How to address the function of a member of a prolific gene family encoding a multifunctional enzyme?: The case of a Catharanthus roseus class III peroxidase involved in alkaloid biosynthesis

Class III peroxidases (PRXs) are characteristic of plants and they form a gene family counting 73 genes in Arabidopsis. They arc highly plastic, multifunctional enzymes able to oxidise a wide spectrum of substrates in vitro, a fact that makes it difficult to confirm each individual function. In Catharanthus roseus, we have purified, characterized and cloned a basic PRX capable of performing, in vitro, the dimerization reaction leading to alpha-3',4'-anhydrovinblastine (AVLB), the direct precursor of two anticancer alkaloids. This basic PRX was the single PRX detected in leaves, and cytochemical and homology studies indicated a subcellular localization in the vacuole, together with the alkaloids. The merging of all gathered information points to a function of the enzyme as AVLB synthase, possibly involving a mechanism of metabolic channelling.