3-Dcoxy-D-arabino heptulosonate 7-phosphate synthase from Zea mays : General properties and regulation by tryptophan

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In extracts from Zea mays shoots, the presence of thiol compounds in the extraction buffer was necessary to get an active 3 deoxy-D-arabino heptulosonic acid 7-phosphate (DAHP) synthase. Its pH optimum for activity was about 7.5. Of the different cations tested, only Mn^<++> was an activator. Enzyme stability was optimal in Tris-HCl buffer, pH 7.5, that contained a reducing agent, Mn^<++> and a polyol. Contrary to other reports, phosphoenolpyruvate (PEP) did not stabilize the preparation significantly. The synthase exhibited high affinities for both erythrose-4-phosphate (Km: 0.24 mM) and PEP (Km: 0.31 mM) . Its specific activity was highest in young shoots. Corn DAHP synthase was inhibited in vitro by tryptophan. Moreover, the enzyme was retarded on a tryptophan agarose affinity column, but it was removed with the bulk of protein from the same support when eluted with buffer containing tryptophan. Inhibition which was easily lost during storage at 4℃ was pH dependent and increased during development. Maximal inhibition, about 60% with I mM tryptophan, was observed in extracts from 8 day-old shoots. Phenylalanine and tyrosine were not inhibitory, and no synergistic effects were observed when the aromatic amino acids were tested in combination. Isoenzymes could not be demonstrated.
日本植物生理学会の論文