Glutamine synthetase dissociation

Many enzymes (see Chapters 14 to 16) derive at least some of their catalytic power from oligomeric associations of monomer subunits. This can happen in several ways. The monomer may not constitute a complete enzyme active site. Formation of the oligomer may bring ail the necessary catalytic groups together to form an active enzyme. For example, the active sites of bacterial glutamine synthetase are formed from pairs of adjacent subunits. The dissociated monomers are inactive. 

Isotopic labeling has also been cleverly used to demonstrate the existence of enzyme-bound intermediates that do not readily dissociate into solution. The enzyme glutamine synthetase catalyzes the formation of glutamine from ATP and ammonia possibly through a tightly bound glutamyl phosphate intermediate. 

Similar complexes were observed in rat livers and in other tissues [145-149]. The extensively purified complex of glutamine-dependent carbamoyl phosphate synthetase, aspartate carbamoyltransferase and dihydro-orotase from rat liver had a sedimentation coefficient of 27 S (approximately 900000 daltons). Treatment of the complex with pancreatic elastase caused a selective inactivation of carbamoyltransferase with concomitant dissociation of the complex [159]. 

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