All civilizations, from antiquity to present time, have used fragrances for a variety of purposes. Before the advent of organic synthesis, fragrances were often limited to those found in the form of oils, balsams, exudates, and resins.1 With development of organic synthesis during the 19th Century, fragrance chemistry advancedinto industrial synthesis and distribution. Since that time, a great deal has been achieved in both the understanding of the biology of smell, olfaction, as well as the development of new and unique fragrances. Biology of Olfaction Odorant binding proteins (OBPs) are small, water soluble proteins that are approximately 19 kDa in size.2 OBPs belong to a family of carrier proteins known aslipocalins and reversibly bind a variety of different odorants in the nM to µM range.2 The ability for OBP to bind such a wide array of odorants, as well as the selective localization in the lateral nasal cavity, suggest a physiological role in olfaction.3 Bovine OBP was crystallized as a homodimer in which the two subunits are held together by noncovalent bonds.4 Each monomer has an eight strandedβ-barrel with an α-helix. In the dimer, the α-helix of one monomer completes the β-barrel of the other as shown in Figure 1. Based on a wealth of structure-activity relationship data, there are three
Figure 1. Crystal structure of Bovine OBP homodimer. possible roles OBP could play in the process of olfaction.5 The first would be to act as a buffer, lowering the concentration of odorants that bind tothe olfactory receptors.6 This action would trap most of the molecules that would otherwise inactivate the olfactory receptors for a long period of time. A second role of OBP could be to serve as a carrier protein like the lipocalins.7 Most odorants are small hydrophobic molecules, poorly soluble in the aqueous mucus surrounding the olfactory receptors. OBP could bind odorants and transport themto the receptor. The third role OBP could play in olfaction is that of a transducer.8 In this scenario, OBP would bind to the odorant and interact with the receptor as a complex. This model allows for discrimination of odors by OBP and not purely by the receptors in the olfactory epithelium.
The olfactory receptors belong to the largest group of receptors for neurotransmitters,Gprotein-coupled receptors (GPCRs). GPCRs are membrane bound proteins containing seven αhelical domains that are connected by six loops (three extracellular and three intracellular).9,10 These receptors are thought to be bound to the olfactory cilia of sensory neurons whose axons project into the olfactory bulb in the brain.11-14 In order to elucidate the pathway for olfactory signal transduction, 18different members of a multigene family that encodes seven transmembrane proteins whose expression is restricted to the olfactory epithelium were characterized.15 Shown in Figure 2 is the protein encoded by cDNA clone I15, which is
Figure 2. Expression of I15, G-Protein Coupled Receptor believed to transverse the cell membrane seven times. Amino acids in white circles illustrate positions at which 60%or more of the clones share the same residues, whereas those in black illustrate more variable residues. The high degree of variability in domains III, IV, and V suggest that odorant molecules may bind between these domains. This would allow for the discrimination of odorants by different GPCRs. The signal transduction process involving these receptors is shown in Figure 3. It is believed thatodorants bind to the membrane bound
Figure 3. Mechanism of signal transduction in olfactory epithelium. receptor which itself is bound to a GTP (guanosine triphosphate) binding protein also known as a G-protein. Binding of the odorant causes the α domain (Gα) to dissociate from the β and γ domains. Gα then binds to adenylyl cyclase which converts ATP into cAMP. cAMP acts as a second messenger...