Biology 1002B Study Guide - Final Guide: Color Vision, Trichromacy, Rhodopsin
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19 Apr 2017
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Colour Vision
What is trichromatic vision
Trichromatic vision: three types of cones in the retina, each with different rhodopsin (pigment) but they
all bind to the same retinal
o Blue (short wavelength), green (medium wavelength), yellow (long wavelength)
o It is possible to match all colours of the visible spectrum by mixing these 3 colours
Trichromats: organisms with 3 different rhodopsins
o Can distinguish colour better than organisms with less (dichromats, monochromats)
What accounts for the difference in absorption characteristics of the different rhodopsins
o Since the same retinal is used, there is no change in the biosynthetic pathway
▪ Retinal is not a protein, so we an’t hange it diretly
▪ We don’t want to hange the entire iosyntheti pathway and all the enzymes involved
to create different retinals
o The difference is the protein that the retinal binds to
▪ Have different sequences -> different folding -> different light sensitivity
▪ Lysine: specific highly conserved amino acids/residues that bind to retinal
Glutamic acid stabilizes the complex
▪ The protein is much larger than retinal -> stronger influence
Major similarities and differences as revealed in protein alignment of SW, MW and LW opsins
o Red shows homology in all 3, blue shows differences
o All 3 have conserved retinal binding site and counter ion spot (glutamic acid)
o MW and LW are much more similar, SW is the outlier
▪ MW and LW diverged recently from a gene duplication event close together on the X
chromosome (gave rise to trichromacy)
▪ SW was on chromosome 7
Importance of lysine at position 296 and Glutamic acid at position 113 of opsins
Lysine at 296: the location of retinal binding
Glutamic acid at 113: stabilizes the lysine – retinal bond
o Hydrogen bonding between N+ and amino acids (-)
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