The unusual pK a of the rhodopsin chromophore : Is this how nature minimizes photoreceptor noise?

Abstract

The active site of the visual pigment, rhodopsin, contains a retinyl polyene chromophore that is bound to the protein backbone via a protonated Schiffbase linkage to lysine 296 (Fig. 1). When this chromophore absorbs light, it undergoes photochemistry (from an 11 -cis to an 11-trans conformation) and initiates a complex series of dark reactions which ultimately generate a nerve impulse (for a recent review see reference 1). The efficiency of this process is quite high (the quantum yield for converting a photon of light into a nerve impulse is about 67%). This high efficiency contrasts with the extremely low rate of thermal activation of the protein, which in the human visual system, generates a false (dark) signal every 0.01 s for each photoreceptor cell. Because each photoreceptor cell contains lIO rhodopsin molecules, the dark noise rate is impressively low, 1O-1 events rhodopsin' s-'. This characteristic is in part responsible for the ten log units of operating range that is achieved by the eye. The question ofhow nature controls dark noise is a subject of active debate ( 1, 8-14). An article by Steinberg et al. ( 15) in this issue presents a detailed study of the PKa of the Schiff base proton on the chromophore within the binding site of the visual pigment, rhodopsin. By using a series of model retinal chromophores with electron-withdrawing substituents, these authors concluded that the apparent PKa of the protonated Schiff base is 16 or greater. This PKa value is significantly larger than corresponding values in model compounds or related proteins (see below). Although it was not possible to definitively rule out the possibility that the Schiff base linkage is not accessible for titration from the aqueous bulk medium, more recent studies of proton exchange rates in rhodopsin suggest that this linkage is accessible (R. Callender et al., manuscript in preparation). In this article we explore the potential relevance of a high PKa value on photoreceptor noise. There is growing evidence that the mechanism for thermal activation of rhodopsin is a two step process ( 1, 14). The first step is deprotonation ofthe 11 -cis protonated Schiff base chromophore. The second step is thermal 11 -cis to 11-trans isomerization of the chromophore.