Rhodopsins basics

Most of the G-protein-coupled receptors are homologous with rhodopsin however, other quantitatively minor families as well as some individual receptors do not share any of the structural features common to the rhodopsin family (Figure 2.3). The most dominant of these are the glucagon/VIP/caldtonin receptor family, or family B (which has approximately 65 members), and the metabotropic glutamate receptor family, or family C (which has approximately 15 members), as well as the frizzled/smoothened family of receptors. Thus, the only structural feature that all G-protein-coupled receptors have in common is the seven-transmembrane helical bundle. Nevertheless, most non-rhodopsin-like receptors do have certain minor structural features in common with the rhodopsin-like receptors — for example, a disulfide bridge between the top of TM-III and the middle of extracellular loop-3, and a cluster of basic residues located just below TM-VI. 

Binding of these ligands does not occur in a concave groove located on the surface of the receptor protein as otherwise often imagined. As described in Section 2.2.1, the x-ray structure of rhodopsin showed that retinal is bound deep in the seven-helical structure with major interaction points in TM-III and TM-VI, as well as the covalent attachment point in TM-VII. In fact, rhodopsin interacts with basically all transmembrane segments. Importantly, side-chains from the transmembrane helices cover the retinal molecule on all sides, and its binding site is found deep in the middle of... 

An additional basic difference between rhodopsin and bacter-iorhodopsin is associated with the structure of the pigment in the membrane. In contrast to the well-established rotational and translational mobility of the rhodopsin molecule in the viscous disk membrane (31-35), X-ray diffraction methods have shown that bacteriorhodopsin in the purple membrane is organized as a rigid two-dimensional hexagonal lattice with a 63 A unit cell (36-38). ... 

Figure 28-3 Classification and basic architecture of cell-surface receptors that couple to G-proteins. Panel A lists the major families and groups of GPCRs.The mammalian receptors are confined to families A, B, and C. Family A is the largest and includes the diverse odorant receptors and prototypic GPCRs, such as rhodopsin and the p-adrenergic receptor. Panel B shows a schematic structure of one of the most extensively characterized GPCRs, the p-adrenergic receptor. Major structural features are indicated and are expanded on in the text, ffrom Conn PM, Melmed. eds.Textbook of endocrinology.Towanta Nj Humana Press 1997.)...

McGee LH, Lau D, Zhou S, et al. Rescue of photoreceptor degeneration in S334ter(4) mutant rhodopsin transgenic rats by adeno-associated virus (AAV)-mediated delivery of basic fibroblast growth factor. Invest Ophthalmol Vis Sci 1999 40 S936. 

Information and control. Stimuli external to a cell, such as hormone signals or light, are detected by specific proteins, receptors and the photosystem, for example, that transfer signals to the cell interior. The visual protein rhodopsin is located in the membranes of retinal cells. Other proteins control the basic cellular fmictions of transcription and translation. 

With the double aim of reducing computational costs as well as of understanding the most basic constituents in photodynamics processes, it has been common to investigate model systems, which are supposed to be the minimal unit that can stiU reproduce the photodynamics observed in the complete molecule. In the following sections, we shall review three classes of molecules and their model systems (1) protonated Schiff bases, models for rhodopsin chro-mophores (2) heteroaromatic rings, models for nucleic acid bases and (3) formamide, model for peptide bonds. 

---