Rhodopsin structural changes caused

A photochemical change in the structure of rhodopsin causes it to activate the G protein called transducin so that it binds GTP. 

Fig. 4.13 (A) Crystal structure of bovine rhodopsin viewed from a perspective approximately normal to the membrane. The polypeptide backbone is represented by a ribbon model (gray) and the retinylidine chromophore by a licorice model (black). The coordinates are Irran Protein Data Bank file IfSS.pdb [69]. Some parts of the protein that protrude from the phosopholipid bilayta- of the membrane are omitted fw clarity. (B) The 11-cfr-retinyUdine chromophore is attached to a lysine residue by a protonated Schiff base linkage. Excitation results in isomerization around the 11-12 bond to give an all-rrfl s structure. (C, D) Schematic depictions of a field of rhodopsin molecules in a rod cell disk membrane, viewed nmmal to the membrane. The short arrows in the shaded ovals represent the transition dipoles of individual rhodopsin molecules. (Each disk in a human retina contains approximately 1,000 rhodopsins.) The transition dipoles lie approximately in the plane of the membrane, but have no preferred orientation in this plane. A polarized excitation flash (horizontal double-headed arrow in C) selectively excites molecules that are oriented with their transition dipoles parallel to the polarization axis, causing some of them to isomerize and changing their absorption spectrum (empty ovals in D). (E) Smoothed records of the absorbance changes at 580 run as a function of time, measured with probe light polarized either parallel or perpendicular to the excitation [14]. The vertical arrow indicates the time of the flash. The absorbance change initially depends on the polarization, but this dependence disappears as rhodopsin molecules rotate in the membrane...

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