English: General reaction mechanism of catalysed by a catalytic triad (black): nucleophilic substitution at a carbonyl substrate (red) by a second substrate (blue). First, the enzyme's nucleophile (X) attacks the carbonyl to form a covalently linked acyl-enzyme intermediate. This intermediate is then attacked by the second nucleophile (X') of the second substrate. If the second nucleophile is the hydroxyl of water, the result is hydrolysis, otherwise the result is group transfer of X'.