Pyridoxine-5 -phosphate synthase

Yeh, J.I. et al (2002) Multistate binding in pyridoxine 5 -phosphate synthase 1.96 A crystal structure in complex with 1-deoxy-D-xylulose phosphate. Biochemistry, 41 (39), 11649-11657. 

Vitamin Bi is an essential co-factor for several enzymes of carbohydrate metabolism such as transketolase, pyruvate dehydrogenase (PDH), pyruvate decarboxylase and a-ketoglutarate dehydrogenase. To become the active co-factor thiamin pyrophosphate (TPP), thiamin has to be salvaged by thiamin pyrophosphokinase or synthesized de novo. In Escherichia coli and Saccharomyces cerevisiae thiamin biosynthesis proceeds via two branches that have to be combined. In the pyrimidine branch, 4-amino-5-hydroxymethy-2-methylpyrimidine (PIMP) is phosphorylated to 4-amino-2-methyl-5-hydroxymethyl pyrimidine diphosphate (PIMP-PP) by the enzyme HMP/HMP-P kinase (ThiD) however, the step can also be catalyzed by pyridoxine kinase (PdxK), an enzyme also responsible for the activation of vitamin B6 (see below). The second precursor of thiamin biosynthesis, 5-(2-hydroxyethyl)-4-methylthiazole (THZ), is activated by THZ kinase (ThiM) to 4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P), and then the thia-zole and pyrimidine moieties, HMP-PP and THZ-P, are combined to form thiamin phosphate (ThiP) by thiamin phosphate synthase (ThiE). The final step, pyrophosphorylation, yields TPP and is carried out by thiamin pyrophosphorylase (TPK). 

In the first path, the PLP precursor pyridoxine 5-phosphate (PNP) is biosynthesized from 3-hydroxy-1-aminoacetone phosphate 1 and DXP 2 (Figure 6.1). DXP 2 is formed from pyruvate 3 and glyceraldehyde 3-phosphate 4, catalyzed by deoxy-D-xylulose 5-phosphate synthase (DXS). The 3-hydroxy-1-aminoacetone phosphate 1 is obtained from the erythrose 4-phosphate 5 in four steps. The first step involves the oxidation of erythrose 4-phosphate 5, mediated by erythrose 4-phosphate dehydrogenase (GapB), to erythronate 4-phosphate 6. The latter is further oxidized by D-erythronate 4-phosphate dehydrogenase (PdxB) to 3-hydroxy-4-phosphohydroxy-a-ketobutyrate 7. Transamination reaction between... 

Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin Bs), which is a cofactor for the cystathionine synthase reaction. Some patients respond to the administration of pharmacological doses of pyridoxine (25-100 mg daily) with a reduction of plasma homocysteine and methionine. Pyridoxine responsiveness appears to be hereditary, with sibs tending to show a concordant pattern and a milder clinical syndrome. Pyridoxine sensitivity can be documented by enzyme assay in skin fibroblasts. The precise biochemical mechanism of the pyridoxine effect is not well understood but it may not reflect a mutation resulting in diminished affinity of the enzyme for cofactor, because even high concentrations of pyridoxal phosphate do not restore mutant enzyme activity to a control level. 

Homocysteine is metabolized in the liver, kidney, small intestine and pancreas also by the transsulfuration pathway [1,3,89]. It is condensed with serine to form cystathione in an irreversible reaction catalyzed by a vitamin B6-dependent enzyme, cystathionine-synthase. Cystathione is hydrolyzed to cysteine that can be incorporated into glutathione or further metabolized to sulfate and taurine [1,3,89]. The transsulfuration pathway enzymes are pyridoxal-5-phosphate dependent [3,91]. This co-enzyme is the active form of pyridoxine. So, either folates, cobalamin, and pyridoxine are essential to keep normal homocysteine metabolism. The former two are coenzymes for the methylation pathway, the last one is coenzyme for the transsulfuration pathway [ 1,3,89,91 ]. 

Cystathionine synthase has pyridoxal phosphate as cofactor a radically different form of treatment was introduced in 1967 giving pyridoxine at a dosage level of 50 to 200 mg per day [40]. The concentrations of methionine and homocysteine in the blood, and of homocystine and homocysteine-cysteine mixed disulphide in the urine, fell sharply on such treatment in... 

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