A new carbohydrate-active oligosaccharide dehydratase is involved in the degradation of ulvan

Marcus Bäumgen,Theresa Dutschei,Daniel Bartosik,Christoph Suster,Lukas Reisky,Nadine Gerlach,Christian Stanetty,Marko D Mihovilovic,Thomas Schweder,Jan-Hendrik Hehemann,Uwe T Bornscheuer

doi:10.1016/j.jbc.2021.101210

Abstract

Marine algae catalyze half of all global photosynthetic production of carbohydrates. Owing to their fast growth rates, Ulva spp. rapidly produce substantial amounts of carbohydrate-rich biomass and represent an emerging renewable energy and carbon resource. Their major cell wall polysaccharide is the anionic carbohydrate ulvan. Here, we describe a new enzymatic degradation pathway of the marine bacterium Formosa agariphila for ulvan oligosaccharides involving unsaturated uronic acid at the nonreducing end linked to rhamnose-3-sulfate and glucuronic or iduronic acid (Δ-Rha3S-GlcA/IdoA-Rha3S). Notably, we discovered a new dehydratase (P29_PDnc) acting on the nonreducing end of ulvan oligosaccharides, i.e., GlcA/IdoA-Rha3S, forming the aforementioned unsaturated uronic acid residue. This residue represents the substrate for GH105 glycoside hydrolases, which complements the enzymatic degradation pathway including one ulvan lyase, one multimodular sulfatase, three glycoside hydrolases, and the dehydratase P29_PDnc, the latter being described for the first time. Our research thus shows that the oligosaccharide dehydratase is involved in the degradation of carboxylated polysaccharides into monosaccharides.

Highlights

The initial degradation step catalyzed by the ulvan lyases (P10_PLnc and P30_PL28) leads to the formation of several oligosaccharides with diverse composition, see Fig. S1 [10]
We were able to complement the complex ulvan degradation pathway previously described by Reisky et al [10] by elucidating an alternative enzyme cascade, which is able to fully degrade uronic-acid-containing oligosaccharides resulting from an incomplete degradation by ulvan lyases
In addition to the previously described ulvan-degrading enzymes (P10_PL40, P17_GH2, P18_S1_7, P20_GH78, P24_GH3, P27_GH43, P30_PL28, P31_GH39, P32_S1_8, P33_GH105, P36_S1_25), we were able to elucidate the function of three further enzymes in the ulvan utilization comprising two glucoside hydrolases (P34_GH3, P36_GH78) and an oligosaccharide dehydratase (P29_PDnc)

Summary

Introduction

We elucidated the degradation cascade for ulvan consisting of 12 carbohydrate-active enzymes, including two polysaccharide lyases, three sulfatases, and seven glycoside hydrolases [10]. It desulfates the rhamnose residue at the nonreducing end so that this sulfatase is active on ulvan trisaccharides with the general structure Rha3S-XXX-Rha3S [10]. Analogous to the already established pathway, Rha-GlcA/IdoA-Rha3S is degraded by the α-L-rhamnosidase domain of P36_GH78 leading to the removal of the rhamnose residue at the nonreducing end

Results

Conclusion