Rhodopsins as light-detectors: Of visual pigments and human beings
Koutalos, Ioannis
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Permalink
https://hdl.handle.net/2142/20284
Description
Title
Rhodopsins as light-detectors: Of visual pigments and human beings
Author(s)
Koutalos, Ioannis
Issue Date
1991
Doctoral Committee Chair(s)
Ebrey, Thomas G.
Department of Study
Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Education, Sciences
Biophysics, General
Language
eng
Abstract
Rhodopsin is the light-sensitive pigment of the photoreceptor cells of higher organisms. Photon absorption by rhodopsin initiates a biochemical amplification cascade that results in an electrical signal. Rhodopsin is composed of the protein opsin, and the chromophore retinal, linked to it by a protonated Schiff base. Titration of the octopus opsin with 11-cis retinal gives an extinction coefficient of 27000 $\pm$ 3000 M$\sp{-1}$ cm$\sp{-1}$ at 475 nm, not very different from retinal's ethanol. The absorption maxima of bovine and octopus artificial pigments, formed by regenerating opsins with the dihydro series of chromophores, support a color regulation model in which the chromophore-binding site of the protein has two negative charges: one directly hydrogen bonded to the Schiff base nitrogen and another near carbon-13. The interaction of the second charge with the chromophore in octopus rhodopsin is weaker than in bovine rhodopsin, while in octopus metarhodopsin is as strong as in bovine rhodopsin. The interaction of the first charge with the nitrogen is approximately the same for the chromophore in the three pigments and in solution. The difference in the intrinsic Schiff base pKs of octopus metarhodopsin (8.44 $\pm$ 0.12), octopus rhodopsin (10.65 $\pm$ 0.10), and retinal Schiff base in polar solvent (around 7.0) is attributed to the preorganization of the chromophore-binding site of the pigments. On the basis of pH-dependent absorption changes, the surface density of octopus photoreceptor membranes is measured to be $-$1.6 $\pm$ 0.1 electronic charges per 1000 A$\sp2$. The first photoproduct of octopus rhodopsin, bathorhodopsin, is studied by Resonance Raman spectroscopy and deuterated retinals. No significant opsin-chromophore interactions are detected for the ground state, though significant perturbations are inferred for the excited state. My reflections foreground the details of the activity that produced this knowledge, in order to provide the appropriate local context. Scientific knowledge is demonstrated to be coproduced with human beings, material objects, human activities, representations, and social interactions. Knowledge emerges as irrevocably embedded in human engagements with the material world, and as such intrinsically human. On this basis, an interdisciplinary approach toward the selection and pursuit of research problems is advanced.
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