Structure Odorant receptors

Vomeronasai organ a structure possessing odorant receptors for pheromones. 

Molecular structure must be implicated as odorants bind specifically with the sensory receptors called odorant receptors (ORs). The olfactory mucus has proteins called odorant binding proteins (OBPs) that dissolve the odorant molecule in the aqueous/lipid interface of the mucus. The OBPs act as binder molecules to assist the transfer of odorant to the receptor and increase its relative concentration in the mucus relative to inhaled air. They also function to remove used odorants for breakdown and free up the receptor to detect other molecules. 

When the odorant binds to the odorant receptor it changes the receptor structure and activates an olfactory protein called a G protein. This in turn converts ATP (adenosine triphosphate) to cAMP (cyclic adenosine monophosphate) that allows opening of ion channels, causing the receptor to become depolarized. Depolarization is an electrical change that triggers a nerve impulse. Impulses from the nasal receptors are sent along the olfactory nerve to the brain. 

Whereas there are thousands of odorant molecules and odorant receptors, there are only some tens of OBPs in any one species. The three-dimensional structure of a bovine odorant-binding protein has been solved. Structural information has provided a clue to how a single dimeric OBP can accomodate so many different odorant molecules. But it is not clear whether OBPs are the only proteins carrying odorants to their receptors, nor is it clear how they find the right receptor. Moreover, at present it remains questionable whether OBPs have other functions, for example as scavengers, removing odorants. 

Most olfaction models in the literature are far too simplistic and too mechanical in nature, and none of them have succeeded in accounting for all of the observations about olfaction. As described, recent advances in our understanding have confirmed that odor perception, as predicted by Polak (19), starts with a combinatorial mechanism at the receptor level (1) and involves pattern recognition in the higher brain (4). No single odorant-receptor interaction will be the sole determinant of odor percept, and even knowledge of the pattern elicited at the olfactory bulb is insufficient to enable prediction of the cortical image of odor. Therefore, structure/odor models are and, for the foreseeable future, will remain statistical tools rather than mechanistic indicators. 

Odor is subjective even at the most basic level. It is unlikely that any two humans (except identical twins) use the same set of receptors in their epithelial array (7). Differences between individuals in subsequent neuroprocessing, because of physiologic and experiential factors, increase interindividual differences in odor perception. Therefore, information going into structure/odor models is either relevant for one individual or is an average figure. Comparison of data from one individual to another or from one average to another is always suspect and can be totally irrelevant. 

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.)...

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