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Articles

Pea Leaf Glutamine Synthetase

Regulatory Properties ¹

^a Department of Biology, Rensselaer Polytechnic Institute, Troy, New York 12181

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.

Carbamyl phosphate was an effective inhibitor only when Mn²⁺ was present, and did not compete with substrates. Inorganic phosphate and pyrophosphate caused significant inhibition of the Mg²⁺-dependent activity.

¹ This work was supported by a grant from the National Research Council of Canada.