Structure-function analyses of a PL24 family ulvan lyase reveal key features and suggest its catalytic mechanism

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Ulvan, a major cell wall polysaccharide from green algae (genus *Ulva*), is degraded by marine bacterial enzymes called ulvan lyases.
• Two families, PL24 and PL25, of ulvan-degrading lyases have been identified, with limited structural and mechanistic information available.

Purpose of the Study:

• To determine the crystal structure of the PL24 family ulvan lyase LOR_107.
• To elucidate the structural features and catalytic mechanism of LOR_107.
• To understand the substrate-binding interactions of LOR_107.

Main Methods:

• X-ray crystallography was used to determine the structure of LOR_107.
• Site-directed mutagenesis was employed to identify key catalytic residues.
• Substrate-binding interactions were analyzed using structures of inactive mutants complexed with a tetrasaccharide.

Main Results:

• The crystal structure of LOR_107 revealed a seven-bladed β-propeller fold with a distinct substrate-binding canyon.
• Mutagenesis identified essential catalytic residues within the canyon.
• Structural analysis of substrate-bound mutants indicated a two-step binding mechanism.
• LOR_107 shares structural similarity with PL25 family enzymes despite low sequence identity.
• A novel catalytic mechanism, differing from the typical His/Tyr mechanism, was proposed for LOR_107.

Conclusions:

• The structure of LOR_107 provides the first atomic view of a PL24 family ulvan lyase.
• LOR_107 employs a unique substrate-binding mode and a potentially novel catalytic mechanism.
• Structural homology between distantly related enzyme families suggests convergent evolution in polysaccharide degradation.