The solvent polarity dependent conformational equilibrium of the carboxylic ionophore narasin: a proton NMR study

Role of Na+and K+in Enzyme Function

Metal complexation is a key mediator or modifier of enzyme structure and function. In addition to divalent and polyvalent metals, group IA metals Na+and K+play important and specific roles that assist function of biological macromolecules. We examine the diversity of monovalent cation (M+)-activated enzymes by first comparing coordination in small molecules followed by a discussion of theoretical and practical aspects. Select examples of enzymes that utilize M+as a cofactor (type I) or allosteric effector (type II) illustrate the structural basis of activation by Na+and K+, along with unexpected connections with ion transporters. Kinetic expressions are derived for the analysis of type I and type II activation. In conclusion, we address evolutionary implications of Na+binding in the trypsin-like proteases of vertebrate blood coagulation. From this analysis, M+complexation has the potential to be an efficient regulator of enzyme catalysis and stability and offers novel strategies for protein engineering to improve enzyme function.

The effects of lipid composition on the binding of lasalocid A to small unilamellar vesicles

The binding of the carboxylic ionophore lasalocid A (X537A) to small unilamellar phospholipid vesicles of varying composition was examined in an effort to determine what structural features of the phospholipid membrane influence the ionophore-membrane interaction. Apparent dissociation constants (Kapp) were calculated for both the acidic and anionic forms of the ionophore using the change in fluorescence intensity observed for lasalocid A upon addition of phospholipid vesicles. The Kapp for binding to fluid phase dimyristoylphosphatidylcholine (DMPC) vesicles is 46 microM for the anion and 14 microM for the acid. While the phase transition of DMPC had no effect on the Kapp of the anion, an increase was observed in the Kapp of the acid below the phase transition temperature. The Kapp of the anion was not affected by the incorporation of 10% dimyristoylphosphatidylethanolamine (DMPE), but increased slightly upon incorporation of cholesterol. The pKa values of the ionophore were the same in DMPC and DMPC/DMPE membranes. Incorporation of the negative lipids phosphatidylglycerol, phosphatidic acid, or phosphatidylethanolamine (at pH 9.4 where PE carries a negative charge) decreases binding of the anion in accord with the increase in surface potential estimated from Gouy-Chapman theory. The CD spectrum of membrane-bound lasalocid A anion indicated the ionophore to be in an extended acyclic conformation on the membrane surface with the C-1 carboxylate rotated out of the plane of the salicylate ring. The out-of-plane rotation of the carboxylate may be the result of facial binding by the amphiphilic ionophore on the membrane surface or of weak ion pairing to the polar lipid head groups. These results suggest that the primary determinants of binding of the anionic ionophore on the membrane surface are packing density of the polar head groups and membrane surface potential. There is no evidence of strong hydrogen bond formation between the lipid polar head groups and the ionophore as has previously been suggested.

The role of molecular conformation in ion capture by carboxylic ionophores: a circular dichroism study of narasin A in single-phase solvents and liposomes

Conformational and thermodynamic aspects of cation binding by the carboxylic ionophore narasin A were studied by circular dichroism (CD). In single-phase solvents, dramatic increases in the maximum differential absorption (delta epsilon) of the C-11 carbonyl were observed upon the binding of K+, Na+ and protons to the free anionic form. These changes were associated with major shifts in the conformation equilibrium between extended and pseudocyclic conformers of narasin. Similar CD changes observed upon the binding of K+ to narasin A in dimyristoylphosphatidylcholine vesicles provided evidence that in the membrane environment, comparable conformation changes were associated with ion binding. Variation of the polar and protic properties of single-phase solvents was also found to influence the delta epsilon of the cation bound species of narasin A, supporting previous evidence for polarity-mediated modulation of conformation. Comparison of cation binding affinities indicated that in both single-phase solvents and liposomes, narasin had a marked equilibrium selectivity for K+ over Na+.