Olfaction and olfactory enzymes – OLFZYME

To study the flavor molecules metabolization in link with their perception, several enzymes have been studied with a particular focus on the glutathione transferase family for which the role of both olfactory and taste has been demonstrated. To achieve this goal, a combination of molecular approaches was carried out allowing on the one hand the production and purification of these pure enzymes from bacterial systems. Then, we have studied their capacity to metabolize flavor molecules (odorant and taste molecules) and finally the resolution of their structure in complex with these molecules at the atomic scale. Indeed, among the metabolized aroma molecules, cinnamaldehyde, a molecule with a cinnamon scent was obtained in complex with a nasal metabolizing enzyme. Immunohistochemistry approaches were used to show in the rodent model that these enzymes are localized in different tissues of the oral sphere with differences depending on the enzyme. These analyzes have also been extended to humans to confirm that they are also present there with interesting expression differences in humans compared to rodents. Electron microscopy and neural activity recording techniques also supported the OlfZyme hypothesis.

The outstanding results of the OlfZyme project are numerous 1- the demonstration of the enzymes role in sensory perception which allows us to improve the aromatic formulation of perfumes and food products, 2 - the resolution of the first structure of one of these enzymes at the atomic scale in complex with an odorant molecule, 3 - the mapping of the localization of these enzymes in the oral sphere, 4 - the recording of the effect of this metabolization on olfactory neurons and the demonstration of the possibility of modulating this phenomenon by adding another enzyme.

OlfZyme allow new strategy in food formulation for aromas as well as for bitter perceived molecules in the population. Additionally, results on people suffering of taste disorder open new strategy for food formulation for this specific population.

The OlfZyme project has so far made it possible to produce around ten international scientific publications in major journals. These publications have demonstrated our initial hypothesis devlopped in the OlfZyme project supporting the role of enzymes in the oral sphere in sensory perception. In addition, these results have been communicated to the scientific community through conferences as well as to the general public on the occasion of events such as the Science Festival or conferences.

The process conducting to the both, orthonasal and retronasal odour perceptions includes: the biochemical odorant detection, the signal transduction and the signal integration. To detect odorant molecules, we are equipped of approximately 350 olfactory receptors that are expressed in the olfactory epithelium, and a combinatorial activation of olfactory receptors is at the basis of the generated signal. The odorant molecule triggers the olfactory receptors by crossing directly the olfactory mucus or after a salivary step. The olfactory mucus is an aqueous interface between the olfactory receptors and odorants mainly constituted of mucins and many enzymes. In mammalian, the first step of odorant detection can be modulated by the salivary and mucosal enzymatic activity. Interestingly, in human these enzymes can also be inhibited by odorants, and odorant acting as an enzyme inhibitor could modify the smell perception of another odorant. Among the enzymes, glutathione transferases (GSTs), namely, GSTA1 and GSTP1, have been identified in both, the olfactory mucus and the saliva. We assume that GSTA1 and GSTP1 may have a dual role in both, orthonasal and retronasal odorant perception. The role of GSTs in retronasal perception includes an additional salivary step compare to the orthonasal perception. During the salivary step the GSTs can modify the aromas composition, the aromas speed release and consequently their perception.

The choice of these both enzymes as a model presents an additional opportunity with their polymorphism in more than 10% of the human population. The polymorphism may result in a lower GST enzyme activity, a lack of activity or a change of the molecule spectrum interacting with the GSTs. Even if the impact of the genetic polymorphism of the two olfactory GSTs on the sense of smell has not been studied yet, the link between these enzymes and human diet choice is already established.

Understanding the role of these enzymes in olfaction by using a combination of different approaches, will bring crucial information on the role of these enzymes in human olfaction directly acting in the human mucosa and indirectly acting in human saliva. More particularly, OLFZYME should provide a unique opportunity
(1) to characterize the binding capacity of these enzymes towards odorants,
(2) to determine the impact of the polymorphism on the odorant binding,
(3) to measure GSTs activity toward odorant in biological sample as saliva and mucosa,
(4) to determine using cell assay and imaging and human olfactory epithelium if these enzymes are able to interact with olfactory receptors, and if yes how,
(5) finally provide a proof of concept that smell perception can be modulate based on the salivary and / or olfactory mucus GSTs activity

In term of societal impact, OLFZYME will allow
(1) formulating food with superior sensory characteristics by rationally designing a mixture of odorants able to modulate the human smell perception;
(2) to identify target populations with increased health risks due to GSTs coding genes polymorphism-induced poor diet habits.

To achieve its objectives, OLFZYME proposes a multidisciplinary approach associating expertise in cell culture, protein biochemistry, imaging technics, analytical chemistry, bioinformatics and sensory analysis. Moreover, OLFZYME is a fundamental research project, which will give the opportunity to a young researcher to develop his own research thematic and lead in the future his own research group.