4nk6
From Proteopedia
Crystal Structure of the periplasmic alginate epimerase AlgG
Structural highlights
FunctionALGG_PSESM Bifunctional protein that converts poly(beta-D-mannuronate) to alpha-L-guluronate and that is also part of a periplasmic protein complex that serves as a scaffold that leads the newly formed alginate polymer through the periplasmic space to the outer membrane secretin AlgE (By similarity). Publication Abstract from PubMedPseudomonas aeruginosa is an opportunistic pathogen that forms chronic biofilm infections in the lungs of cystic fibrosis patients. A major component of the biofilm during these infections is the exopolysaccharide alginate, which is synthesized at the inner membrane as a homopolymer of 1-4-linked beta-D-mannuronate. As the polymer passages through the periplasm 22-44% of the mannuronate residues are converted to alpha-L-guluronate by the C5 epimerase AlgG to produce a polymer of alternating beta-D-mannuronate and alpha-L-guluronate blocks and stretches of polymannuronate. To understand the molecular basis of alginate epimerization the structure of Pseudomonas syringae AlgG has been determined at 2.1A resolution and the protein functionally characterized. The structure reveals that AlgG is a long right-handed parallel beta-helix with an elaborate lid structure. Functional analysis of AlgG mutants suggest that His319 acts as the catalytic base, and that Arg345 neutralizes the acidic group during the epimerase reaction. Water is the likely catalytic acid. Electrostatic surface potential and residue conservation analysis, in conjunction with activity and substrate docking studies, suggest that a conserved electropositive groove facilitates polymannuronate binding and contains at least nine substrate binding subsites. These subsites likely align the polymer in the correct register for catalysis to occur. The presence of multiple subsites, the electropositive groove and the non-random distribution of guluronate in the alginate polymer suggest that AlgG is a processive enzyme. Moreover, comparison of AlgG and the extracellular alginate epimerase AlgE4 of A. vinelandii provides a structural rationale for the differences in their Ca2+ dependency. Catalytic mechanism and mode of action of the periplasmic alginate epimerase AlgG.,Wolfram F, Kitova EN, Robinson H, Walvoort M, Codee JD, Klassen JS, Howell PL J Biol Chem. 2014 Jan 7. PMID:24398681[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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