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

Tsai, Ming-Li and Hadt, Ryan G. and Marshall, Nicholas M. and Wilson, Tiffany D. and Lu, Yi and Solomon, Edward I. (2013) Axial interactions in the mixed-valent Cu_A active site and role of the axial methionine in electron transfer. Proceedings of the National Academy of Sciences of the United States of America, 110 (36). pp. 14658-14663. ISSN 0027-8424. PMCID PMC3767567. doi:10.1073/pnas.1314242110.

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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 Cu_A sites is less clear. Recently, the mutation of the axial Met to Leu in a Cu_A site engineered into azurin (Cu_A Az) was found to have a limited effect on E^0 relative to this mutation in blue copper (BC). Detailed low-temperature absorption and magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance studies on Cu_A 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 H_2O coordination to the Cu center in the M123L mutant in Cu_A but not in the equivalent BC variant. The comparable stabilization energy of the oxidized over the reduced state in Cu_A and BC (Cu_A ∼ 180 mV; BC ∼ 250 mV) indicates that the S(Met) influences E(0) similarly in both. Electron delocalization over two Cu centers in Cu_A 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 E^0 while maintaining a low reorganization energy required for rapid electron transfer under physiological conditions.

Item Type:Article
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URLURL TypeDescription CentralArticle Information
Hadt, Ryan G.0000-0001-6026-1358
Lu, Yi0000-0003-1221-6709
Solomon, Edward I.0000-0003-0291-3199
Alternate Title:Axial interactions in the mixed-valent CuA active site and role of the axial methionine in electron transfer
Additional Information:© 2013 National Academy of Sciences. Contributed by Edward I. Solomon, July 30, 2013 (sent for review May 31, 2013) This work was funded by National Science Foundation Grants CHE-0948211 (to E.I.S.) and CHE-1058959 (to Y.L.) and National Institutes of Health Grant DK-31450 (to E.I.S.). M.-L.T. received support from the Postdoctoral Research Abroad Program sponsored by the National Science Council, Taiwan (Republic of China), and R.G.H. acknowledges a Gerhard Casper Stanford Graduate Fellowship and the Achievement Rewards for College Scientists Foundation. Author contributions: M.-L.T., R.G.H., Y.L., and E.I.S. designed research; M.-L.T., R.G.H., N.M.M., and T.D.W. performed research; N.M.M., T.D.W., and Y.L. contributed new reagents/analytic tools; M.-L.T., R.G.H., Y.L., and E.I.S. analyzed data; and M.-L.T., R.G.H., and E.I.S. wrote the paper. The authors declare no conflict of interest. This article contains supporting information online at
Funding AgencyGrant Number
National Science Council (Taipei)UNSPECIFIED
Stanford UniversityUNSPECIFIED
Subject Keywords:energy transduction pathway; reduction potential; spectroscopy
Issue or Number:36
PubMed Central ID:PMC3767567
Record Number:CaltechAUTHORS:20180612-125104163
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Official Citation: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 Proceedings of the National Academy of Sciences Sep 2013, 110 (36) 14658-14663; DOI: 10.1073/pnas.1314242110
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87009
Deposited By: George Porter
Deposited On:12 Jun 2018 20:17
Last Modified:15 Nov 2021 20:44

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