Pheromone degradation degrading enzymes

Our understanding of pheromone reception had undergone dramatic change just prior to 1987 with the proposal that Pheromone Binding Proteins (PBPs) and pheromone degrading enzymes transported and inactivated pheromonal signals... 

Vogt R. G., Riddiford L. M. and Prestwich G. D. (1985) Kinetic properties of a sex pheromone-degrading enzyme the sensillar esterase of Antheraea polyphemus. Proc. Natl. Acad. Sci. USA 82, 8827-8831. 

An antennal-specific aldehyde oxidase (AOX) of M. sexta (MsexAOX) was the next identified pheromone-degrading enzyme (Rybczynski el al., 1989). The activity of MsexAOX was visualized on non-denaturing PAGE, and was shown to be antennal specific but present in sensilla of both male and female antennae. MsexAOX was observed as a dimer with a combined estimated molecular mass of 295 kDa. M. sexta uses a multicomponent pheromone consisting exclusively of aldehydes including bombykal (Starratt el al., 1979 Tumlinson el al., 1989, 1994) MsexAOX was shown to degrade bombykal to its carboxylic acid. Both TLC and spectrophotometric assays were established and a variety of substrates and inhibitors were characterized. Making adjustments for the concentrations and volumes within a sensillum lumen, the in vivo half-life of pheromone was estimated at 0.6 msec in the presence of this enzyme (Rybczynski el al., 1989). 

Vogt R. G. and Riddiford L. M. (1986b) Scale esterase a pheromone degrading enzyme from the wing scales of the silk moth Antheraea polyphemus. J. Chem. Ecol. 12, 469-482. 

Figure 16.1 The three levels of molecular recognition in the pheromone olfactory system of insects. Pheromone adsorbs on the cuticle, where it enters the sensillum lymph through pores (1). The first level of molecular recognition occurs when the PBP binds and desorbs the pheromone from the cuticle (2). PBP transports the pheromone through the lymph to the receptor, where the second level of recognition occurs (3). The third level of recognition involves the pheromone-degrading enzymes, which rapidly inactivate pheromone that has dissociated from the PBP (4). PBP-pheromone and/or pheromone alone may also be removed by an endocytotic process, possibly mediated by SNMP (5). Finally, intracellular enzymes may be involved in further removal of pheromone (6).

An antenna remains in a plume 1 s and an antenna is not an isolated system, as is required to reach equilibrium. The kinetic properties of the PBP-ligand complexes may be more important to the function of PBPs as potential filters than the equilibrium dissociation constants. Thus, ligands with very fast association rate constants and very slow dissociation rate constants are more likely to be bound at the pore surfaces and to traverse the sensillar lymph unharmed by the powerful pheromone-degrading enzymes in the lymph (see below). Thus, in order to understand the function of PBPs, it is essential to obtain more data on binding kinetics. 

Mayer, M. S., S. M. Ferkovich, and R. R. Rutter Localization and reactions of a pheromone degradative enzyme isolated from an insect antenna. Chemical Senses and Flavor 2, 51—61 (1976). 

Ishida, Y. and Leal, W. S. Chiral discrimination of the Japanese beetle sex pheromone and a behavioral antagonist by a pheromone-degrading enzyme. Proc Natl Acad Sci USA 105, 9076-9080 (2008). 

Mayer, 1975 Taylor et al., 1981). Although no specific enzymes were identified, these efforts drew attention to the issues of pheromone degradation as an important component of the pheromone detection process. 

Tasayco M. L. and Prestwich G. D. (1990a) Aldehyde oxidizing enzymes in an adult moth. In vitro study of pheromone degradation in Heliothis virescens. Arch. Biochem. Biophys. 278, 444 151. 

Figure 15.8 Schematic representation of the proposed mechanisms for mode of action of OBPs in the perireceptor events. Pheromone (or other semiochemicals) enters the sensillar lymph through cuticular openings (pore tubules), is solubilized by an odorant-binding protein, transported to the olfactory receptors, and protected from degrading enzymes. Interaction with negatively charged sites at the surface of the dendrites triggers a conformational change that leads to the formation of a C-terminal a-helix. The insertion of this helix into the binding cavity ejects the pheromone to the olfactory receptors.

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