Functional and structural analysis of a cyclization domain in a cyclic β-1,2-glucan synthase

Area of Science:

• Biochemistry
• Microbiology
• Enzymology

Background:

• Cyclic β-1,2-glucans (CβGs) are crucial in bacterial interactions with hosts.
• The cyclization mechanism of CGS enzymes remains poorly understood.
• Understanding CGS function is vital for deciphering bacterial pathogenesis and symbiosis.

Purpose of the Study:

• To elucidate the mechanism of cyclization by the CGS enzyme.
• To characterize the functional and structural properties of the CGS cyclization domain.
• To determine the substrate requirements and reaction pattern of CGS.

Main Methods:

• Functional analysis of the isolated cyclization domain (TiCGS_Cy).
• Product identification using ¹H-NMR spectroscopy.
• Action pattern analysis with varying β-1,2-glucooligosaccharide substrates.
• Structural comparison with related enzymes.

Main Results:

• TiCGS_Cy alone produces CβGs from linear β-1,2-glucans.
• The enzyme exclusively performs transglycosylation, not hydrolysis.
• A minimum hexasaccharide chain length is required, with longer chains preferred.
• Structural analysis revealed similarities to β-1,2-glucanases but distinct catalytic residues.

Conclusions:

• The cyclization domain of CGS possesses unique enzymatic activity.
• A novel reaction mechanism for CGS-mediated cyclization is proposed.
• The CGS enzyme family is classified as a new glycoside hydrolase family, GH189.