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PLoS One
2009 Sep 15;49:e7032. doi: 10.1371/journal.pone.0007032.
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Characterization of an enantioselective odorant receptor in the yellow fever mosquito Aedes aegypti.
Bohbot JD, Dickens JC.
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Enantiomers differ only in the left or right handedness (chirality) of their orientations and exhibit identical chemical and physical properties. In chemical communication systems, enantiomers can be differentially active at the physiological and behavioral levels. Only recently were enantioselective odorant receptors demonstrated in mammals while their existence in insects has remained hypothetical. Using the two-microelectrode voltage clamp of Xenopus oocytes, we show that the yellow fever mosquito, Aedes aegypti, odorant receptor 8 (AaOR8) acts as a chiral selective receptor for the (R)-(-)-enantiomer of 1-octen-3-ol, which in the presence of other kairomones is an attractant used by blood-sucking insects to locate their hosts. In addition to steric constraints, chain length and degree of unsaturation play important roles in this recognition process. This is the first characterization of an enantioselective odorant receptor in insects and the results demonstrate that an OR alone, without helper proteins, can account for chiral specificity exhibited by olfactory sensory neurons (OSNs).
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19753115 ???displayArticle.pmcLink???PMC2737144 ???displayArticle.link???PLoS One
Figure 1. AaOR8 discriminates between the two enantiomers of 1-octen-3-ol.(A) The odorant 1-octen-3-ol occurs in two configurations: (R) and (S). Asterisk indicates the chiral center. (B) Response traces of AaOR8 to each enantiomer are recorded in nano-ampere (nA). For space considerations, time scales differ. (C) Concentration-response plots of AaOR8 to each enantiomer of 1-octen-3-ol (n = 6). Odorant concentrations were plotted on a logarithmic scale. Each point represents the mean and vertical current response; error bars are s.e.m. Responses to 10−5 M 1-octen-3-ol are highlighted in red.
Figure 2. Strong preference of AaOR8 towards (R)-(—)-1-octen-3-ol.The concentration-response plot for (R)-(—)-1-octen-3-ol was repeated in each panel for comparative purposes. (A) Importance of C3 as a chiral center. Concentration-response plots of AaOR8 to 1-octen-3-one (n = 6). (B) Shifting the chiral center from C3 to C4 reduces AaOR8 sensitivity. Concentration-response plots of AaOR8 to 1-octen-4-ol (n = 8). (C) Side chain length affects AaOR8 sensitivity. Concentration-response plots of AaOR8 to 1-nonen-3-ol and 1-hepten-3-ol (n = 8 to 9). (D) The double bond is critical for recognition by AaOR8. Concentration-response plots of AaOR8 to 3-octanol (n = 6). (E) EC50 ranking profile of AaOR8 for octenol related compounds. Asterisk, p<0.05; two asterisks, p<0.01 and three asterisks, p<0.001. Odorant concentrations were plotted on a logarithmic scale. Each point represents the mean and error bars indicate s.e.m.
Figure 3. Relative activity of AaOR8 towards (R)-(—)-1-octen-3-ol and related compounds.Changes relative to (R)-(—)-1-octen-3-ol are shaded in grey. Presence and absence of a specific chemical feature are indicated by + and -, respectively. The formula of racemic compounds does not assume the 3 dimensional orientation of the residues attached to the chiral center. Note that the least active isomers are those lacking the proper chirality at the 3-position (C3*). Relative activities of each odorant (EC50s) are reflected by area of solid circles.
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