The Inward H⁺ Pathway in Bacteriorhodopsin: The Role of M₄₁₂ and P(n)₅₆₀ Intermediates*

In purple bacteriorhodopsin sheets adsorbed onto the phospholipid‐impregnated collodion film, electrogenic stages are identified correlating with decays of the M and N(P)‐type intermediates. It is concluded that both M → N and N → bR transitions are electrogenic. The M decay is shown to be of a complex kinetics. In purple sheets, the lower the light intensity, the higher the rate of “slow M” decay. Such a dependence, which is absent from monomeric bacteriorhodopsin in proteoliposomes and from Triton X‐100‐solubilized protein, may be explained by the inhibiting effect of a light‐induced conformation change in a bacteriorhodopsin molecule upon the M decay in some other bacteriorhodopsin molecules within the same sheet. The light intensity‐independent “slow M” decay in solubilized bacteriorhodopsin is shown to correlate with the decay of the N intermediate and H + uptake after the flash. In contrast to “fast M”, “slow M” is pH dependent, closely resembling in this respect the N intermediate. It is suggested that there is a fast light‐independent equilibration between M and N so that “slow M” represents the portion of the M pool that monitors the N concentration. The M → N equilibrium is assumed to be involved in the effect of the light‐induced electric field on the M decay. No direct effect of light on the equilibrium was found.