Stabilization of the quaternary structure of transcarboxylase by cobalt (II) ions.

When dilute solutions of transcarboxylase are incubated at 25 degrees C in an alkaline 50 mM buffer, the enzyme rapidly loses activity. This loss of activity is accompanied by the dissociation to enzymatically inactive subunits. The inclusion of 2 mM Co2+ in the buffer reduces both dissociation and the loss of enzymatic activity. This stabilization does not take place with 2 mM Mg2+, Mn2+, Fe2+, Ni2+, Ca2+, or Cu2+, but there is a slight protection by Zn2+. At Co2+ concentrations of less than 2 mM, the stabilization decreases. The cobalt involved in the stabilization is not that required for catalysis as evidenced by the fact that the "catalytic" cobalt does not exchange with added free Co2+ under the conditions that prevent loss of enzymatic activity. The stabilizing effects of Co2+ were also observed toward inactivation with guanidinium chloride and by heat. It is proposed that Co2+ shifts that equilibrium of the dissociation of transcarboxylase toward the associated form and thus enzymatic activity is retained at alkaline pH.