The spectrum of colors a monkey can detect depends on how many kinds of light sensitive opsin proteins that the cone cells in its eye are able to produce. Each cone cell contains only one kind of opsin. Dichromats have two opsin genes and can make two opsin proteins – the short wave sensitive opsin protein (SWS) and the medium wave sensitive opsin protein (MWS). Trichromats have three opsin genes, one coding for an additional opsin protein – the long wave sensitive opsin protein (LWS).
An opsin protein is a transmembrane protein in a cone cell. It is stimulated when a photon of light enters the eye and activates a chromophore molecule, a retinal, embedded in the opsin. When the retinal chromophore is activated it changes shape (configuration) from an 11-cis arrangement to an all-trans arrangement. Once the chromophore is in an all-trans state, the opsin protein it is associated with is stimulated and the cone cell emits a signal that ultimately travels to the brain indicating that light has been detected. Because the three different types of opsin (SWS, MWS and LWS) are structurally different, they each interact with the retinal chromophore in a slightly different way. This results in chromophore activation and opsin stimulation at different wavelengths for each different type of opsin. For SWS opsin proteins, retinal chromophores are most efficiently activated when light entering the eye has a wavelength of 420 nm (blue; see the spectrum below). Retinal chromophores in SWS opsin proteins can be activated for light as short as ~360 nm and as long as ~540 nm but retinal activation is most efficient (i.e. it peaks) at 420 nm. For MWS opsin proteins, retinal chromophores can be activated by light with wavelengths between ~400 nm and 650 nm, peaking at 534 nm (green). For LWS opsin proteins, retinal chromophores can be activated by light with wavelengths between 400 nm and 680 nm, peaking at 564 nm (darker green). The longer wavelengths allow the observer to perceive red.
The Opsin Protein
The opsin protein is composed of a string of amino acids. Each green dot in the 2D visualization represents one amino acid. The LWS opsin functionally differs from the MWS opsin in only three places in the amino acid sequence. When amino acid # 180 is an alanine, amino acid # 277 is a phenylalanine and amino acid # 285 is an alanine the resulting opsin protein has an stimulation maximum of ~ 534 nm (i.e. it is medium wave sensitive, MWS). When amino acid # 180 is a serine, amino acid # 277 is a tyrosine and amino acid # 285 is a threonine the resulting opsin protein has a stimulation maximum of ~ 564 nm (i.e. it is long wave sensitive, LWS).
The responses to light of each opsin protein (SWS, MWS and LWS) in trichromats are shown to the right. Note how similar the curves look for MWS and LWS. The LWS stimulation curve is shifted 30 nm to the right of the MWS stimulation curve. Dichromats, who lack the LWS, are not able to “see” colors of longer wavelength, and thus miss out on some of the redder hues.