Formal reduction potential of 3,5-difluorotyrosine in a structured protein: insight into multistep radical transfer.
The reversible Y-O•/Y-OH redox properties of the α3Y model protein allow access to the electrochemical and thermodynamic properties of 3,5-difluorotyrosine. The unnatural amino acid has been incorporated at position 32, the dedicated radical site in α3Y, by in vivo nonsense codon suppression. Incorporation of 3,5-difluorotyrosine gives rise to very minor structural changes in the protein scaffold at pH values below the apparent pK (8.0±0.1) of the unnatural residue. Square-wave voltammetry on α3(3,5)F2Y provides an E°'(Y-O•/Y-OH) of 1026±4 mV versus the normal hydrogen electrode (pH 5.70±0.02) and shows that the fluoro substitutions lower the E°' by -30±3 mV. These results illustrate the utility of combining the optimized α3Y tyrosine radical system with in vivo nonsense codon suppression to obtain the formal reduction potential of an unnatural aromatic residue residing within a well-structured protein. It is further observed that the protein E°' values differ significantly from peak potentials derived from irreversible voltammograms of the corresponding aqueous species. This is notable because solution potentials have been the main thermodynamic data available for amino acid radicals. The findings in this paper are discussed relative to recent mechanistic studies of the multistep radical-transfer process in Escherichia coli ribonucleotide reductase site-specifically labeled with unnatural tyrosine residues.
Department of Chemistry, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.
Kanchana R
KR
Ravichandran
Li
L
Liang
JoAnne
J
Stubbe
Cecilia
C
Tommos
D000595:Amino Acid Sequence; D004579:Electron Transport; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry*; D007700:Kinetics; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D012264:Ribonucleotide Reductases / Q000737:chemistry*; D013194:Staining and Labeling; D013816:Thermodynamics; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry
D029968:Escherichia coli Proteins; D005609:Free Radicals; C025200:3,5-difluorotyrosine; D014443:Tyrosine; D012264:Ribonucleotide Reductases