Proanthocyanidin Biosynthesis in Plants

Abstract

Leucoanthocyanidin reductase (LAR) catalyzes the synthesis of catechin, aninitiating monomer of condensed tannin or proanthocyanidin (PA) synthesis,from 3,4--leucocyanidin and thus is the first committed step in PAbiosynthesis. The enzyme was purified to near homogeneity from PA-rich leavesof the legume (Jacq.) DC, partially sequenced andthe corresponding cDNA cloned. The identity of the enzyme was confirmed byexpressing active recombinant LAR in and in tobaccoand white clover. The enzyme is a monomer of 43 kDa (382 amino acids) and ismost active synthesizing catechin (specific activity of ∼10 μmolmin mg of protein) but alsosynthesizes afzelechin and gallocatechin. LAR is most closely related to theisoflavone reductase group of plant enzymes that are part of theReductase-Epimerase-Dehydrogenase (RED) family of proteins. Unlike all otherplant isoflavone reductase homologues that are about 320 amino acids long, LARhas an additional 65-amino acid C-terminal extension whose function is notknown. Curiously, although makes PA, there is no obviousLAR orthologue in the genome. This may be becauseseems to produce only an epicatechin, rather than a dualcatechin/epicatechin-based PA similar to many other plants.