Untangling the Reaction Mechanism of the Polysaccharide Lyase PL42 Using QM/MM Metadynamics Simulations.

Area of Science:

• Biochemistry
• Enzymology
• Computational Chemistry

Background:

• Polysaccharide lyase family 42 (PL42) enzymes catalyze the cleavage of specific motifs in polysaccharides like gum arabic via β-elimination.
• Previous studies proposed a catalytic mechanism for the PL42 enzyme FoRham1, but it lacked validation on a free-energy surface.

Purpose of the Study:

• To computationally investigate and validate the reaction mechanism of the PL42 enzyme FoRham1 at the atomic level.
• To analyze the catalytic role of key amino acid residues and the substrate's conformational dynamics during β-elimination.

Main Methods:

• Combined quantum mechanics/molecular mechanics (QM/MM) with well-tempered metadynamics and umbrella sampling.
• Atomic-level analysis of the reaction pathway and free-energy landscape.
• Investigation of substrate conformation and product-ring geometry evolution.

Main Results:

• Confirmed a concerted yet highly asynchronous syn β-elimination mechanism.
• Identified His85 as the catalytic base/acid, assisted by Asp83 and His105, with Arg166 stabilizing the glucuronate carboxylate.
• Determined the reactive substrate conformation as an inverse-chair ¹C₄ state and mapped the sugar pathway.
• Calculated a refined energy barrier of 17.7 kcal mol^-1, consistent with experimental data.

Conclusions:

• The study validates the proposed catalytic mechanism for FoRham1, highlighting a distinct histidine-based catalytic system within PL42.
• Provides insights into the conformational dynamics of polysaccharide lyase mechanisms, linking them through product-ring geometries.
• Expands the understanding of β-elimination catalysis in polysaccharide degradation.