Color vision: Studies on gecko cone visual pigment
Liang, Jie
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Permalink
https://hdl.handle.net/2142/20413
Description
Title
Color vision: Studies on gecko cone visual pigment
Author(s)
Liang, Jie
Issue Date
1994
Doctoral Committee Chair(s)
Ebrey, Thomas G.
Department of Study
Biophysics and Computational Biology
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Health Sciences, Ophthalmology
Biophysics, General
Language
eng
Abstract
Cone visual pigments are responsible for color vision. Although a large amount is known about rod pigments, relatively little is known about cone pigments. This thesis describes the identification and characterization of the photointermediates of gecko cone visual pigment P521: batho (ca. 570 nm), BSI (blue-shifted intermediate) (ca. 520 nm), lumi (ca. 516 nm) meta I (480 nm), and meta II (ca. 380 nm). The biologically active form of the rod visual pigment is its meta II intermediate, so special focus has been on the meta I to meta II transition of the gecko cone pigment. The formation of meta II does not have single exponential kinetics, but can be better characterized as having a fast and a slow component. High pH favors faster kinetics of meta II formation but with lower amounts. High temperature is found to favor a faster rate and larger amounts of meta II formation. The pK$\sb{\rm a}$ is 8.7 for the meta I $\rightleftharpoons$ meta II equilibrium, which is higher compared to that of the rod pigment (6.4) in comparable conditions. However, the fast component of the meta II formation is found to be different and has a somewhat lower pK$\sb{\rm a}$ (e.g. 6.4).
An obligatory condition for the meta II intermediate formation is Schiff base deprotonation. Using fluorinated retinal analogs, the binding affinity of Schiff base proton of native gecko cone pigment P521 pK$\sb{\rm a}$ is determined to be 9.9. In invertebrate visual pigment of octopus the pK$\sb{\rm a}$ is 10.4. Cl$\sp-$ has profound impact on the behavior of long wavelength absorbing cone pigments. Cl$\sp-$ binding to the early BSI and batho intermediate is studied, as well as its effect on Schiff base proton binding to pigment in ground state. I tentatively conclude that there is a major reduction in the binding constant for Cl$\sp-$ from ground state ($K\sb{D}$ $\le$ 2 mM) to the batho ($K\sb{D}$ $\approx$ 90 mM) and BSI ($K\sb{D}$ $\approx$ 30 mM) states. Furthermore, Cl$\sp-$ has no effect on the Schiff base pK$\sb{\rm a}$.
I have also made the interesting observation that, unlike all other known biological membranes, the gecko photoreceptor membrane seems to carry a net positive charge. The possible implications of such a positive surface charge density will be discussed.
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