Dehydration of Macromolecules II. Protective Effects of Certain Anions on Ribulosediphosphate Carboxylase Subjected to Low Water Potentials in Vitro

Abstract

Studies of the instability of ribulosediphosphate carboxylase at low water potentials, generated in a pressure-membrane apparatus, have been continued. The presence of phosphate, pyrophosphate, citrate, or ribulose diphosphate during treatment protects the protein against fragmentation. Tris-sulphate, sodium sulphate, tetra-alkyl ammonium salts, or polyvinylpyrrolidone do not afford protection.The results are considered in terms of water-structuring by ions, the binding of ions to the protein, and stabilizing of the protein by ions. Reported effects of ions on enzyme kinetics do not explain the difference between sulphate and phosphate with respect to protection. It is concluded that protective ions must be bound to cationic sites on the protein but must also structure adjacent water. The additional structured water presumably prevents fragmentation of the protein that can occur at low water potentials.

Water Status of Rhizobia in Relation to their Susceptibility to Desiccation and to their Protection by Montmorillonite

“…The equation states that A F is proportional to the area under the curve in a plot of W/(P/P,) against P/P,. Steer (1973) noted that the presence of residual salt markedly affected the water sorption isotherms of various proteins. To determine whether this was an important factor in the water sorption characteristics of both groups of rhizobia, the isotherms of salt-free suspensions of the bacteria were obtained.…”