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Solution structure of the CBM10 cellulose binding module from Pseudomonas xylanase A
S Raghothama1, P J Simpson, L Szabó
1Department of Molecular Biology and Biotechnology, Krebs Institute, University of Sheffield, Sheffield S10 2TN, U.K.
Biochemistry
|February 2, 2000
Summary
Researchers determined the structure of a key carbohydrate-binding module (CBM) from Pseudomonas fluorescens xylanase A using NMR. This CBM features a unique fold with aromatic residues, facilitating enzyme attachment to plant polysaccharides.
Area of Science:
- Biochemistry
- Structural Biology
- Enzymology
Background:
- Plant cell wall hydrolases possess modular structures with catalytic and carbohydrate-binding modules (CBMs) for substrate attachment.
- Xylanase A from Pseudomonas fluorescens subsp. cellulosa (Pf Xyn10A) contains a family 10 catalytic domain and two CBMs (N-terminal family IIa and internal family 10).
Purpose of the Study:
- To elucidate the three-dimensional structure of the 45-residue family 10 CBM from Pf Xyn10A in solution.
- To characterize the structural features responsible for the CBM's interaction with polysaccharide substrates.
Main Methods:
- Nuclear Magnetic Resonance (NMR) spectroscopy was employed to determine the solution structure of the family 10 CBM.
- Structural analysis focused on identifying secondary structures, residue distribution, and potential binding surfaces.
Main Results:
- The family 10 CBM adopts a structure comprising two antiparallel beta-sheets (one with two strands, one with three) and a short alpha-helix.
- A high density of exposed aromatic residues (Tyr8, Trp22, Trp24) forms a flat surface, indicative of a polysaccharide-binding site.
- The fold resembles the oligonucleotide/oligosaccharide-binding (OB) fold but evolved convergently, lacking sequence similarity and differing in binding site orientation.
Conclusions:
- The determined structure provides insights into the molecular basis of substrate recognition and binding by plant cell wall hydrolase CBMs.
- The unique fold and aromatic residue arrangement highlight the functional adaptation of this CBM for efficient xylan binding.
- Convergent evolution is suggested for this CBM fold, distinct from the canonical OB fold, emphasizing diverse evolutionary pathways for carbohydrate binding.