Metarhodopsin intermediates of the gecko cone pigment P521

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Abstract
Cone visual pigments are responsible for color vision. Using low-temperature spectroscopy and linear/logarithmic time scale flash photolysis, we studied the photochemistry of the cone visual pigment P521 from the lizard Gekko gekko. We found both meta I and meta II intermediates in this cone pigment; meta I absorbs at 480 nm, and meta II absorbs at ca. 380 nm. The formation of meta I is a fast process with a lifetime of 2.8 microseconds for the slow component (pH 6.70, 8.0 degrees C, 2% digitonin) compared to 2.1 microseconds for bovine rhodopsin under these conditions. The formation of meta II does not have single-exponential kinetics but can be better characterized by fast and slow components. High pH favors faster kinetics of meta II formation, but less is formed. The amount of meta II formed has a pKa of 8.7. The fast component of the meta II formation seems to have a somewhat lower pKa (e.g., 6.4). Temperature also affects meta II formation, with high temperature favoring a faster rate and larger amounts. The higher pKa of the meta I to meta II transition in gecko P521 compared to a rod pigment like bovine rhodopsin (pKa = 6.4) probably is due to a cysteine residue at position 211 in gecko rather than a histidine residue in bovine rhodopsin.

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