STUDIES ON SUBUNIT STRUCTURE OF OVINE BRAIN GLUTAMINE SYNTHETASE

Ovine brain glutamine synthetase is isolated in an octameric form exhibiting a sedimentation coefficient, S2O.W, of 15.0 S and possessing a molecular weight of 525,000 ± 25,000. In the presence of 2 M urea at 25°, 1 m urea at 35°, 20% dimethylformamide, 20% dimethyl sulfoxide, or at values of pH greater than 8.1 in low ionic strength media, the enzyme dissociates reversibly to a form, probably a tetramer, exhibiting a sedimentation coefficient of 8.6S. Such dissociation is prevented by adenosine triphosphate and Mg2+. Association of the 8.6S species to re-form the octamer is promoted by removal of the dissociating agent or by addition of adenosine triphosphate and Mg2+. Reversible dissociation in 1 M urea is temperature dependent. Studies at various values of pH suggest that a functional group of the enzyme with a pK of about 8.1 is involved in the reversible dissociation phenomenon. The data suggest that dissociation of the octamer to the 8.6S form does not involve extensive alteration of the tertiary structure of the enzyme, and that certain reagents are quite specific for disrupting the linkage between tetramers. The findings are consistent with a model of the octameric enzyme in which two heterologously linked tetramers are held together by weaker isologous bonds. Treatment of the octameric enzyme with maleic anhydride, acetic anhydride, tetranitromethane, diazonium-1H-tetrazole, or adjustment of the pH to values greater than 9.8 yields a catalytically inactive monomeric species [s20,w = 2.8 S; mol wt 65,000]. © 1969, American Chemical Society. All rights reserved.