Pyridoxal-containing enzymes

Pyridoxal-containing enzymes accept only a single enantiomer of an amino acid. The bond oriented perpendicular to the n system of the Schiff base becomes most labile as shown in Figure 7.3.4 for deprotonation. Enzyrnes may not only control the stereochemistry of chemical reaction by orienting the amino acid substituents, but also select one out of the five reactions sketched in Scheme 7.3.15. 

The dehydro amino acid esters react with various aldehydes, including pyri-doxal, to form imines. These imines (4) are believed to be involved in several reactions of pyridoxal-containing enzymes. ... 

E. Ohler, E. Prantz and U. Schmidt (287, 352) have studied the addition of mercaptans to the arylidene-dehydrovaline esters (72) with reference to cysteine biosynthesis, which according to most recent ideas can proceed in two ways, catalysed by pyridoxal-containing enzymes (111, 414, see also 101). 

Oxidation of purine derivative, oxidation of allopurinol to alloxanthene FAD dependent oxidative deamination of monoamines, e.g. primaquine Pyridoxal dependent, copper containing enzyme. Conversion of allylamine to acrolein... 

L-Canaline is an ineffective antimetabolite of L-ornithine since it has little ability to antagonize ornithine-dependent reactions. On the other hand, it forms a covalently bound Schiff-base complex with the pyridoxal phosphate moiety of Bg-containing enzymes. As such it is a potent inhibitor of many decarboxylases and aminotransferases that utilize this vitamin. 

Canaline is a potent inhibitor of all seven pyridoxal phosphate-containing enzymes studied by Rahiala et (27) but it lacks adverse effects on three ornithine-utiTTzing enzymes lacking a Bg cofactor. Finally, in jack bean, Canavalia ensiformis, ornithine carbamoyl transferase can form 0-ureido-L-homoserine from canaline and carbamoyl phosphate as it does citrulline from ornithine and carbamoyl phosphate. Nevertheless, neither compound inhibited formation of the reaction products (31). 

Biotin-Containing Enzymes. 2. The Mechanism of Biotin Action 738 Box 14-C The Vitamin B6 Family Pyridoxine, Pyridoxal, and Pyridoxamine... 

The subsequent cleavage of cystathionine to yield cysteine, a-ketobutyrate and NH4+ is catalyzed by y-cystathionase, a pyridoxal-phosphate-containing enzyme. This transsulfura-tion pathway is one of the routes used for methionine catabolism. 

Fig. 5). This copper-containing enzyme requires the coenzyme pyridoxal phosphate, derived from vitamin B6 (see Topic M2), for activity. The aldehyde group on allysine then reacts spontaneously with either the side-chain amino group of Lys or with other allysine residues on other polypeptide chains to form covalent interchain bonds. 

A Saturation of Biochemical Function. A reliable biochemical indicator is required. For niacin, which NAD" - or NADP -containing enzyme should be selected Which transaminase will be the indicator for pyridox-ine Which function of vitamin A should be j selected for retinol, vision in the rods or cell differentiation As noted from Table 8.2, many of the assays for vitamin status have significant limitations to estimate reliable doses. 

Labilization of bonds of an amino acid bound to pyridoxal phosphate-containing enzymes. Given the appropriate apoenzyme, any atom or group on the carbon atom proximal to the Schiff base can be cleaved. 

Structures of compounds that inhibit pyridoxal phosphate-containing enzymes. 

Canaline is the product of the hydrolytic cleavage of canavanine with the simultaneous formation of urea. Canaline is an ornithine analogue which also shows neurotoxicity in the adult sexta where it adversely affects central nervous system functions (jj ). It also is a potent inhibitor of vitamin B -containing enzymes (20-22). It forms a stable Schiff base with the pyridoxal phosphate moiety of the enzyme and drastically curtails enzymatic activity. Pyridoxal phosphate-containing enzymes are vital to insects because they function in many essential transamination and decarboxylation reactions. Ornithine is an important metabolic precursor for insect production of glutamic acid and proline (23). 

The vitamin is transformed into pyridoxal phosphate (PLP), which, as a pyridine, is basic at the ring nitrogen, and in the active form, is V-protonated. Enzymes containing PLP have various functions, all connected with amino acids. Amongst other activities, PLP-containing enzymes can (i) effect transfer of an amino group from an a-amino acid to an a-keto acid (ii) bring about decarboxylation of an a-amino acid or... 

