Rapid Release of Retinal from a Cone Visual Pigment following Photoactivation

As part of the visual cycle, the retinal chromophore in both rod and cone visual pigments undergoes reversible Schiff base hydrolysis and dissociation following photobleaching. We characterized light-activated release of retinal from a short-wavelength-sensitive cone pigment (VCOP) in 0.1% dodecyl maltoside using fluorescence spectroscopy. The half-time (<i>t</i>_1/2) of release of retinal from VCOP was 7.1 s, 250-fold faster than that of rhodopsin. VCOP exhibited pH-dependent release kinetics, with the <i>t</i>_1/2 decreasing from 23 to 4 s with the pH decreasing from 4.1 to 8, respectively. However, the Arrhenius activation energy (<i>E</i>_a) for VCOP derived from kinetic measurements between 4 and 20 °C was 17.4 kcal/mol, similar to the value of 18.5 kcal/mol for rhodopsin. There was a small kinetic isotope (D₂O) effect in VCOP, but this effect was smaller than that observed in rhodopsin. Mutation of the primary Schiff base counterion (VCOP^D108A) produced a pigment with an unprotonated chromophore (λ_max = 360 nm) and dramatically slowed (<i>t</i>_1/2 ∼ 6.8 min) light-dependent retinal release. Using homology modeling, a VCOP mutant with two substitutions (S85D and D108A) was designed to move the counterion one α-helical turn into the transmembrane region from the native position. This double mutant had a UV–visible absorption spectrum consistent with a protonated Schiff base (λ_max = 420 nm). Moreover, the VCOP^S85D/D108A mutant had retinal release kinetics (<i>t</i>_1/2 = 7 s) and an <i>E</i>_a (18 kcal/mol) similar to those of the native pigment exhibiting no pH dependence. By contrast, the single mutant VCOP^S85D had an ∼3-fold decreased retinal release rate compared to that of the native pigment. Photoactivated VCOP^D108A had kinetics comparable to those of a rhodopsin counterion mutant, Rho^E113Q, both requiring hydroxylamine to fully release retinal. These results demonstrate that the primary counterion of cone visual pigments is necessary for efficient Schiff base hydrolysis. We discuss how the large differences in retinal release rates between rod and cone visual pigments arise, not from inherent differences in the rate of Schiff base hydrolysis but rather from differences in the properties of noncovalent binding of the retinal chromophore to the protein