Axial interactions in the mixed-valent Cua active site and role of the axial methionine in electron transfer

Ming Li Tsai, Ryan G. Hadt, Nicholas M. Marshall, Tiffany D. Wilson, Yi Lu, Edward I. Solomon

Research output: Contribution to journalArticlepeer-review


Within Cu-containing electron transfer active sites, the role of the axial ligand in type 1 sites is well defined, yet its role in the binuclear mixed-valent CuA sites is less clear. Recently, the mutation of the axial Met to Leu in a CuA site engineered into azurin (CuA Az) was found to have a limited effect on E0 relative to this mutation in blue copper (BC). Detailed low-temperature absorption and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance studies on CuA Az (WT) and its M123X (X = Q, L, H) axial ligand variants indicated stronger axial ligation in M123L/H. Spectroscopically validated density functional theory calculations show that the smaller ΔE 0 is attributed to H2O coordination to the Cu center in the M123L mutant in CuA but not in the equivalent BC variant. The comparable stabilization energy of the oxidized over the reduced state in CuA and BC (CuA ∼ 180 mV; BC ∼ 250 mV) indicates that the S(Met) influences E0 similarly in both. Electron delocalization over two Cu centers in CuA was found to minimize the Jahn - Teller distortion induced by the axial Met ligand and lower the inner-sphere reorganization energy. The Cu-S(Met) bond in oxidized Cu A is weak (5.2 kcal/mol) but energetically similar to that of BC, which demonstrates that the protein matrix also serves an entatic role in keeping the Met bound to the active site to tune down E0 while maintaining a low reorganization energy required for rapid electron transfer under physiological conditions.

Original languageEnglish (US)
Pages (from-to)14658-14663
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number36
StatePublished - Sep 3 2013


  • Energy transduction pathway
  • Reduction potential
  • Spectroscopy

ASJC Scopus subject areas

  • General


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