Purification and studies of some physicochemical properties of glutamine synthetase of Neurospora crassa

Abstract

Glutamine synthetase (EC 6.3.1.2) of N. crassa was purified to near homogeneity by chromatography on a glutamate-Sepharose affinity column. Its properties, including molecular weight, subunit structure, amino acid composition and approximate .alpha.-helix content, were examined. In the native state, this enzyme was demonstrated by gel filtration to be an octamer of MW 360,000 and as having a sedimentation coefficient of 13.2 S by sedimentation velocity measurements. Circular dichroism spectra in the far UV range suggest an approximate .alpha.-helix content of 23-24%. The subunit generated by treatment with urea was 45,000 daltons by gel filtration methods and a MW of 46,000 was calculated for the monomer obtained by sodium dodecyl sulfate (SDS) treatment and electrophoresis in SDS-polyacrylamide gels. Interprotomeric cross-linking experiments, using diimidoesters, suggest the presence of 2 noncovalently linked tetramers comprising the native octameric structure. Amino acid analyses revealed the presence of 6 tryptophans, 4 half cystines and 9 methionine residues per monomer of 45,000 daltons.