Chemical cross-linking stabilizes the enzymic activity and quaternary structure of formyltetrahydrofolate synthetase

Clostridium cylindrosporum formyltetrahydrofolate synthetase tetramers cross-linked with dimethyl suberimidate remained active in the absence of the monovalent cations normally required for enzymic activity and the tetrameric conformation. The modified enzyme was analyzed by sodium dodecyl sulfate electrophoresis, sedimentation velocity, and gel permeation chromatography. Under the experimental conditions used, the enzyme was only partially cross-linked; 74% of the enzyme was cross-linked dimer or monomer. Nonetheless, the modified enzyme is able to retain enzymic activity and the tetrameric structure under conditions where native enzyme would be completely dissociated and inactivated. The result suggests that cross-linked dimers strongly associate with each other and with monomers. Flame emission spectroscopy indicates that cross-linked enzyme contains two monovalent cations per tetramer.