Tuning reduction potentials of type 1 copper center in azurin by replacing a histidine ligand with its isostructural analogues

• Published in: Journal of inorganic biochemistry Vol. 234; p. 111863
• Main Authors: Yu, Yang, Marshall, Nicholas M., Garner, Dewain K., Nilges, Mark J., Lu, Yi
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.09.2022
• Subjects: Blue copper; Cupredoxins; Electron transfer; Protein engineering; Redox; Unnatural amino acids; Redox; Blue copper; Electron transfer; Protein engineering; Cupredoxins; Unnatural amino acids
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/j.jinorgbio.2022.111863

Type 1 copper proteins have a conserved ligand set of one cysteine and two histidines, with many proteins, such as azurin, also containing an axial methionine. While the cysteine and methionine in azurin have been replaced with their respective isostructural analogues of unnatural amino acids to reveal their roles in tuning electronic structures and functional properties, such as reduction potentials (E°′), the histidine ligands have not been probed in this way. We herein report the substitution of His117 in azurin with three unnatural isostructural analogues, 5-nitrohistidine(Ntr), thiazolylalanine(SHis) and 1-methylhistidine(MeH) by expressed protein ligation. While UV–vis absorption and electron paramagnetic resonance spectroscopies confirm that isostructural replacement results in minimal structural change in the Cu(II) state, the E°′ of these variants increases with increasing pKa of the δ nitrogens of the imidazole. This counter-intuitive relationship between E°′ of the protein and pKa of the sidechain group suggests additional factors may play a role in tuning E°′. Replacing His117 in azurin with three isostructural analogues, 5-nitrohistidine(Ntr), thiazolylalanine(SHis) and 1-methylhistidine(MeH) resulted in linear tuning of reduction potentials of the type 1 Cu2+/Cu+ couple. [Display omitted] •The histidine ligand in azurin is probed with its unnatural amino acid analogs.•Minimal structural perturbation was observed by spectroscopic studies.•Redox potentials of the variants increase with increasing pKa's of the histidine analogs.•The correlation between redox potentials and pKa's of the histidines is counter-intuitive.•This finding suggests additional factors may play a role in tuning the redox potentials.