The regulation of glutamine synthesis in the food yeast Candida utilis: the purification and subunit structure of glutamine synthetase and aspects of enzyme deactivation.

Abstract

SUMMARY: Yeast glutamine synthetase was purified and shown to be an octameric globular protein (s = 15·4, f/f 0 = 1·29, mol. wt = 390000). It consists of two weakly-bound half molecules (s = 8·7, f/f 0, = 1·35, mol. wt = 180500) and relatively harsh treatment is required to dissociate these octamers into component monomers (s = 3·8). Deactivation of the enzyme in vivo may include changes in the conformation of the native enzyme with its separation into two ‘ tight ’ tetramers, followed by their dissociation into component monomers and dimers. In the presence of Mg2+ and glutamate, monomers re-aggregate to oligomers which, although having transferase activity, are devoid of synthetase activity. The relevance of these observations in relation to control of glutamine synthetase activity in yeast is discussed.