How vertebrate and invertebrate visual pigments differ in their mechanism of photoactivation

In vertebrate visual pigments, a glutamic acid serves as a negative counter ion to the positively charged chromophore, a protonated Schiff base of reti nal, When photoisomerization leads to the Schiff base deprotonating, the an ionic glutamic acid becomes protonated, forming a neutral species that acti vates the visual cascade. We show that in octopus rhodopsin, the glutamic a cid has no anionic counterpart. Thus, the "counterion" is already neutral, so no protonated form of an initially anionic group needs to be created to activate. This helps to explain another observation-that the active photopr oduct of octopus rhodopsin can be formed without its Schiff base deprotonat ing, In this sense, the mechanism of light activation of octopus rhodopsin is simpler than for vertebrates, because it eliminates one of the steps req uired for vertebrate rhodopsins to achieve their activating state.