Rhodopsin

The Opsin present in human Rod cells. It is mostly unrelated to color vision or CVD.

Human Rhodopsin is a type of Ciliary Opsin, designated with the class Rh1. It was the last class of the vertebral visual opsins to evolve (yes, even after Rh2). All instances of rhodopsin in the class Rh1 have a very stable Peak Wavelength, as in very few deviate away from 500nm. This puts the maximum sensitivity of human Scotopic Vision at 500nm.

The naming of Rhodopsin is quite frustrating. First, the “Rhod-” of rhodopsin has nothing to do with the name of the photoreceptor it is found in – the rod. Instead, the “rhod-” comes from the greek word for pink/rose, “rhodon”. Secondly, rhodopsin usually refers to a subset of visual opsins to differentiate it from the Photopsins. However, “Rhodopsin” is also used (more classically) to refer to the entire unit comprising an opsin (the protein) and its chromophore (retinal). So while rhodopsin is a type of opsin, an opsin is part of rhodopsin. Lunacy! The classical definition of “Rhodopsin” has mostly been replaced by “Visual Pigment”.