An Unusual His/Asp Dyad Operates Catalysis in Agar-Degrading Glycosidases

Overview

Journal ACS Catal

Publisher American Chemical Society

Date 2024 Nov 21

PMID 39569157

Authors

Mert Sagiroglugil

Alba Nin-Hill

Elizabeth Ficko-Blean

Carme Rovira

Affiliations

Soon will be listed here.

Abstract

Agarose motifs, found in agars present in the cell walls of red algae, consist of alternating units of d-galactose (G) and α-3,6-anhydro-l-galactose (LA). Glycoside hydrolases from family 117 (GH117) cleave the terminal α-1,3-glycosidic bonds, releasing LA units. Structural studies have suggested that these enzymes use unconventional catalytic machinery, involving a histidine (His302) as a general acid rather than a carboxylic residue as in most glycosidases. By means of quantum mechanics/molecular mechanics metadynamics, we investigated the reaction mechanism of GH117, confirming the catalytic role of His302. This residue shares a proton with a neighbor aspartate residue (Asp320), forming a His/Asp dyad. Our study also reveals that, even though the sugar unit at the subsite (LA) can adopt two conformations, and , only the latter is catalytically competent, defining a → [ ] → (→ ) conformational itinerary. This mechanism may be applicable to similar enzymes with a His/Asp dyad in their active sites, such as GH3 β--acetylglucosaminidase and GH156 sialidase. These insights enhance our understanding of glycosidase catalytic strategies and could inform the engineering of enzymes for the more efficient processing of seaweed.

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