In plant secondary metabolism, b-acetal ester-dependent acyltransferases, such as the 1-O-sinapoyl-b-glucose:l-malate sinapoyltransferase (SMT; EC ), are homologous to serine carboxypeptidases. Mutant analyses and modeling of Arabidopsis SMT (AtSMT) have predicted amino acid residues involved in substrate recognition and catalysis, confirming the main functional elements conserved within the serine carboxypeptidase pro-tein family.