Organic Cosolutes Increase the Catalytic Efficiency of Phosphoenolpyruvate Carboxylase, From Cynodon Dactylon (L.) Pers., Apparently Through Self-Association of the Enzymic Protein

Abstract

Several organic cosolutes (glycerol, sorbitol, betaine, proline, polyethylene glycol, polyvinylpyrrolidone) increase, to a large extent and in a concentration-dependent manner, the apparent affinity of phosphoenolpyruvate carboxylase for phosphoenolpyruvate, whereas the maximum activity remains unaffected. In absence of cosolutes, a similar response is obtained as the concentration of the enzymic protein in the assay medium is increased. The effect of the organic additives does not depend on the osmotic potential or the viscosity of the medium and it could be best interpreted with the exclusion volume theory. It is inferred that the inclusion of an appropriate cosolute in the assay medium promotes the self-association of the enzymic protein and, therefore, mimics the intracellular situation, where the enzyme is much concentrated. In light of these results, it is suggested that the physiological relevance of past data concerning the non-saturating activity and the regulation of the enzyme in vivo should be reevaluated.