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bioRxiv · 10.1101/2022.08.29.505653

Charge neutralization and β-elimination cleavage mechanism of family 42 L-rhamnose-α-1,4-D-glucuronate lyase revealed using neutron crystallography

Yano, N.·Kondo, T.·Kusaka, K.·Yamada, T.·Arakawa, T.·Sakamoto, T.·Fushinobu, S.
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Abstract

Gum arabic (GA) is widely used as an emulsion stabilizer and edible coating, and consists of a complex carbohydrate moiety with a rhamnosyl-glucuronate group capping the non-reducing ends. Enzymes that can specifically cleave the glycosidic chains of GA and modify their properties are valuable tools for structural analysis and industrial application. Cryogenic X-ray crystal structure of GA-specific L-rhamnose--1,4-D-glucuronate lyase from Fusarium oxysporum (FoRham1), belonging to the polysaccharide lyase (PL) family 42, has been previously reported. To determine the specific reaction mechanism based on its hydrogen-containing enzyme structure, we performed joint X-ray/neutron crystallography of FoRham1. Large crystals were grown in the presence of L-rhamnose (a reaction product), and neutron and X-ray diffraction datasets were collected at room temperature up to 1.80 and 1.25 [A] resolutions, respectively. The active site contained L-rhamnose and acetate, the latter being a partial analog of glucuronate. Incomplete H/D exchange between Arg166 and acetate suggested that a strong salt-bridge interaction was maintained. Doubly deuteronated His105 and deuteronated Tyr150 supported this interaction. The unusually hydrogen-rich environment functions as a charge neutralizer for glucuronate and stabilizes the oxyanion intermediate. The NE2 atom of His85 was deprotonated and formed a hydrogen bond with the deuterated O1 hydroxy of L-rhamnose, indicating the function of His85 as the base/acid catalyst for bond cleavage via {beta}-elimination. Asp83 functions as a pivot between the two catalytic histidine residues by bridging them, and this His-His-Asp structural motif is conserved in the three PL families. Significance StatementAlthough hydrogen transfer plays an important role in enzymatic reactions, hydrogen atoms are generally invisible in macromolecular X-ray crystallography. In the reaction of polysaccharide lyases, substrate activation by negative charge stabilization of uronic acid and base/acid-catalyzed {beta}-elimination reaction have been postulated. Here, we report the neutron crystallography of polysaccharide lyase. Joint X-ray/neutron crystallography of L-rhamnose--1,4-D-glucuronate lyase from Fusarium oxysporum (FoRham1) complexed with L-rhamnose was performed, and the hydrogen and deuterium atoms were visualized at a high resolution. FoRham1 catalyzes the specific cleavage of the cap structure of gum arabic, which is useful for various applications in the food, cosmetic, and pharmaceutical industries. A detailed catalytic mechanism for FoRham1 was proposed based on the key structural features of its active site.

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Yano, N., Kondo, T., Kusaka, K., Yamada, T., Arakawa, T., Sakamoto, T., Fushinobu, S.. 2022-08-29. Charge neutralization and β-elimination cleavage mechanism of family 42 L-rhamnose-α-1,4-D-glucuronate lyase revealed using neutron crystallography. https://doi.org/10.1101/2022.08.29.505653

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