Abbreviations: GPCR, g (TP-binding) protein coupledreceptors; HMM, hidden markov models; OR, olfactory receptor(s); NMR, nuclear magnetic resonance; TM, transmembrane; RMSD, root mean square displacement; RMSF, root mean square fluctuations
OpinionThe publication of the discovery of olfactory receptors (OR) in 1991 1 sparked burgeoning research efforts in the domain of chemoreception. The need to study the role of olfactory receptors in olfaction became imperative, following the publication of the initial drafts of the human genome. Olfactory or olfactory-like receptors mined from the human genome constituted its largest family, possessing between 350 and 400 functional ORs genes. [2][3][4] Interestingly, the functional genes in the OR super family were a little more than third of the approximately, 1000 olfactory receptor repertoire mined from the genome the remaining are non-functional or pseudogenes. This raises very important questions as to how the sense of smell in human beings has evolved. Rodent 5,6 and canine 7 OR repertoires (consisting of about 1200 OR genes), when mined from their genomes, showed, that approximately, half of these genes were pseudogenic, or otherwise, non-functional. More recently, the OR repertoire from the African elephant genome listed 5000 ORs, of which, 40% are functional.
8One of the primary challenges in the study of ORs is elucidating the mechanism(s) by which a few hundred ORs in mammals discriminate several thousand odorants from the environment, which singly or in combination produce distinct and identifiable odors. OR-odorant interactions are promiscuous: one OR is known to bind multiple odorants; odorants, in turn, are known to interact with more than one OR.Research in deorphanizing ORs-identifying odorants or odors that are likely to bind an OR-has adopted a two-pronged approach. The primary approach is experimental, where excitatory responses are identified following the exposure of olfactory receptor neurons (one receptor corresponds to one neuron) to a panel of odorants. [9][10][11][12] Excitatory responses that arise from these interactions over a range of concentrations of odorants (typically organic chemical compounds) are then measured. This opinion paper is primarily concerned with the secondarycomputational-approach. This approach has often been a companion to the experimental functional analysis approach. 13,14 But it is increasingly coming into its own as an independent sub-domain of study. Computational studies involve studying the interactions between ORs and odorants, in silico. This paper will culminate in the recommendation to revisit the established paradigms related to the computational approaches for deorphanizing ORs.
The challenges facing in silico approachesORs are membrane bound proteins. They belong to a class of fairly ubiquitous proteins called G(TP-binding) Protein Coupled Receptors (GPCRs). These receptors, following excitatory responses to stimuli (an interaction with an odorant or odor) bind to a G-protein and initiate a cytoplasmic signal...