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Selectivity of odorant-binding proteins from the southern house mosquito tested against physiologically relevant ligands.

Yin J, Choo YM, Duan H, Leal WS - Front Physiol (2015)

Bottom Line: Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5.Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1.Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biology, University of California, Davis Davis, CA, USA.

ABSTRACT
As opposed to humans, insects rely heavily on an acute olfactory system for survival and reproduction. Two major types of olfactory proteins, namely, odorant-binding proteins (OBPs) and odorant receptors (ORs), may contribute to the selectivity and sensitivity of the insects' olfactory system. Here, we aimed at addressing the question whether OBPs highly enriched in the antennae of the southern house mosquito, Culex quinquefasciatus, contribute at least in part to the selective reception of physiologically relevant compounds. Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5. Based on dissociation constants, we surmised that CquiOBP2 is a carrier for the oviposition attractant skatole, whereas CquiOBP1 and CquiOBP5 might transport the oviposition pheromone MOP, a human-derived attractant nonanal, and the insect repellent picardin. Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1. Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands. These findings, along with previous experimental evidence suggesting that CquiOBP1 does not detect nonanal in vivo, suggest that OBP selectivity may not be clearly manifested in their dissociation constants.

No MeSH data available.


Related in: MedlinePlus

Docking of ligands into CquiOBP1. (A) Re-docked MOP (cyan) and MOP as observed in the CquiOBP1-MOP crystal complex (Mao et al., 2010). (B) Nonanal bound to the main cavity and stabilized by a hydrogen bond with the main chain of Phe-123. (C) Picaridin bound in the same cavity, but having a different orientation and the hydroxyl group forming a hydrogen bond with the side chain of Tyr-10. Figures were generated with Sybyl 7.3.
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Figure 4: Docking of ligands into CquiOBP1. (A) Re-docked MOP (cyan) and MOP as observed in the CquiOBP1-MOP crystal complex (Mao et al., 2010). (B) Nonanal bound to the main cavity and stabilized by a hydrogen bond with the main chain of Phe-123. (C) Picaridin bound in the same cavity, but having a different orientation and the hydroxyl group forming a hydrogen bond with the side chain of Tyr-10. Figures were generated with Sybyl 7.3.

Mentions: To assess the suitability of dock simulations in providing insights into the interactions of nonanal and picaridin with CquiOBP1, we first re-docked MOP into CquiOBP1 and compared this structure with the previously reported crystal structure (Figure 4A). The position of the polar moiety of MOP in the binding pocket matched that observed in the crystal structure. As previously described, MOP has its long lipid “tail” bound in a hydrophobic tunnel formed between helices 4 and 5 and only its lactone/acetate head is housed in the central cavity (Mao et al., 2010). There was a slight difference between the position of the hydrophobic moiety of MOP in the simulated and crystal structures, but this part of the molecule is flexible and different conformations could be accommodated in the hydrophobic tunnel. Docked nonanal was stabilized by a hydrogen bond between the oxygen atom of the carbonyl as a hydrogen bond acceptor and the (-NH-) group of Phe-123 in the backbone as hydrogen bond donor. The short hydrophobic tail of nonanal was only partially accommodated in the hydrophobic tunnel (Figure 4B). Interestingly, nonanal bound at the periphery of the central cavity even further away from the center than MOP (Figures 5A,B). Likewise, picaridin was accommodated at a similar location (Figure 5C), but having a different orientation. The polar moiety of picaridin was stabilized in the central binding cavity by a hydrogen bond between the hydroxyl group of picaridin (acceptor) and the side chain of Tyr-10 (donor) (Figure 4C). Despite the fact that all ligands are accommodated in the same binding pocket the orientations of the ligands in the binding pocket were different (Figures 5D,E). Additionally, MOP occupied the hydrophobic channel more so than the other ligands (Figure 5E). That MOP fits better in the binding cavity of CquiOBP1 is reflected in the Cscore of 8.51, as compared to 5.55 and 5.47 for picaridin and nonanal, respectively. These scores correlate well with the above-described dissociation constants.


Selectivity of odorant-binding proteins from the southern house mosquito tested against physiologically relevant ligands.

