During avocado fruit ripening, decreasing levels of the flavonoid epicatechin have been reported to modulate the metabolism of preformed antifungal compounds and the activation of quiescent Colletotrichum gloeosporioides infections. Epicatechin levels decreased as well when C. gloeosporioides was grown in the presence of epicatechin in culture. Extracts of laccase enzyme obtained from decayed tissue and culture media fully metabolized the epicatechin substrate within 4 and 20 h, respectively. Purified laccase protein from C. gloeosporioides showed an apparent MW of 60,000, an isoelectric point at pH 3.9, and maximal epicatechin degradation at pH 5.6. Inhibitors of fungal laccase such as EDTA and thioglycolic acid reduced C. gloeosporioides symptom development when applied to ripening susceptible fruits. Isolates of C. gloeosporioides with reduced laccase activity and no capability to metabolize epicatechin showed reduced pathogenicity on ripening fruits. On the contrary, Mexican isolates with increasing capabilities to metabolize epicatechin showed early symptoms of disease in unripe fruits. Transcript levels of cglac1, encoding C. gloeosporioides laccase, were enhanced during fungal development in the presence of epicatechin at pH 6.0, where avocado fruits are susceptible to fungal attack. But transcript increase was not detected at pH 5.0, where the fruit is resistant to fungal attack. The present results suggest that biotransformation of epicatechin by C. gloeosporioides in ripening fruits is followed by the decline of the preformed antifungal diene compound, resulting in the activation of quiescent infections.

A transition-state analogue inhibitor that covalently reversibly binds to an enzyme formally consists of two parts: the chemical site, CS and the recognition site, RS. We have experimentally and theoretically demonstrated that the trend of binding affinity in a series of isoselective inhibitors (with identical RS and different CS fragments) depends mainly on their CS fragments. Isoselective inhibitors have the same affinity trend toward different enzymes of the same family with a common catalytic mechanism. Thus, very good correlation between experimentally determined and theoretically calculated Ki values was demonstrated. A practical outcome is the application of the described method as a tool for an expert analysis in virtual screening of inhibitor libraries and in the design of new enzyme inhibitors.

Mechanistic studies of catalysis and the inhibition of serine and cysteine proteases afford new and sometimes surprising insights, challenging conventional dogmas in enzymology. The intrinsic source of the difference in the catalytic mechanisms of serine and cysteine hydrolases, the origin of the stability of the enzymeinhibitor complex in serine proteases, and the structures and mechanisms of catalysis and inhibition in cysteine proteases are not just intellectually interesting; our findings provide a mechanistic basis to understand the trend in the binding affinity of “warheads” of reversible covalent (reaction coordinate analogue, RCA) inhibitors. The theoretically derived covalent descriptors W1 and W2 differentiate serine and cysteine hydrolases and account for the energetic contribution of the new covalent bond in the enzymeinhibitor complex. The W1 and W2 descriptors are at the heart of our enzyme mechanism based method (EMBM); a new computer‐assisted drug design tool for the filtration of inhibitor warheads by activity. EMBM is unique because it accounts for both covalent and noncovalent interactions of RCA inhibitors with their target enzymes.

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