Pourbaix Diagram, Proton-Coupled Electron Transfer, and Decay Kinetics of a Protein Tryptophan Radical: Comparing the Redox Properties of W32• and Y32• Generated Inside the Structurally Characterized ?3W and ?3Y Proteins.

ABSTRACT: Protein-based "hole" hopping typically involves spatially arranged redox-active tryptophan or tyrosine residues. Thermodynamic information is scarce for this type of process. The well-structured ?3W model protein was studied by protein film square wave voltammetry and transient absorption spectroscopy to obtain a comprehensive thermodynamic and kinetic description of a buried tryptophan residue. A Pourbaix diagram, correlating thermodynamic potentials (E°') with pH, is reported for W32 in ?3W and compared to equivalent data recently presented for Y32 in ?3Y ( Ravichandran , K. R. ; Zong , A. B. ; Taguchi , A. T. ; Nocera , D. G. ; Stubbe , J. ; Tommos , C. J. Am. Chem. Soc. 2017 , 139 , 2994 - 3004 ). The ?3W Pourbaix diagram displays a pKOX of 3.4, a E°'(W32(N•+/NH)) of 1293 mV, and a E°'(W32(N•/NH); pH 7.0) of 1095 ± 4 mV versus the normal hydrogen electrode. W32(N•/NH) is 109 ± 4 mV more oxidizing than Y32(O•/OH) at pH 5.4-10. In the voltammetry measurements, W32 oxidation-reduction occurs on a time scale of about 4 ms and is coupled to the release and subsequent uptake of one full proton to and from bulk. Kinetic analysis further shows that W32 oxidation likely involves pre-equilibrium electron transfer followed by proton transfer to a water or small water cluster as the primary acceptor. A well-resolved absorption spectrum of W32• is presented, and analysis of decay kinetics show that W32• persists ?104 times longer than aqueous W• due to significant stabilization by the protein. The redox characteristics of W32 and Y32 are discussed relative to global and local protein properties.

SUBMITTER: Glover SD 

PROVIDER: S-EPMC5762255 | biostudies-literature | 2018 Jan

REPOSITORIES: biostudies-literature