Synthesis of Long-Chain Chitooligosaccharides by a Hypertransglycosylating Processive Endochitinase of Serratia proteamaculans 568

Abstract

ABSTRACT We describe the heterologous expression and characterization of a 407-residue single-domain glycosyl hydrolase family 18 chitinase ( Sp ChiD) from Gram-negative Serratia proteamaculans 568 that has unprecedented catalytic properties. Sp ChiD was optimally active at pH 6.0 and 40°C, where it showed a K m of 83 mg ml −1 , a k cat of 3.9 × 10 2 h −1 , and a k cat / K m of 4.7 h mg −1 ml −1 on colloidal chitin. On chitobiose, the K m , k cat , and k cat / K m were 203 μM, 1.3 × 10 2 h −1 , and 0.62 h −1 μM −1 , respectively. Hydrolytic activity on chitooligosaccharides (CHOS) and colloidal chitin indicated that Sp ChiD was an endo-acting processive enzyme, with the unique ability to convert released chitobiose to N -acetylglucosamine, the major end product. Sp ChiD showed hyper transglycosylation (TG) with trimer-hexamer CHOS substrates, generating considerable amounts of long-chain CHOS. The TG activity of Sp ChiD was dependent on both the length and concentration of the oligomeric substrate and also on the enzyme concentration. The length and amount of accumulated TG products increased with increases in the length of the substrate and its concentration and decreased with increases in the enzyme concentration. The Sp ChiD bound to insoluble and soluble chitin substrates despite the absence of accessory domains. Sequence alignments and structural modeling indicated that Sp ChiD would have a deep substrate-binding groove lined with aromatic residues, which is characteristic of processive enzymes. Sp ChiD shows a combination of properties that seems rare among family 18 chitinases and that may resemble the properties of human chitotriosidase.