Substrate specificities of tobacco chitinases

Ten tobacco chitinases (1,4-N-acetyl-β-D-glucosaminide glycanhydrolase, EC 3.2.1.14) were purified from tobacco leaves hypersensitively reacting to tobacco mosaic virus. The 10 enzymes, which belong to five distinct structural classes of plant chitinases, were incubated with several potential substrates such as chitin, a β-1,4 N-acetyl-D-glucosamine (GlcNAc) polymer, chitosan (partially deacetylated chitin), chitin oligomers of variable length and bacterial cell wall. Tobacco chitinases are all endo-type enzymes that liberate oligomers from chitin and are capable of processing the chito-oligomers further at differential rates. Chitin reaction products were separated and quantified by HPLC and differential kinetics of oligomer accumulation and degradation were observed with the distinct classes of chitinases. Depending on the substrate to be hydrolysed, each isoform displayed a different spectrum of activity. For example, class I isoforms were the most active on chitin and (GlcNAc)_4–6 whereas class III basic isoforms were the most efficient in inducing bacterial lysis. Class V and class VI chitinases were shown to more readily hydrolyse chitin oligomers than the chitin polymer itself. Together, these data indicate that the 10 tobacco chitinases represent complementary enzymes which may have synergistic effects on their substrates. This paper discusses their implication in plant defense by attacking pathogen's structural components and in plant development by maturing signal molecules.