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Evidence for the proximity of a cysteinyl and a tyrosyl residue in the active sita of 6-phosphogluconate dehydrogenase
No full textThe treatement of 6-phosphogluconate dehydrogenase from Candida utilis with Dansyl chloride causes the modification of one amino acid residue per enzyme subunit and the inactivation of the enzyme. Either a cysteine or a tyrosine residuecan be modified, depending on the pH of the reaction mixture. The dansyl residue can be transferred fron one residue to the other suggesting that the vtwo amino acid residues are close in the tridimensional structure of the active site of the enzym
6-phosphogluconate dehydrogenase: structural symmetry and functional asymmetry
No full text6-phosphogluconate dehydrogenase catalyzes the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate through an oxidation, a decarboxylation and a tautomerization. The two subunits in the crystals of the dimeric sheep liver enzyme have the same conformation, as the apoenzyme and with the substrate or coenzyme. An hypothesis is now advanced that in solution, during catalysis, the two subunits have a different alternating role and thus a functional asymmetry
The stabilization by a coenzyme analog of a conformational change induced by substrate in 6-phosphogluconate dehydrogenase
No full textThe reaction of NADP+ with periodate yields a coenzyme analog that can be bound to the NADP+ binding site of 6-phosphogluconate dehydrogenase from Candida utilis. This coenzyme analog can be irreversibly bound to the enzyme by reduction with sodium borohydride. The binding of one molecule of inhibitor to only one of the two subunits of the enzyme causes the inactivation of this subunit but does not alter the catalytic activity of the other subunit. Thus the two subunits do not have apparent catalytic interactions. When the reaction between the enzyme and the coenzyme analog is carried out in the presence of the substrate, the covalent modification of only one subunit causes the inactivation of both subunits. In this case the two subunits show an extreme negative cooperativity. It is suggested that the binding of the substrate induces in the enzyme molecule a conformational change that is stabilized by the irreversible binding of the coenzyme analog. © 1979
The active site of 6-phosphogluconate dehydrogenase. A phosphate binding site and its surroundings
No full textTetrahedral anions bind to a phosphate binding site of 6-phosphogluconate dehydrogenase from Candida utilis, inhibit the enzyme competitively with the 6-phosphogluconate, decrease the reactivity of the SH groups, and mimic the protective effect of 6-phosphogluconate against some inactivating agents. The reaction of the enzyme with butanedione results in the inactivation of the enzyme associated with the modification of a single arginine residue per subunit. This arginine residue may be involved in the binding of the phosphate to the enzyme. Inactivation of the enzyme, upon reaction with permanganate, appears to be due to the oxidation to cysteic acid of a single cysteine residue per enzyme subunit. The reaction of the enzyme with either periodate or hexachloroplatinate causes the loss of the catalytic activity. This inactivation, due to an affinity labeling, is correlated with the oxidation of two SH groups per subunit to an S-S bridge. Photoinactivation of the enzyme by pyridoxal 5′-p..
A further non-redox role for NADPH in the mechanism of action of 6-pho-sphogluconate dehydrogenase from lamb liver
No full textA multiple role for the coenzyme in the mechanism of action of 6-phosphogluconate dehydrogenase
No full text6-Phosphogluconate dehydrogenase from Candida utilis
catalyzes the oxidative decarboxylation of 2-deoxy-6-phosphogluconate.
The 3-keto-2-deoxy-6-phosphogluconate, an
intermediate of the reaction, is reduced to 2-deoxy-6-phosphogluconate
and decarboxylated to I-deoxyribulose 5-phosphate
when incubated with the enzyme and TPNH. The
decarboxylation process does not occur in the absence of
the reduced coenzyme, which does not have, in this step,
an oxidation-reduction role.
Since TPNH also has a non-redox role in a tritium exchange
reaction catalyzed by the enzyme, it appears that the
coenzyme has a multiple role in the mechanism of action of
6-phosphogluconate dehydrogenase: a redox role in the dehydrogenation
and another (or others) role(s) in the decarboxylation
and tritium exchange reactions.
The hydroxyl group present at carbon 2 of 6-phosphogluconate
seems to have a dual role in the mechanism of
action of the enzyme: one in the binding of the substrate to
the enzyme, another in enhancing the decarboxylation of the
dehydrogenation product.
These findings are discussed with relations to the mechanism
of action of isocitrate dehydrogenase and of the malic
enzyme.
The enzymatic oxidative decarboxylation of 2-deoxy-6-
phosphogluconate is a new step for the metabolism of the
metabolic inhibitor 2-deoxyglucose
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