Varietal thiols are considered for years as key aroma compounds in many wines. Their main origin is the cleavage during alcoholic fermentation of S-conjugate precursors present in grapes and musts, even if the levels of precursors already identified struggle to completely explain the levels of thiols found in wine.
In this bioconversion process, yeast is the key since the cleavage of thiol precursors has been demonstrated to be due to the internal C-S lyase activity of the yeast strains. Whatever the intrinsic capacity of the yeast and the regulation mechanisms identified for the transportation of thiol precursor into the cell, the conversion yield remains very low, estimated in most cases at 1%. In this context, the use of exogenous C-S lyases could be an alternative option to reveal a larger part of the thiol aroma.
Our study focused on the characterization of a recombinant C-S lyase model obtained by from Lactobaccilus delbrueckii subsp. bulgaricus, expressed in E. coli [1] to investigate the possibility of using such enzymes in different matrices such as wine, beer or directly hops, known to be rich in thiol precursors.
A spectrophotometric method was developed for the quantification of the C-S lyase activity, using commercially available S-4-nitrophenyl-L-cysteine (Cys(4NP)). This method was then used to study the specificity of substrate and potential competitor recognition, still using Cys(4NP) but also G-4NP, Cys(4NP)-Gly and γGlu-Cys(4NP) conjugates, which were specifically synthesized in our lab, based on our previous results [2].
The C-S lyase we used was able to cleave efficiently Cys(4NP), but not glutathione and dipeptide 4NP-S-conjugates. As expected, pH emerged as a key parameter: no reaction at wine pH (2.8-3.5), low activity at beer pH (4.5-5.6) and > 80% of activity at pH above 6. Assays with N-Ac-Cys(4NP) confirmed that a free amine group on the substrate was compulsory for recognition by the enzyme and subsequent cleavage of the substrate. Free cysteine has also been demonstrated to compete with Cys(4NP) resulting in a dramatic decrease in conversion efficiency.
These first results documented the possibility of using such enzyme in the different matrices, highlighting the constraints for the subsequent identification of C-S lyase more suitable to wine or beer productions
Authors: Clerat Luigi1, Vives Eric1, Cavelier Florine2, Remond Emmanuelle2 and Schneider Rémi3
1PhyMedExp – Physiologie & médecine expérimentale du Cœur et des Muscles [U 1046]
2Institut des biomolécules Max Mousseron (IBMM) – UMR-5247 – CNRS
3Oenobrands Montpellier FR
*corresponding author: luigi.clerat@inserm.fr
References:
[1] Allegrini, A.; Astegno, A.; La Verde, V.; Dominici, P. Characterization of C-S lyase from Lactobacillus delbrueckii subsp. bulgaricus ATCC BAA-365 and its potential role in food flavor applications. J. Biochem. 2017, 61, 349−360.
[2] Bonnaffoux, H., Roland, A., Rémond, E., Delpech, S., Schneider, R., & Cavelier, F. (2017). First identification and quantification of S-3-(hexan-1-ol)-γ-glutamyl-cysteine in grape must as a potential thiol precursor, using UPLC-MS/MS analysis and stable isotope dilution assay. Food Chemistry, 237, 877–886.
Keywords: C-S lyase, varietal thiol precursors, wine aroma, S-conjugates
