Abstract
Agars are sulfated galactans from red macroalgae and are composed of a d-galactose (G unit) and l-galactose (L unit) alternatively linked by α-1,3 and β-1,4 glycosidic bonds. These polysaccharides display high complexity, with numerous modifications of their backbone (e.g. presence of a 3,6-anhydro-bridge (LA unit) and sulfations and methylation). Currently, bacterial polysaccharidases that hydrolyze agars (β-agarases and β-porphyranases) have been characterized on simple agarose and more rarely on porphyran, a polymer containing both agarobiose (G-LA) and porphyranobiose (GL6S) motifs. How bacteria can degrade complex agars remains therefore an open question. Here, we studied an enzyme from the marine bacterium Zobellia galactanivorans (ZgAgaC) that is distantly related to the glycoside hydrolase 16 (GH16) family β-agarases and β-porphyranases. Using a large red algae collection, we demonstrate that ZgAgaC hydrolyzes not only agarose but also complex agars from Ceramiales species. Using tandem MS analysis, we elucidated the structure of a purified hexasaccharide product, L6S-G-LA2Me-G(2Pentose)-LA2S-G, released by the activity of ZgAgaC on agar extracted from Osmundea pinnatifida By resolving the crystal structure of ZgAgaC at high resolution (1.3 Å) and comparison with the structures of ZgAgaB and ZgPorA in complex with their respective substrates, we determined that ZgAgaC recognizes agarose via a mechanism different from that of classical β-agarases. Moreover, we identified conserved residues involved in the binding of complex oligoagars and demonstrate a probable influence of the acidic polysaccharide's pH microenvironment on hydrolase activity. Finally, a phylogenetic analysis supported the notion that ZgAgaC homologs define a new GH16 subfamily distinct from β-porphyranases and classical β-agarases.
Highlights
Agars are sulfated galactans from red macroalgae and are composed of a D-galactose (G unit) and L-galactose (L unit) alternativelylinkedby␣-1,3and-1,4glycosidicbonds.Thesepolysaccharides display high complexity, with numerous modifications of their backbone (e.g. presence of a 3,6-anhydro-bridge (LA unit) and sulfations and methylation)
We studied an enzyme from the marine bacterium Zobellia galactanivorans (ZgAgaC) that is distantly related to the glycoside hydrolase 16 (GH16) family -agarases and -porphyranases
Using a large red algae collection, we demonstrate that ZgAgaB complex (Fig. 10). Trp110 (ZgAgaC) hydrolyzes agarose and complex agars from Ceramiales species
Summary
The main cell wall polysaccharides of marine red macroalgae are unique sulfated galactans, carrageenans or agars [1] These polysaccharides consist of a backbone of galactopyranose units linked by alternating ␣-1,3 and -1,4 linkages. The regular structure of the backbone of red algal galactans is often masked by additional chemical modifications, such as ester sulfate groups, methyl groups, or pyruvic acid acetal groups (6 –8). This complexity has been taken into account in carrageenan nomenclature, and traditionally carrageenans are identified by a Greek prefix, indicating the major component of the sample [7].
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.