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  • Title: The monovalent cation-induced association of formyltetrahydrofolate synthetase subunits: a solvent isotope effect.
    Author: Harmony JA, Himes RH, Schowen RL.
    Journal: Biochemistry; 1975 Dec 02; 14(24):5379-86. PubMed ID: 1191644.
    Abstract:
    In the presence of specific monovalent cations (K+, Cs+, NH4+), inactive monomers of formyltetrahydrofolate synthetase associate to a catalytically active tetramer. The rate and extent of association of enzyme monomers prepared from C. cylindrosporum are enhanced 3.3-and about 50-fold, respectively, by the substitution of D2O for H2O. Both rate and equilibrium solvent isotope effects are due to a decrease in D2O of the dissociation constant of the monomer-cation complex. Analysis of rate and equilibria data obtained in solvent mixtures of varying deuterium/protium ratios indicates that the isotope effect may be due to the change in bonding of a single monomer proton during the association process. The data are most consistent with a model in which this proton is in a very weak potential in the cation-free monomer and is converted to a "normal" water-like proton in the monomer-cation complex.
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