The kinetics of formation of both the tyrosinate ion (from its absorption at 296 nm) and the deprotonated Schiff base (M412) (from its absorption at 404 nm) are studied simultaneously at different pH values (7-11) and temperatures (5-250C). Two formation rates are observed for M412 in agreement with previous observations. The slow one is dominant under physiological conditions and is found to be slightly faster than that for the tyrosinate formation. This is in disagreement with the proposal that the tyrosinate formation is a prerequisite to the deprotonation of the Schiff base (M412). The ratio of the amplitudes of the fast and slow components is found to be sensitive to pH and, at any pH, it can be used to calculate an amino acid pKa value of 9.6. This is explained by proposing the existence of two sites for the protonated Schiff base within the protein. In one site, the Schiff base is near the neutral form of an amino acid residue with a pK. value of 9.6 (giving rise to the slow component), while in the other, it is near its conjugate base. The formation of the tyrosinate ion as well as the formation of the slow and fast components of M412 all have activation energies that are comparable to H-bond energies. A model is suggested to account for this and the comparable deprotonation rates of tyrosine and the slow component of the protonated Schiff base. It involves the reduction of their pKa by their exposure to a positively charged species.Bacteriorhodopsin (bR) is a protein found in the cell membrane of the halophile Halobacterium halobium that transforms visible light into chemical energy (1-4). It is composed of one molecule of retinal covalently bound to the E-amino group of a lysine residue in the surrounding protein via a protonated Schiff base linkage (5,6). When bR absorbs light, it undergoes the following simplified photochemical cycle:Following the L550 -* M412 transformation, protons are ejected from the cell, thus generating an electrochemical gradient. This proton gradient directly drives some metabolic processes such as ATP synthesis and other endergonic processes (1-3, 7-10).The exact mechanism of this proton pump still remains unclear. However, accumulated evidence indicates that the M412 intermediate plays a key role in the photochemical cycle of bR (1, 11). M412 is unique in the unprotonated nature of its retinal-lysine Schiff base bond, as compared with the protonated structure characterizing the parent pigment and all other intermediates (1,(12)(13)(14)(15)(16). Some studies have inferred that the Schiff base deprotonation is closely associated with the proton pump mechanism (17, 18). Studies of the effect of chemically modified tyrosines on the rate of decay of M412 suggest a coupling between some tyrosine residues and M412 (19)(20)(21)(22).Information relating to the molecular aspect of M412 formation has been obtained by Kalisky et al. (17). Their low temperature photolysis studies showed that at temperatures below -80'C, the yield of M412 was markedly increased by a change in the...

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The transient absorption changes occurring at 297 nm during the photocycles of the deionized and acidified bacteriorhodopsins (blue membranes) were studied. As opposed to what happens during the photocycle of the purple membrane, for the blue membranes only the fast absorption increase corresponding to trans-cis isomerization of the retinal chromophore is present; the slow rise attributed to the tyrosine deprotonation is not observed; The addition of different salts to the deionized membrane restores the original color and causes a tyrosine deprotonation during the photocycle. This suggests that the presence of cations is required for the deprotonation of tyrosine as it is for the deprotonation of the retinylidene Schiff base. These results are discussed in terms of the recently proposed cation model for the observed deprotonation processes in the photocycle of bacteriorhodopsin.Bacteriorhodopsin (bR), the only protein in the purple membrane of Halobacterium halobium, contains one molecule of retinal covalently bound to the e-amino group of a lysine residue via a protonated Schiff base (1, 2). When it absorbs light, it undergoes a photochemical cycle (3) during which protons are pumped from the inside to the outside of the cell resulting in a pH gradient across the cell membrane. -This proton gradient drives metabolic processes such as ATP synthesis (4-9).The absorption spectrum of bR shows a maximum at 570 nm. However, when the membrane is deionized on a cationexchange column-(10) or treated with NaCl (10, 11), EDTA (11,12) In the purple membrane photocycle, a tyrosine deprotonation occurs on a time scale similar to that for the L550-tO-M412 transformation (18-23). The effect of chemically modified tyrosines on the rate of decay of M412 suggests a coupling between some tyrosines and M412 (24)(25)(26)(27). Recently (22), the kinetics offormation of both the tyrosinate ion and M412 were studied simultaneously as a function of pH and temperature. The results have shown two formation rates for M412, with the slower one dominating under physiological conditions and with a rate slightly faster than that for the tyrosine deprotonation. This eliminated the possibility that the tyrosinate formation is a prerequisite for the Schiff base deprotonation (M412 formation) (28). The ratio of the amplitudes of the fast and slow M412 components is very sensitive to pH and, at any pH, can be used to calculate a pKa value of about 9.6. This was explained (22) by assuming two different sites for the protonated Schiff base within the protein. In one site, the Schiff base is near an amino acid with a pKa of 9.6-e.g., tyrosine. This gives rise to the slow M412 component. In the second site, the Schiff base is near the conjugate base of the amino acid and this gives rise to the fast M412 component. A model was then proposed to account for the comparable rate ofdeprotonation ofboth the tyrosine and the slow component of the protonated Schiff base (22). In this model, a positively charged species comes in close proximity to bot...