Pyridoxal phosphate enzyme requirements

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

Whereas many cognate apoenzymes can be reconstituted with iron/sulfur clusters by simple and essentially alchemistic procedures using Fe + and sulfide ions under anaerobic conditions, a highly complex enzymatic machinery is used in vivo for the synthesis of iron/sulfur clusters and their transfer to the target enzymes. Sulfide ions required for cluster synthesis are obtained from cysteine (1) via a persulfide of a protein-bound cysteine residue (2) pyridoxal phosphate is required for the formation of the persulfide intermediate (Fig. 1) (12). 

Aminomutases. The enzymes L-p-lysine mutase (which is also D-a-lysine mutase) and D-omithine mutase catalyze the transfer of an co-amino group to an adjacent carbon atom (Table 16-1). Two proteins are needed for the reaction pyridoxal phosphate is required and is apparently directly involved in the amino group migration. In the P-lysine mutase the 6-amino group of L-P-lysine replaces the pro-S hydrogen at C-5 but with inversion at C-5 to yield (3S, 5S)-... 

Pyridoxal phosphate is required for the enzymes catalyzing the reaction... 

Experiments made on pyridoxal-deficient animals suggested that pyridoxal phosphate and iron are required for the biosynthesis of porphyrin. It was later established that pyridoxal phosphate is required for the formation of -amino levulinic acid, probably by participating in the formation of active glycine. The condensation of succinate and glycine leads to the formation of a very labile a-amino-j -keto adipic acid. The participation of a-amino-jS-keto adipic acid as an intermediate in the reaction was established in experiments proving the acid to be an efficient precursor of porphyrin biosynthesis in vitro. An enzyme system capable of catalyzing the succinyl CoA-glycine condensation and the decarboxylation of the intermediate to yield amino levulinic acid has also been obtained from a particular fraction of chicken erythrocyte. In liver, an enzyme has been found in the mitochondria [132]. 

This is the excretion product of 3-hydroxykynurenic acid, an intermediate in the conversion of tryptophan to nicotinic acid. Pyridoxal phosphate is required as a cofactor for the enzyme, kynureninase, which catalyses the conversion of 3-hydroxykynurenic acid to 3-hydroxyanthrinilic acid. In patients with pyridoxine deficiency, 3-hydroxykynurenic acid accumulates and is excreted in the urine as xanthurenic acid. Xanthurenic acid can therefore be measured in urine (especially after giving an oral typtophan load) in order to detect pyridoxine deficiency. 

The enzyme is specific for the L-threo form of the amino acid. As found with other enzymes of this type, pyridoxal phosphate is required as a coenzyme and free sulfhydryl groups are essential for its activity. 

Hydrolases represent a significant class of therapeutic enzymes [Enzyme Commission (EC) 3.1—3.11] (14) (Table 1). Another group of enzymes with pharmacological uses has budt-ia cofactors, eg, in the form of pyridoxal phosphate, flavin nucleotides, or zinc (15). The synthases, and other multisubstrate enzymes that require high energy phosphates, are seldom available for use as dmgs because the required co-substrates are either absent from the extracellular space or are present ia prohibitively low coaceatratioas. 

This soluble enzyme requires pyridoxal phosphate for the conversion of L-dopa to 3,4-dihydroxyphenylethyl-amine (dopamine). Compounds that resemble L-dopa, such as a-methyldopa, are competitive inhibitors of this reaction. a-Methyldopa is effective in treating some kinds of hypertension. 

GOT (AST is the more recent abbreviation) catalyzes the transamination of 1-aspartic acid in the presence of a-ketoglut-aric acid, with pyridoxal phosphate being a required co-enzyme. The reaction is ... 

By contrast, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (Am 4x 10 " M) than its synthesis and indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e.g. tryptophan and tyrosine) and in view of its low substrate specificity is known as a general L-aromatic amino-acid decarboxylase. 

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

GABA transaminase is a mitochondrial enzyme which, like GAD, requires pyridoxal phosphate as co-factor. It is present in both neurons and glia and while secondary to... 

Hydrogen sulfide is a well known general metabolite produced on sulfate reduction by certain bacteria. Moreover, organic forms of sulfur can give rise to HS , hence H2S in certain bacteria. Thus, cysteine desulfhydrase (EC 4.4.1.1, cystathionine y-lyase) converts L-cysteine to H2S, pyruvate, and NH3. This enzyme shows a requirement for pyridoxal phosphate and the unstable ami-noacrylic acid is an intermediate (Equation 1) in the reaction ... 

This pyridoxal phosphate-requiring enzyme has been studied in several bacteria and X-ray crystal structures are available.35 The coryneform bacterium, Brevi-bacterium linens, is common on the surface of several cheeses, including Limburger and those of the Trappist type. The methionine y-lyase of this organism has been purified to homogeneity36 and the relevant gene, mgl (from MGL, abbreviation for methionine y-lyase) has been cloned and analyzed.37... 

