In the human eye the three types of the rhodopsin (blue, green, red) are responsible for color vision. All of these color pigments bound to a single chromophore: 11-cis-retinal. It is not still clear that how the interaction of this chromophore with these different opsins leads to different wavelength spectrum. Since working with rhodopsins (membrane proteins) are challenging therefore in this study Cellular retinol binding protein II (hCRBPII) small, cytosolic and solubleprotein has been selected to be used as rhodopsin mimic. The first step was generating of the active lysine in the binding pocket that can bond to all-tran-sretinal and forms the Schiff base. Systematic mutations on this protein followed by X-ray crystallography lead us conversion of hCRBPII to mimic of rhodopsin that covers the full visual spectrum. Crystal structure of several holo mutants bound to retinal at high resolution illustrate that changes in the ordered water networks (making water net work, removing eater net work or changing in dipole moment) in the binding pocket leads to tremendous changes of the absorbance of protein bound to retinal. The mutants that do not effect on the ordered water networks do not make the big differences in wavelength. The crystal structures also showedthat in this system despite rhodopsin the counter anion is not necessary and the positive charge of the protonated Schiff base can be stabilized with the other interactions. Based on these data we proposed the mechanism of the mechanism of wavelength regulation in one rhodopsin mimic. We could also correlate the pKa of protonated Schiff base with the water networks and wavelength. We have observed that the electrostatic interactions between the amino acids and alsopolar groups (water) in the binding pocket have an undeniable role on spectral tuning. By introducing one specific mutation on hCRBPII, this protein forms a very stable dimer and domain swapped protein. This unique protein folding mechanism so far has been observed in only around 40 different proteins and is believed to be a mechanism of evolution of the dimers and oligomers. The high resolution crystal structure of the dimer and domain swapped hCRBPII shows that we could trapped or produce a very stable partially folded protein and with this data we were able to explain the mechanism of protein folding In hCRBPII.
Read
