Background: Type II pyridoxal 5'-phosphate decarboxylases are an important group of phylogenetically diverse enzymes involved in amino acid metabolism. Within plants, this group of enzymes is represented by aromatic amino acid decarboxylases, glutamate decarboxylases and serine decarboxylases. Additional evolutionary divergence of plant aromatic amino acid decarboxylases has resulted in further subcategories with distinct substrate specificities and enzymatic activities. Despite shared homology, no such evolutionary divergence has been characterized within glutamate decarboxylases or serine decarboxylases (SDC).
Results: Comparative analysis of two previously characterized serine decarboxylase-like (SDC-like) enzymes demonstrates distinct substrate specificities despite their highly conserved primary sequence. The alternate substrate preference of these homologous SDC-like proteins indicated that functional divergence might have occurred with in SDC-like proteins. In an effort to identify additional SDC-like functional divergence, two uncharacterized SDC-like enzymes were recombinantly expressed and characterized.
Conclusions: An extensive biochemical analysis of two serine decarboxylases-like recombinant proteins led to an interesting discovery; both proteins catalyze the formation of acetaldehyde derivatives from select hydrophobic amino acids substrates. Specifically, Medicago truncatula [GenBank: XP_003592128] and Cicer arietinum [GenBank: XP_004496485] catalyze the decarboxylation and oxidative deamination of phenylalanine, methionine, leucine and tryptophan to generate their corresponding acetaldehydes. The promiscuous aldehyde synthase activity of these proteins yields novel products of 4-(methylthio) butanal, 3-methylbutanal (isovaleraldehyde) and indole-3-acetaldehyde from methionine, leucine and tryptophan respectively. A comparative biochemical analysis of the Medicago truncatula and Cicer arietinum enzymes against two previously characterized SDC-like enzymes further emphasizes the unusual substrate specificity and activity of these novel aldehyde synthases. Due to the strong substrate preference towards phenylalanine, it is likely that both enzymes function as phenylacetaldehyde synthesis in vivo. However, due to their significant sequence divergence and unusual substrate promiscuity these enzymes are functionally and evolutionary divergent from canonical phenylacetaldehyde synthesis enzymes. This work further elaborates on the functional complexity of plant type II PLP decarboxylases and their roles in secondary metabolite biosynthesis.