title: Aromatic-Mediated Carbohydrate Recognition in Processive Serratia marcescens Chitinases.
abstract: Microorganisms use a host of enzymes, including processive glycoside hydrolases, to deconstruct recalcitrant polysaccharides to sugars. Processive glycoside hydrolases closely associate with polymer chains and repeatedly cleave glycosidic linkages without dissociating from the crystalline surface after each hydrolytic step; they are typically the most abundant enzymes in both natural secretomes and industrial cocktails by virtue of their significant hydrolytic potential. The ubiquity of aromatic residues lining the enzyme catalytic tunnels and clefts is a notable feature of processive glycoside hydrolases. We hypothesized that these aromatic residues have uniquely defined roles, such as substrate chain acquisition and binding in the catalytic tunnel, that are defined by their local environment and position relative to the substrate and the catalytic center. Here, we investigated this hypothesis with variants of Serratia marcescens family 18 processive chitinases ChiA and ChiB. We applied molecular simulation and free energy calculations to assess active site dynamics and ligand binding free energies. Isothermal titration calorimetry provided further insight into enthalpic and entropic contributions to ligand binding free energy. Thus, the roles of six aromatic residues, Trp-167, Trp-275, and Phe-396 in ChiA, and Trp-97, Trp-220, and Phe-190 in ChiB, have been examined. We observed that point mutation of the tryptophan residues to alanine results in unfavorable changes in the free energy of binding relative to wild-type. The most drastic effects were observed for residues positioned at the "entrances" of the deep substrate-binding clefts and known to be important for processivity. Interestingly, phenylalanine mutations in ChiA and ChiB had little to no effect on chito-oligomer binding, in accordance with the limited effects of their removal on chitinase functionality.
authors.affiliation: Department of Chemical and Materials Engineering, University of Kentucky , Lexington, Kentucky 40506-0046, United States.
authors.forename: Suvamay
authors.initials: S
authors.lastname: Jana
authors.affiliation: Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences , Ås 1430, Norway.
authors.forename: Anne Grethe
authors.initials: AG
authors.lastname: Hamre
authors.affiliation: Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences , Ås 1430, Norway.
authors.forename: Patricia
authors.initials: P
authors.lastname: Wildberger
authors.affiliation: Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences , Ås 1430, Norway.
authors.forename: Matilde Mengkrog
authors.initials: MM
authors.lastname: Holen
authors.affiliation: Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences , Ås 1430, Norway.
authors.forename: Vincent G H
authors.initials: VG
authors.lastname: Eijsink
authors.affiliation: National Bioenergy Center, National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Gregg T
authors.initials: GT
authors.lastname: Beckham
authors.affiliation: Department of Chemistry, Biotechnology, and Food Science, Norwegian University of Life Sciences , Ås 1430, Norway.
authors.forename: Morten
authors.initials: M
authors.lastname: Sørlie
authors.affiliation: Department of Chemical and Materials Engineering, University of Kentucky , Lexington, Kentucky 40506-0046, United States.
authors.forename: Christina M
authors.initials: CM
authors.lastname: Payne
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020134:Catalytic Domain; D002686:Chitin / Q000737:chemistry / Q000378:metabolism*; D002688:Chitinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D056004:Molecular Dynamics Simulation; D016297:Mutagenesis, Site-Directed; D012706:Serratia marcescens / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D002686:Chitin; D002688:Chitinases; C000601657:chitinase B, Serratia marcescens; C094590:chitinase ChiA