... 

A key step in the synthesis of the spiroketal subunit is the convergent union of intermediates 8 and 9 through an Evans asymmetric aldol reaction (see Scheme 2). Coupling of aldehyde 9 with the boron enolate derived from imide 8 through an asymmetric aldol condensation is followed by transamination with an excess of aluminum amide reagent to afford intermediate 38 in an overall yield of 85 % (see Scheme 7). During the course of the asymmetric aldol condensation... 

Macko, S.A., Estep, M.L.E., Engel, M.H. and Hare, RE. 1986 Kinetic fractionation of stable nitrogen isotopes during amino acid transamination. Geochimica et Cosmochimica Acta 50 2143-2146. 

Benzannulated NHPs are straightforwardly accessible from AUV-disubsti luted o-phenylenediamines either via base-induced condensation with substituted dichlorophosphines [25] or PC13 [26], or via transamination with tris(dialkylamino) phosphines [13, 14, 27], respectively. An analogous NH-substituted derivative was obtained in low yield via transamination of o-phcnylcncdiaminc with ethoxy-bis(diethylamino)phosphine [28], and condensation of o-phenylenediamine with excess tris(diethylamino)phosphine furnished a l,3-bis(phosphino)-substituted heterocycle [29], Intermediates with one or two NH functions were detectable by spectroscopy but could not be isolated in pure form under these conditions. However, 2-chloro-benzo-l,3,2-diazaphospholene and the corresponding 1-phenyl derivative were prepared in acceptable yield via condensation of PC13 with o-phenylenediamine under microwave irradiation [30], or with A-phenyl-o-phenylenediamine under reflux [27], respectively, in the absence of additional base. The formation of tetrameric benzo-NHPs during transamination of A-alkyl-o-phenylenediamines with P(NMe2)3 has already been mentioned (cf. the section entitled 1,3,2-Diazaphospholes and 1,3,2-Diazaphospholides ). 

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. 

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. 

Back in 1937, Braunstein and Kritzman and, independently, Herbst, had proposed transamination might proceed via a Schiff s base formation. The essential lability of the H atom on the a-C atom was shown with deuterium labelling (1942). a-2H alanine released 2H into the medium during transamination. The label did not appear in glutamate, the end-product. 

In a muscle at rest, most of the 2-oxo acids produced from transamination of branched chain amino acids are transported to the liver and become subject to oxidation in reactions catalysed by branched-chain 2-oxo acid dehydrogenase complex. During periods of exercise, however, the skeletal muscle itself is able to utilize the oxo-acids by conversion into either acetyl-CoA (leucine and isoleucine) or succinyl-CoA (valine and isoleucine). 

For some amino acids, the transamination reaction occurs during the course of, rather than at the beginning of, the catabolic pathway (Appendix 8.3). 

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. 

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. 

In addition to pulmonary toxicity, nausea/vomiting, lactic acidosis and transaminitis, UCN-01 induced insulin resistance during Phase I clinical trials. As shown recently with rat adipose cells, this effect may be due to UCN-01 inhibition of PKB Thr-308 phosphorylation—no effect on Ser-473 was observed in this study—and subsequent blockade of GLUT4 translocation in response to insulin [104]. If this mode of action is confirmed in the ongoing clinical trials and contrary to what was observed in the PDKl hypomorphic mice (vide supra) [100], insulin resistance may represent an important hurdle in the development of PDKl inhibitors and, in general, of any agent that blocks the PI3K/PKB pathway in adipose and muscle cells. 

Figure 9-1. Molecular interconversions in handling of ammonia. The major enzyme responsible for interconversion of glutamate and a-ketoglutarate is glutamate dehydrogenase. No free ammonia is ever present during direct transfer of amino groups from alanine or aspartate via transamination to produce glutamate. ALT, alanine aminotransferase AST, aspartate aminotransferase.

Glutamate synthase is not present in animals, which, instead, maintain high levels of glutamate by processes such as the transamination of a-ketoglutarate during amino acid catabolism. 

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