Cysteinylated thiol precursors

These assay methods have made it possible to determine the location of cysteinylated thiol precursors in Sauvignon Blanc grapes (Figure 7.12). The 4-MMP and 4-MMPOH precursors are mainly located in the flesh (approximately 80%), while the skin and flesh contain equal amounts of 3-MH precursor (Peyrot des Gachons et al., 2002a). Similarly, a majority (60%) of the 3-mercaptohexan-... 

Methods to measure cysteinylated aroma precursors in must were developed by Peyrot des Gachons et al. (2000) and Murat et al. (2001b). They allowed the location of thiol precursors to be determined in Sauvignon blanc grapes (Fig. 8B.7). Around 80% of P-4MSP is in the juice and 50% of P-3SH in the skin. The skin contact improves the must aromatic potential, but its main effect is on the P-3SH content (Fig. 8B.8). Similarly, in Cabernet Sauvignon and Merlot grapes, 60% of the P-3SH is located in the skins This method has been also applied to study the influence of... 

Tominaga, T, Dubourdieu, D. (2000). Identification of cysteinylated aroma precursors of some volatil thiols in Passion fruit juice. J. Agric. Food Chem., 48, 2874-2876. 

Pena-GaUego, A., Flemandez-Orte, P., Cacho, J., Ferreira, V. (2012). 5-cysteinylated and S-glutathionylated thiol precursors in grapes - a review. Food Chemistry, 131,1-13. 

Tominaga T., Dubourdieu D. Identification of cysteinylated aroma precursors of certain volatile thiols in passion fruit juice. Journal of Agricultural and Food Chemistry, 48 2874-2876 (2000). 

The thiol-template mechanism is utilized in other enzymes involved in production of peptide-based antibiotics. Actinomycin synthetase II (ACMSII) and b-L-(a-aminoadipolyl)-L-cysteinyl-D-valine (ACV) synthetase catalyze the stereoinversion of valine residues vithin peptide-based antibiotics, and employ ATP and the PAN cofactor in a mechanism similar to that depicted in Fig. 7.14 [58, 59]. ACMSII catalyzes the stereoinversion of a valine within the tripeptide 4-MHA-L-Thr-D-Val (MHA, 4-methyl-3-hydroxyanthranilic acid), which is a precursor for the antibiotic actinomycin D. ACV synthetase catalyzes the stereoinversion of the valine within ACV, which is a precursor for penicillin and cephalosporin [60-63]. ACV synthetase has been shown to have much broader substrate specificity, also accepting non-natural substrates [64, 65]. 

From an organoleptic standpoint, S-cysteine conjugates are responsible for the nnnsnal sensation known as Sanvignon Blanc aftertaste, perceived on tasting Sanvignon Blanc grapes or mnst. Enzymes in the month probably have a /S-lyase activity capable of releasing volatile thiols from their cysteinylated precursors in jnst a few seconds. 

Much larger quantities of cysteinylated precursors of volatile thiols are present in the must... 

Fig. 7.13. Formation of volatile thiols from their cysteinylated precursors in Sauvignon Blanc must during alcoholic fermentation...

The peptidic nature of penicillin was not recognized immediately after its isolation. In its jS-lactam bicyclic ring structure the tripeptide -(L-a-aminoadipoyl)-L-cysteinyl-D-valine is hidden but in 1960 it was discovered in extracts of Penicillium by Arnstein et al. [11]. Experiments on biosynthesis of penicillins in the intact mycelium of the producing fungi did not lead to unequivocal results, due to the poor permeability of the cell wall. Protoplasts, naked cells obtained after enzymatic removal of the outer wall, however, were able to serve in studies with radioactively labeled potential precursors, and in cell-free systems from Cephalosporium acremonium, the route to isopenicillin N was finally revealed [12]. The tripeptide L-a-aminoadipoyl-L-cysteinyl-D-valine first forms the j -lactam moiety and a second, oxidative step closes the thiazolidine ring between the -carbon of valine and the thiol of cysteine (Fig. 9). 

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