Function of a Recombinant Chitinase Derived from a Virulent Aeromonas Hydrophila Isolated from Diseased Channel Catfish

A chitinase was identified in extracellular products of a virulent Aeromonas hydrophila isolated from diseased channel catfish (Ictalurus punctatus). Bioactive recombinant chitinase (rChi-Ah) was produced in Escherichia coli. Purified rChi-Ah had optimal activity at temperature of 42°C and pH 6.5. The affinity (Km) for chitosan was 4.18 mg ml^-1 with Vmax of 162 mg min^-1 mg^-1. With home-made colloidal chitin as substrate, rChi-Ah generated N,N’-diacetyl-glucosamine predominantly. Conversion of chitosan (≥ 75% deacetylated) by rChi-Ah revealed five major products: 2 to 4 units of glucosamine, all of which had at least one acetyl group. It is postulated that N-acetylated glucosamine is the recognition and cleavage site of rChi-Ah; the minimal and maximal cleavages are two and four glucosamine units, respectively. Whether A. hydrophila benefits from the chitinolytic products for survival and/or infection merits further investigation. Additionally, since chitooligosaccharides have many interesting bioactivities, rChi-Ah is a useful chitinolytic enzyme for extensive research in related fields.