Mariam Muradova

Glycoside hydrolases catalyze the hydrolysis of glycosidic bonds in diverse substrates. Oral bacteria can metabolize glycosidic precursors present in foods into aroma compounds. This metabolism has an impact on food sensory perception and presumably involves specific glycosidases belonging the glycoside hydrolases. Comprehensive elucidation of the structural and functional characteristics of glycoside hydrolases in oral bacteria is needed for advancing our understanding of aroma compound metabolism within the oral cavity. In this context, we have identified a glycoside hydrolase coded in the Prevotella sp. genome, exhibiting homology to β-glycosidases of the GH1 family. This enzyme, designated as PsBG1, was successfully expressed and purified for crystallographic investigation. PsBG1 crystals belong to the monoclinic space group P2₁ with unit cell parameters a = 134.2, b = 139.5, c = 172.7 Å and β = 99.8°. The crystal structure of PsBG1 was elucidated using molecular replacement techniques, utilizing a predictive model constructed with AlphaFold2. Analysis revealed that the asymmetric unit comprises three copies of homotetramer, while the predominant oligomeric state in solution is also a homotetramer. Ongoing efforts are focused on the refinement and detailed examination of the PsBG1 structure to clarify its functional significance in the metabolism of aromatic compounds.

Aroma is among of the most important criteria that indicate the quality of food and beverage products. Aroma compounds can be found as free molecules or glycosides. Notably, a significant portion of aroma precursors accumulates in numerous food products as nonvolatile and flavorless glycoconjugates, termed glycosidic aroma precursors. When subjected to enzymatic hydrolysis, these seemingly inert, nonvolatile glycosides undergo transformation into fragrant volatiles or volatiles that can generate odor-active compounds during food processing. In this context, microbial β-glucosidases play a pivotal role in enhancing or compromising the development of flavors during food and beverage processing. β-glucosidases derived from bacteria and yeast can be utilized to modulate the concentration of particular aroma and taste compounds, such as bitterness, which can be decreased through hydrolysis by glycosidases. Furthermore, oral microbiota can influence flavor perception by releasing volatile compounds that can enhance or alter the perception of food products. In this review, considering the glycosidic flavor precursors present in diverse food and beverage products, we underscore the significance of glycosidases with various origins. Subsequently, we delve into emerging insights regarding the release of aroma within the human oral cavity due to the activity of oral microbial glycosidases.

Glutathione transferases (GSTs) are ubiquitous key enzymes with different activities as transferases or isomerases. As key detoxifying enzymes, GSTs are expressed in the chemosensory organs. They fulfill an essential protective role because the chemosensory organs are located in the main entry paths of exogenous compounds within the body. In addition to this protective function, they modulate the perception process by metabolizing exogenous molecules, including tastants and odorants. Chemosensory detection involves the interaction of chemosensory molecules with receptors. GST contributes to signal termination by metabolizing these molecules. By reducing the concentration of chemosensory molecules before receptor binding, GST modulates receptor activation and, therefore, the perception of these molecules. The balance of chemoperception by GSTs has been shown in insects as well as in mammals, although their chemosensory systems are not evolutionarily connected. This review will provide knowledge supporting the involvement of GSTs in chemoperception, describing their localization in these systems as well as their enzymatic capacity toward odorants, sapid molecules, and pheromones in insects and mammals. Their different roles in chemosensory organs will be discussed in light of the evolutionary advantage of the coupling of the detoxification system and chemosensory system through GSTs.