Pea Leaf Glutamine Synthetase

Abstract

Of a variety of purine and pyrimidine nucleotides tested, only ADP and 5′AMP significantly inhibited the Mg²⁺-dependent activity of pea leaf glutamine synthetase. They were less effective inhibitors where Mn²⁺ replaced Mg²⁺. They were competitive inhibitors with respect to ATP, with inhibition constant (Ki) values of 1.2 and 1.8 mm, respectively. The energy charge significantly affects the activity of glutamine synthetase, especially with Mg²⁺. Of a variety of amino acids tested, l-histidine and l-ornithine were the most inhibitory, but significant inhibition was seen only where Mn²⁺ was present. Both amino acids appeared to compete with l-glutamate, and the Ki values were 1.9 mm for l-histidine (pH 6.2) and 7.8 mm for l-ornithine (pH 6.2). l-Alanine, glycine, and l-serine caused slight inhibition (Mn²⁺-dependent activity) and were not competitive with ATP or l-glutamate.