Yin J, Choo YM, Duan H, Leal WS - Front Physiol (2015)

Docking of ligands into CquiOBP1. (A) Re-docked MOP (cyan) and MOP as observed in the CquiOBP1-MOP crystal complex (Mao et al., 2010). (B) Nonanal bound to the main cavity and stabilized by a hydrogen bond with the main chain of Phe-123. (C) Picaridin bound in the same cavity, but having a different orientation and the hydroxyl group forming a hydrogen bond with the side chain of Tyr-10. Figures were generated with Sybyl 7.3.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4343023&req=5

Figure 4: Docking of ligands into CquiOBP1. (A) Re-docked MOP (cyan) and MOP as observed in the CquiOBP1-MOP crystal complex (Mao et al., 2010). (B) Nonanal bound to the main cavity and stabilized by a hydrogen bond with the main chain of Phe-123. (C) Picaridin bound in the same cavity, but having a different orientation and the hydroxyl group forming a hydrogen bond with the side chain of Tyr-10. Figures were generated with Sybyl 7.3.
Mentions: To assess the suitability of dock simulations in providing insights into the interactions of nonanal and picaridin with CquiOBP1, we first re-docked MOP into CquiOBP1 and compared this structure with the previously reported crystal structure (Figure 4A). The position of the polar moiety of MOP in the binding pocket matched that observed in the crystal structure. As previously described, MOP has its long lipid “tail” bound in a hydrophobic tunnel formed between helices 4 and 5 and only its lactone/acetate head is housed in the central cavity (Mao et al., 2010). There was a slight difference between the position of the hydrophobic moiety of MOP in the simulated and crystal structures, but this part of the molecule is flexible and different conformations could be accommodated in the hydrophobic tunnel. Docked nonanal was stabilized by a hydrogen bond between the oxygen atom of the carbonyl as a hydrogen bond acceptor and the (-NH-) group of Phe-123 in the backbone as hydrogen bond donor. The short hydrophobic tail of nonanal was only partially accommodated in the hydrophobic tunnel (Figure 4B). Interestingly, nonanal bound at the periphery of the central cavity even further away from the center than MOP (Figures 5A,B). Likewise, picaridin was accommodated at a similar location (Figure 5C), but having a different orientation. The polar moiety of picaridin was stabilized in the central binding cavity by a hydrogen bond between the hydroxyl group of picaridin (acceptor) and the side chain of Tyr-10 (donor) (Figure 4C). Despite the fact that all ligands are accommodated in the same binding pocket the orientations of the ligands in the binding pocket were different (Figures 5D,E). Additionally, MOP occupied the hydrophobic channel more so than the other ligands (Figure 5E). That MOP fits better in the binding cavity of CquiOBP1 is reflected in the Cscore of 8.51, as compared to 5.55 and 5.47 for picaridin and nonanal, respectively. These scores correlate well with the above-described dissociation constants.

Bottom Line: Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5.Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1.Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biology, University of California, Davis Davis, CA, USA.

ABSTRACT
As opposed to humans, insects rely heavily on an acute olfactory system for survival and reproduction. Two major types of olfactory proteins, namely, odorant-binding proteins (OBPs) and odorant receptors (ORs), may contribute to the selectivity and sensitivity of the insects' olfactory system. Here, we aimed at addressing the question whether OBPs highly enriched in the antennae of the southern house mosquito, Culex quinquefasciatus, contribute at least in part to the selective reception of physiologically relevant compounds. Using a fluorescence reporter and a panel of 34 compounds, including oviposition attractants, human-derived attractants, and repellents, we measured binding affinities of CquiOBP1, CquiOBP2, and CquiOBP5. Based on dissociation constants, we surmised that CquiOBP2 is a carrier for the oviposition attractant skatole, whereas CquiOBP1 and CquiOBP5 might transport the oviposition pheromone MOP, a human-derived attractant nonanal, and the insect repellent picardin. Binding of these three ligands to CquiOBP1 was further analyzed by examining the influence of pH on apparent affinity as well as by docking these three ligands into CquiOBP1. Our findings suggest that CquiOBP1 might discriminate MOP from nonanal/picaridin on the basis of the midpoint transition of a pH-dependence conformational change, and that MOP is better accommodated in the binding cavity than the other two ligands. These findings, along with previous experimental evidence suggesting that CquiOBP1 does not detect nonanal in vivo, suggest that OBP selectivity may not be clearly manifested in their dissociation constants.

No MeSH data available.


Related in: MedlinePlus