Glutamine Synthetase of Pea Leaves: Divalent Cation Effects, Substrate Specificity, and Other Properties 1

AUTOR(ES)

O'Neal, D.

RESUMO

Purified glutamine synthetase from pea seedlings was most active with Mg2+ as the metal activator, but Mn2+ and Co2+ were 45 to 60% and 30 to 45% as effective, respectively, when assayed at the optimal pH for each cation. The Mg2+ saturation curve was quite sigmoid, and evidence indicates that MgATP is the active ATP substance. Co2+ also gave a sigmoidal saturation curve, but when Mn2+ was varied only slightly sigmoidal kinetics were seen. Addition of Mn2+, Ca2+, or Zn2+ at low concentrations sharply inhibited the Mg2+ -dependent activity, partially by shifting the pH optimum. Addition of Co2+ did not inhibit Mg2+-dependent activity. The nucleotide triphosphate specificity changed markedly when Co2+ or Mn2+ replaced Mg2+. Using the Mg2+-dependent assay, the Michaelis constant (Km) for NH4+ was about 1.9 × 10−3 M. The Km for l-glutamate was directly proportional to ATP concentration and ranged from 3.5 to 12.4 mm with the ATP levels tested. The Km for MgATP also varied with the l-glutamate concentration, ranging from 0.14 mm to 0.65 mm. Ethylenediaminetetracetic acid activated the enzyme by up to 54%, while sulfhydryl reagents gave slight activation, occasionally up to 34%.

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