Serine carboxypeptidases contain a conserved catalytic triad of serine, histidine, and aspartic acid active-site residues. These enzymes cleave the peptide bond between the penultimate and C-terminal amino acid residues of their protein or peptide substrates. The Arabidopsis Genome Initiative has revealed that the Arabidopsis genome encodes numerous proteins with homology to serine carboxypeptidases. Although many of these proteins may be involved in protein turnover or processing, the role of virtually all of these serine carboxypeptidase-like (SCPL) proteins in plant metabolism is unknown. We previously identified an Arabidopsis mutant, sng1 (sinapoylglucose accumulator 1), that is defective in synthesis of sinapoylmalate, one of the major phenylpropanoid secondary metabolites accumulated by Arabidopsis and some other members of the Brassicaceae. We have cloned the gene that is defective in sng1 and have found that it encodes a SCPL protein. Expression of SNG1 in Escherichia coli demonstrates that it encodes sinapoylglucose:malate sinapoyltransferase, an enzyme that catalyzes a transesterification instead of functioning like a hydrolase, as do the other carboxypeptidases. This finding suggests that SCPL proteins have acquired novel functions in plant metabolism and provides an insight into the evolution of secondary metabolic pathways in plants.
Serine carboxypeptidases contain a conserved catalytic triad of serine, histidine, and aspartic acid active-site residues. These enzymes cleave the peptide bond between the penultimate and C-terminal amino acid residues of their protein or peptide substrates. The Arabidopsis Genome Initiative has revealed that the Arabidopsis genome encodes numerous proteins with homology to serine carboxypeptidases. Although many of these proteins may be involved in protein turnover or processing, the role of virtually all of these serine carboxypeptidase-like (SCPL) proteins in plant metabolism is unknown. We previously identified an Arabidopsis mutant, sng1 (sinapoylglucose accumulator 1), that is defective in synthesis of sinapoylmalate, one of the major phenylpropanoid secondary metabolites accumulated by Arabidopsis and some other members of the Brassicaceae. We have cloned the gene that is defective in sng1 and have found that it encodes a SCPL protein. Expression of SNG1 in Escherichia coli demonstrates that it encodes sinapoylglucose:malate sinapoyltransferase, an enzyme that catalyzes a transesterification instead of functioning like a hydrolase, as do the other carboxypeptidases. This finding suggests that SCPL proteins have acquired novel functions in plant metabolism and provides an insight into the evolution of secondary metabolic pathways in plants. INTRODUCTIONPlants produce thousands of unique molecules that are collectively referred to as secondary metabolites. Even within the angiosperms, many of these compounds are unique to specific taxa, indicating that the pathways that produce them may have evolved within the last 100,000 years. A central question in the study of plant secondary metabolism concerns how the catalytic diversity of plant secondary metabolism has arisen. What classes of genes and proteins have been co-opted, presumably from their ancestral roles in primary metabolism, to serve as catalysts in the synthesis of secondary metabolites?In Arabidopsis, the phenylpropanoid pathway leads to the production of sinapic acid esters, a group of fluorescent UVprotective secondary metabolites derived from phenylalanine (Figure 1). These compounds are dispensable under laboratory conditions and thus provide targets for the genetic dissection of phenylpropanoid metabolism. The analysis of these compounds is facilitated by their blue fluorescence under UV light both in vivo and after thin-layer chromatography (TLC) (Chapple et al., 1992;Ruegger et al., 1999). Arabidopsis and some other members of the Brassicaceae accumulate three major sinapic acid esters. In the biosynthetic pathway leading to these compounds, sinapoylglucose is the immediate precursor of sinapoylcholine and sinapoylmalate, which accumulate in seeds and leaves, respectively. 1-O -Sinapoylglucose is a  -acetal ester with a high free energy of hydrolysis (Mock and Strack, 1993); it provides the necessary free energy for the transacylation reaction catalyzed by sinapoylglucose:malate sinapoyltransferase (SMT; EC 2.3.1.92) (Strack, 1982), which ...
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