These enzymes invariably involve a cofactor, pyridoxal phosphate (vitamin B6). In addition, pyridoxal phosphate is also required for most decarboxylations, racemizations, or elimination reactions in which an amino acid is a substrate. Pyridoxal phosphate is not involved in decarboxylations in which the substrate is not an amino acid. So if a question... 

Pyridoxal Phosphate.—Analogues of pyridoxal and pyridoxamine 5 -phosphates have frequently been used to probe the size and shape of the active sites of a number of enzymes. For example, the apoenzyme of a tryptophanase from Bacillus alvei will bind pyridoxal 5 -phosphate as well as the 2-nor, 2 -methyl, 2 -hydroxy, 6-methyl, and A-oxide analogues.27 No analogue that has been modified at C-4 binds to the enzyme, confirming the absolute requirement for Schiff-base formation between the... 

As indicated in Section 6.3.3 and Table 6.2 the key control step is mediated by glycogen phosphorylase, a homodimeric enzyme which requires vitamin B6 (pyridoxal phosphate) for maximum activity, and like glycogen synthase (Section 6.2) is subject to both allosteric modulation and covalent modification. 

Muscle glycogen phosphorylase is one of the most well studied enzymes and was also one of the first enzymes discovered to be controlled by reversible phosphorylation (by E.G. Krebs and E. Fischer in 1956). Phosphorylase is also controlled allosterically by ATP, AMP, glucose and glucose-6-phosphate. Structurally, muscle glycogen phosphorylase is similar to its hepatic isoenzyme counterpart composed of identical subunits each with a molecular mass of approximately 110 kDa. To achieve full activity, the enzyme requires the binding of one molecule of pyridoxal phosphate, the active form of vitamin B6, to each subunit. 

Both muscle and liver have aminotransferases, which, unlike deaminases, do not release the amino groups as free ammonium ion. This class of enzymes transfers the amino group from one carbon skeleton (an amino acid) to another (usually a-ketoglutarate, a citric acid cycle intermediate). Pyridoxal phosphate (PLP) derived from vitamin is required to mediate the transfer. 

Following the synthesis of 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, the enzyme aromatic amino acid decarboxylase (also known as 5-HTP or dopa decarboxylase) then decarboxylates the amino acid to 5-HT. L-Aromatic amino acid decarboxylase is approximately 60% bound in the nerve terminal and requires pyridoxal phosphate as an essential enzyme. 

Glutamate can then participate in the formation of other amino acids via the process called transamination. Transamination is the exchange of the amino group from an amino acid to a keto acid, and provides the most common process for the introduction of nitrogen into amino acids, and for the removal of nitrogen from them. The reaction is catalysed by a transaminase enzyme, and the coenzyme pyridoxal phosphate (PLP) is required. 

The active form of vitamin Be, pyridoxai phosphate, is the most important coenzyme in the amino acid metabolism (see p. 106). Almost all conversion reactions involving amino acids require pyridoxal phosphate, including transaminations, decarboxylations, dehydrogenations, etc. Glycogen phosphory-lase, the enzyme for glycogen degradation, also contains pyridoxal phosphate as a cofactor. Vitamin Be deficiency is rare. 

This enzyme [EC 4.4.1.4], also known as alliinase and cysteine sulfoxide lyase, catalyzes the conversion of an 5-alkyl-L-cysteine 5-oxide to an alkyl sulfenate and 2-aminoacrylate. The enzyme requires pyridoxal phosphate. 

This enzyme [EC 2.6.1.39] catalyzes the reversible reaction of 2-aminoadipate with 2-oxoglutarate (or, a-ketoglutarate) to generate 2-oxoadipate and glutamate. The enzyme requires pyridoxal phosphate. 

This enzyme [EC 5.1.1.9] catalyzes the interconversion of L- and D-arginine. The enzyme requires pyridoxal phosphate as a cofactor. 

This enzyme [EC 2.6.1.57] catalyzes the reversible reaction of an aromatic amino acid with a-ketoglutarate to generate an aromatic oxo acid and glutamate. Pyridoxal phosphate is a required cofactor. Methionine can also act as a weak substrate, substituting for the aromatic amino acid. Oxaloacetate substitutes for a-ketoglutarate. 

This enzyme [EC 4.1.1.12], also known as desulfinase, catalyzes the conversion of aspartate to alanine and carbon dioxide. Pyridoxal phosphate is a required cofactor. The enzyme will also catalyze the decarboxylation of aminomalonate as well as the desulfination of 3-sulfino-alanine to sulfite and alanine. 

This enzyme [EC 4.1.99.1], also known as L-tryptophan indole-lyase, catalyzes the hydrolysis of L-tryptophan to generate indole, pyruvate, and ammonia. The reaction requires pyridoxal phosphate and potassium ions. The enzyme can also catalyze the synthesis of tryptophan from indole and serine as well as catalyze 2,3-elimination and j8-replacement reactions of some indole-substituted tryptophan analogs of L-cysteine, L-serine, and other 3-substituted amino acids. 

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