title: Redox properties and catalytic activity of surface-bound human sulfite oxidase studied by a combined surface enhanced resonance Raman spectroscopic and electrochemical approach.
abstract: Human sulfite oxidase (hSO) was immobilised on SAM-coated silver electrodes under preservation of the native heme pocket structure of the cytochrome b5 (Cyt b5) domain and the functionality of the enzyme. The redox properties and catalytic activity of the entire enzyme were studied by surface enhanced resonance Raman (SERR) spectroscopy and cyclic voltammetry (CV) and compared to the isolated heme domain when possible. It is shown that heterogeneous electron transfer and catalytic activity of hSO sensitively depend on the local environment of the enzyme. Increasing the ionic strength of the buffer solution leads to an increase of the heterogeneous electron transfer rate from 17 s(-1) to 440 s(-1) for hSO as determined by SERR spectroscopy. CV measurements demonstrate an increase of the apparent turnover rate for the immobilised hSO from 0.85 s(-1) in 100 mM buffer to 5.26 s(-1) in 750 mM buffer. We suggest that both effects originate from the increased mobility of the surface-bound enzyme with increasing ionic strength. In agreement with surface potential calculations we propose that at high ionic strength the enzyme is immobilised via the dimerisation domain to the SAM surface. The flexible loop region connecting the Moco and the Cyt b5 domain allows alternating contact with the Moco interaction site and the SAM surface, thereby promoting the sequential intramolecular and heterogeneous electron transfer from Moco via Cyt b5 to the electrode. At lower ionic strength, the contact time of the Cyt b5 domain with the SAM surface is longer, corresponding to a slower overall electron transfer process.
authors.affiliation: Technische Universität Berlin, Institut für Chemie, Sekr. PC 14, Strasse des 17. Juni 135, D-10623 Berlin, Germany.
authors.forename: Murat
authors.initials: M
authors.lastname: Sezer
authors.forename: Roberto
authors.initials: R
authors.lastname: Spricigo
authors.forename: Tillmann
authors.initials: T
authors.lastname: Utesch
authors.forename: Diego
authors.initials: D
authors.lastname: Millo
authors.forename: Silke
authors.initials: S
authors.lastname: Leimkuehler
authors.forename: Maria A
authors.initials: MA
authors.lastname: Mroginski
authors.forename: Ulla
authors.initials: U
authors.lastname: Wollenberger
authors.forename: Peter
authors.initials: P
authors.lastname: Hildebrandt
authors.forename: Inez M
authors.initials: IM
authors.lastname: Weidinger
mesh_terms: D055162:Biocatalysis; D055664:Electrochemical Techniques; D004566:Electrodes; D004579:Electron Transport; D004800:Enzymes, Immobilized / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000737:chemistry* / Q000378:metabolism; D017434:Protein Structure, Tertiary; D012834:Silver / Q000737:chemistry; D013059:Spectrum Analysis, Raman
chemical_list: D004800:Enzymes, Immobilized; D012834:Silver; D050862:Oxidoreductases Acting on Sulfur Group Donors; C503599:SUOX protein, human