On the mechanism of proton pumping

The proton pump of purple bacteria is based on the photoinduced trans to cis isomerization of the Schiff base of retinal and the thermal cis-trans reisomerization accompanied by a shift of a proton from the cytoplasm to the extracellular space. Quantum mechanical calculations reveal, in exciting the chromophore, that negative charge is removed from the N atom of the Schiff base, causing a weakening of the HN bond by about 0.5 eV. We assume, in exciting the protonated trans isomer of the Schiff base, that the proton is left behind in the extracellular channel. The Schiff base (after trans to cis isomerization and deexcitation, in which the negative charge is shifted back to the N atom) picks up a proton from the cytoplasmic channel. In the proposed proton pumping mechanism a difficulty of the usually assumed mechanism is avoided, namely the assumption that the cis isomer in the ground state has a pK value increase of ca. 9 pH units only by a change of protein conformation.