Breaking the Carboxyl Rule: LYSINE 96 FACILITATES REPROTONATION OF THE SCHIFF BASE IN THE PHOTOCYCLE OF A RETINAL PROTEIN FROM

Background: Lysine rather than a carboxylic residue is in place of the internal proton donor in the E. sibiricum proton pump. Results: H + uptake precedes reprotonation of the retinal Schiff base. K96A mutation slows it by >100-fold. Conclusion: Lysine 96 facilitates proton delivery to the Schiff base. Significance: This is the first example where lysine mediates proton transfer to the retinal Schiff base. A lysine instead of the usual carboxyl group is in place of the internal proton donor to the retinal Schiff base in the light-driven proton pump of Exiguobacterium sibiricum (ESR). The involvement of this lysine in proton transfer is indicated by the finding that its substitution with alanine or other residues slows reprotonation of the Schiff base (decay of the M intermediate) by more than 2 orders of magnitude. In these mutants, the rate constant of the M decay linearly decreases with a decrease in proton concentration, as expected if reprotonation is limited by the uptake of a proton from the bulk. In wild type ESR, M decay is biphasic, and the rate constants are nearly pH-independent between pH 6 and 9. Proton uptake occurs after M formation but before M decay, which is especially evident in D 2 O and at high pH. Proton uptake is biphasic; the amplitude of the fast phase decreases with a p K a of 8.5 ± 0.3, which reflects the p K a of the donor during proton uptake. Similarly, the fraction of the faster component of M decay decreases and the slower one increases, with a p K a of 8.1 ± 0.2. The data therefore suggest that the reprotonation of the Schiff base in ESR is preceded by transient protonation of an initially unprotonated donor, which is probably the ϵ-amino group of Lys-96 or a water molecule in its vicinity, and it facilitates proton delivery from the bulk to the reaction center of the protein.