Are‐examination of NaCI effects on phosphoenolpyruvate carboxylase at high (physiological) enzyme concentrations

Inhibition of phosphoenolpyruvate carboxylase (EC 4.1.1 31) from the C 4 ‐halophyte Salsola soda L. by NaCI is compeutive to phosphoenolpyruvate (PEP). Physiological (betaine, glycerol) and synthetic (polyethylene glycol) osmotica and the allosteric activator glucose‐6‐phosphate (G6P) increase the apparent affinity of the enzyme for PEP and also alleviate the inhibition by NaCl. Physiological osmotica that either increase the K m(PEP) (proline) or are neutral (sorbitol), do not protect the enzyme against NaCI attack. In the absence of cosolutes and, G6P, the enzyme is self‐protected when its concentration in the assay medium is increased to more physiological values. In addition, the amount of betaine needed for complete protection is inversely related to native protein concentration in the assay. Exogenous protein (bovine serum albumin or bovine skin gelatin) have no effect on either K m(PEP) , or extent of NaCl inhibition. These results can be better explained with the exclusion volume theory and the inferred assumption that both cosolutes and high protein concentration strengthen intrinsic aggregation properties of enzymes. It is suggested that the extremely high phosphoenolpyruvate carboxylase concentration in the cytoplasm and the accumulation of compatible solutes in response to water stress fully protect the enzyme in vivo against the chaotropic effects of NaCI.