title: NMR structure of the R-module: a parallel beta-roll subunit from an Azotobacter vinelandii mannuronan C-5 epimerase.
abstract: In the bacterium Azotobacter vinelandii, a family of seven secreted and calcium-dependent mannuronan C-5 epimerases (AlgE1-7) has been identified. These epimerases are responsible for the epimerization of beta-d-mannuronic acid to alpha-l-guluronic acid in alginate polymers. The epimerases consist of two types of structural modules, designated A (one or two copies) and R (one to seven copies). The structure of the catalytically active A-module from the smallest epimerase AlgE4 (consisting of AR) has been solved recently. This paper describes the NMR structure of the R-module from AlgE4 and its titration with a substrate analogue and paramagnetic thulium ions. The R-module folds into a right-handed parallel beta-roll. The overall shape of the R-module is an elongated molecule with a positively charged patch that interacts with the substrate. Titration of the R-module with thulium indicated possible calcium binding sites in the loops formed by the nonarepeat sequences in the N-terminal part of the molecule and the importance of calcium binding for the stability of the R-module. Structure calculations showed that calcium ions can be incorporated in these loops without structural violations and changes. Based on the structure and the electrostatic surface potential of both the A- and R-module from AlgE4, a model for the appearance of the whole protein is proposed.
authors.affiliation: Department of Life Sciences, Aalborg University, Sohngaardsholmsvej 49, DK-9000 Aalborg, Denmark.
authors.forename: Finn L
authors.initials: FL
authors.lastname: Aachmann
authors.forename: Britt I G
authors.initials: BI
authors.lastname: Svanem
authors.forename: Peter
authors.initials: P
authors.lastname: Güntert
authors.forename: Steffen B
authors.initials: SB
authors.lastname: Petersen
authors.forename: Svein
authors.initials: S
authors.lastname: Valla
authors.forename: Reinhard
authors.initials: R
authors.lastname: Wimmer
mesh_terms: D000464:Alginates / Q000737:chemistry; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000737:chemistry; D016948:Azotobacter vinelandii / Q000201:enzymology*; D002118:Calcium / Q000378:metabolism; D002238:Carbohydrate Epimerases / Q000737:chemistry; D006603:Hexuronic Acids / Q000737:chemistry; D007477:Ions; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015233:Models, Statistical; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D011108:Polymers / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013932:Thulium / Q000737:chemistry
chemical_list: D000464:Alginates; D000596:Amino Acids; D006603:Hexuronic Acids; D007477:Ions; D011108:Polymers; C007896:guluronic acid; D013932:Thulium; C008324:mannuronic acid; D002238:Carbohydrate Epimerases; C086519:mannuronan c-5-epimerase; D002118:Calcium