Studies on glutamine synthetase: purification of the enzyme from mung bean (Phaseolus aureus) seedlings and modulation of the enzyme-antibody reaction by the substrates

Abstract

Glutamine synthetase (L-glutamate : ammonia ligase, EC 6.3.1.2) from Phaseolus aureus (mung bean) seedlings was purified to homogeneity by ammonium sulphate fractionation, DEAE-cellulose chromatography, Sephadex G-200 gel filtration and affinity chromatography on histidine-Sepharose. The enzyme had a molecular weight of 775,000 ± 25,000. The enzyme consisted of identical subunits with an approximate subunit molecular weight of 50,000. Hyperbolic saturation curves were obtained with the substrates, glutamate, ATP and hydroxylamine. Antibody, raised in the rabbit, against mung bean glutamine synthetase, completely inhibited the activity of the enzyme. Preincubation of the enzyme with glutamate and ATP, prior to the addition of the antibody, partially protected the enzyme against inhibition. The K_m values of this enzyme-antibody complex and the native enzyme were identical (glutamate, 2.5mM; ATP, 1 mM; hydroxylamine, 0.5 mM). The K_m values of the partially inhibited enzyme (the enzyme pretreated with antibody prior to the addition of substrates) were 2-fold higher than those of the native enzyme. These results suggested that the substrate-induced conformational changes in the enzyme were responsible for the protection against inhibition of the enzyme activity by the antibody.