Structural insights into the substrate-binding mechanism for a novel chitosanase

Area of Science:

• Biochemistry
• Enzymology
• Structural Biology

Background:

• Chitosanase enzymes cleave β-1,4-glycosidic bonds in chitosan, yielding valuable chito-oligomers for applications in functional foods and cancer therapy.
• Understanding the substrate-binding mechanism of chitosanase is crucial for its application, but lacks high-resolution structural data of enzyme-substrate complexes.

Purpose of the Study:

• To elucidate the substrate-binding mechanism of a novel chitosanase (OU01) from Microbacterium sp.
• To determine the high-resolution crystal structure of the chitosanase-substrate complex.
• To identify key residues involved in substrate binding and catalysis.

Main Methods:

• X-ray crystallography to determine the structure of chitosanase OU01 in complex with hexa-glucosamine (GlcN)6.
• Site-directed mutagenesis to investigate the role of specific amino acid residues in substrate binding and catalytic activity.
• Differential Scanning Fluorimetry (DSF) assays to assess protein stability after mutagenesis.

Main Results:

• The crystal structure of the chitosanase-(GlcN)6 complex revealed extensive interactions within the binding cleft, particularly from positions -2 to +1 of the substrate.
• Mutagenesis studies identified critical residues (Tyr37 and His203) essential for catalytic activity, while others (Thr58, Asp235, Ser27) showed varying degrees of impact.
• Mutations did not significantly affect protein stability, as confirmed by DSF assays.

Conclusions:

• The study provides the first mechanistic interpretation of substrate binding to chitosanase at a molecular level.
• The -2, -1, and +1 subsites are identified as dominant regions for substrate binding and catalysis.
• These findings are critical for the rational design of novel chitosanases for efficient biomass conversion.