6hy3
Three-dimensional structure of AgaC from Zobellia galactanivorans
Structural highlights
FunctionAGAC_ZOBGA Cleaves the beta-1,4-linkages between beta-D-galactose and alpha-L-3,6-anhydro-galactose residues in agarose. Cleaves agarose in a random manner with retention of the anomeric-bond configuration, producing beta-anomers that give rise progressively to alpha-anomers when mutarotation takes place (By similarity). Publication Abstract from PubMedAgars are sulfated galactans from red macroalgae and are composed of D-galactose (G unit) and L-galactose (L unit) alternatively linked by alpha-1,3 and beta-1,4 glycosidic bonds. These polysaccharides display high complexity, with numerous modifications of their backbone (e.g. presence of 3,6-anhydro-bridge (LA unit) and sulfations and methylation). Currently, bacterial polysaccharidases that hydrolyze agars (beta-agarases and beta-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 beta-agarases and beta-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 A) 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 beta-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 beta-porphyranases and classical beta-agarases. The agar-specific hydrolase ZgAgaC from the marine bacterium Zobellia galactanivorans defines a new GH16 protein subfamily.,Naretto A, Fanuel M, Ropartz D, Rogniaux H, Larocque R, Czjzek M, Tellier C, Michel G J Biol Chem. 2019 Mar 7. pii: RA118.006609. doi: 10.1074/jbc.RA118.006609. PMID:30846563[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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