Metabolism of Cysteine Conjugates and Production of Flavor Sulfur Compounds by a Carbon–Sulfur Lyase from the Oral Anaerobe <i>Fusobacterium nucleatum</i>

Abstract

Flavor perception is a key factor in the acceptance or rejection of food. Aroma precursors such as cysteine conjugates are present in various plant-based foods and are metabolized into odorant thiols in the oral cavity. To date, the involved enzymes are unknown, despite previous studies pointing out the likely involvement of carbon-sulfur lyases (C-S lyases) from the oral microbiota. In this study, we show that saliva metabolizes allyl-cysteine into odorant thiol metabolites, with evidence suggesting that microbial pyridoxal phosphate-dependent C-S lyases are involved in the enzymatic process. A phylogenetic analysis of PatB C-S lyase sequences in four oral subspecies of <i>Fusobacterium nucleatum</i> was carried out and led to the identification of several putative targets. FnaPatB1 from <i>F. nucleatum</i> subspecies <i>animalis</i>, a putative C-S lyase, was characterized and showed high activity with a range of cysteine conjugates. Enzymatic and X-ray crystallographic data showed that FnaPatB1 metabolizes cysteine derivatives within a unique active site environment that enables the formation of flavor sulfur compounds. Using an enzymatic screen with a library of pure compounds, we identified several inhibitors able to reduce the C-S lyase activity of FnaPatB1 in vitro, which paves the way for controlling the release of odorant sulfur compounds from their cysteine precursors in the oral cavity.