Active transport modulated by barrier fluctuations

The translocation of substance from one side of a biological membrane to the other, with higher electrochemical potential, happens by active transport. This process is accomplished by specific membrane proteins, via conformational changes, and the problem addressed here is the influence of stochastic fluctuations of the energy barriers that separate the intermediate states. The fluctuations are described as a finite set of independent Markovian dichotomous noises. Their properties are presented, along with the Shapiro-Loginov theorem, which is useful in computing noise-averaged physical quantities. After a short account on Hofmeister phenomena (a generic term used for effects of salts on proteins), a theory is proposed that links them to barrier fluctuations with amplitudes proportional to the salt concentration. Experimental data on the solubility of deoxygenated sickle hemoglobin are fitted by barrier fluctuation theory. The effects of salts that cause monotonous increase of protein solubility, are described by a single dichotomous noise, while for other salts two independent noises are necessary. The theory is further generalized to the case of active proton pumping by bacteriorhodopsin. Each energy barrier of the sequential kinetic model of the photocycle with reversible reactions is supposed to contain a noisy term. The noise-averaged kinetic equations are solved for a trial noise amplitude matrix. Comparison with kinetic data on the M intermediate shows qualitative agreement between theory and experiment. Further absorption-kinetic measurements are necessary to obtain the noise amplitudes via nonlinear least-squares fit, offering the possibility to metricate Hofmeister effects in the context of active transport