Glutamine synthetase from pig brain: binding of adenosine triphosphate

Glutamine synthetase was purified to homogeneity from fresh brain homogenates by a procedure that makes use of the affinity of this ATP requiring ligase to Cibacron 3G-A Blue-Sephadex G-75. It is shown that the interaction of pig brain enzyme with the dye does not concern the active centre but a non-catalytic although specific ATP binding site. Both sides contain a cysteine residue which may react with N-ethylmaleimide. The unprotected succinimidated enzyme is inactive, and aggregates. ATP alone protects only against aggregation, not against inactivation; the ATP/Mg2 complex also hinders inactivation. A tryptic heptadecapeptide isolated from the derivatized enzyme representing the carboxy terminal seems to belong to the active centre; its amino acid sequence was partially determined.