Combined use of microbial enzymes as biocatalysts with chemical synthesis has its origin in the steroid transformation developed in the USA in the early 1950s. Arima and his group [11] invented a unique microbial conversion process, in which the aliphatic side-chain of cholesterol was cleaved to produce a steroid core as a starting material for chemical synthesis of steroid hormones. Yamada et al. discovered the reverse reaction of the pyridoxal-containing L-amino acid lyases and applied them to synthesize L-tryptophan and l-DOPA [12] from pyruvate, ammonia and corresponding aromatic compounds. Since these early achievements, a variety of unique processes with newly screened microbial enzymes as biocatalysts have been invented. 

The most versatile of the coenzymes is perhaps pyridoxal phosphate (PEP). The PEP containing enzymes catalyze a wide variety of reactions such as racemization, transamination, [3- and a-decarboxylation, and interconversion of side chains. The first step of all these reactions is the transition between an internal aldimine intermediate to an external aldimine intermediate, which involves the condensation of PEP with an external amino acid substrate to form a Schiff base. The internal aldimine intermediate can then either undergo a-decarboxylation to convert the amino acid substrate into amines and aldehydes, or lose the a-hydrogen... 

All of the nitrogen in heme is derived from glycine and all of the carbons are derived from succinate and glycine. Thus, the process by which heme is synthesized is also called the succinate-glycine pathway. The first step in the process is catalyzed by a pyridoxal phosphate-containing enzyme, 5-aminolevulinic acid synthetase (ALA synthetase)... 

It is possible to write a simple mechanism for pyruvamide containing enzymes catalyzing decarboxylation reactions of a-amino acids, and it was therefore suspected that the pyruvamide was behaving in a manner analogous to that shown in Scheme 17. This reaction scheme bears many similarities to pyridoxal containing... 

Vitamin B6 is one of the most versatile enzyme cofactors. Pyridoxal phosphate-containing proteins are found in each lUB enzyme category except ligases (category 6). [Tong and Davis (1995) reported that 2-amino-3-ketobutyrate-CoA ligase is a pyridoxal phosphate enzyme. However, the... 

Mammalian tissues contain enzymes that catalyze the nonoxidative deamination of serine, threonine, and homoserine. Since the postulated reaction mechanism involves a dehydration before the deamination, these enzymes are called dehydrases. L-Serine, L-threonine, and L-homoserine dehydrases have been partially purified and all are specific for the L-amino acid. Serine and threonine dehydrases require pyridoxal phosphate, ATP, and glutathione for activity. Pyridoxal phosphate requires the homoserine enzyme, but the need for ATP and glutathione has not been demonstrated. The reaction is likely to involve the formation of a Schiff base. The homoserine dehydrase has been... 

In Scheme 13.40 and as noted above, the action of the iron-containing enzyme tyrosine 3-monooxygenase (EC 1.14.16.2) is shown to effect the conversion of tyrosine (Tyr, Y) and oxygen (O2) to 3,4-dihydroxyphenylalanine (L-dopa), while the cofactor tetrahydrobiopterin undergoes oxidation to 4a-hydroxytetrahydrobiop-terin. Then, the general aromatic-L-amino acid decarboxylase (EC 4.1.1.28), an enzyme that uses pyridoxal as a cofactor, effects the decarboxylation of the bisphe-noUc add to the corresponding amine, dopamine [3,4-dihydroxyphenethylamine, 2-(3,4-dihydroxyphenyl)ethanamine]. 

Orientational dependency of pyridoxal phosphate containing enzyme catalyzed reactions... 

Werle and Pechmann 184) have presented evidence that diamine oxidase from plants could be activated by pyridoxal. The rate of inhibition of enzyme systems by isonicotinic acid hydrazide has been used as a means of detecting the presence of pyridoxal phosphate as a coenzyme, particularly when it is tightly bound to the apoenzyme 205, 206). Davison 166) has shown that the rates of inhibition of an established pyridoxal phosphate containing enzyme, cysteinesulfinic acid decarboxylase 207, 208), and diamine oxidase are similar and that pyridoxal phosphate will reactivate diamine oxidase after inhibition with isonicotinic acid hydrazide. On this basis it has been suggested that pyridoxal phosphate is probably involved as a coenzyme for diamine oxidase. 

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