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¹⁵N-¹H Residual Dipolar Coupling Analysis of Native and Alkaline-K79A Saccharomyces cerevisiae Cytochrome c

Michael Assfalg ^*, Ivano Bertini ^*, Paola Turano ^*, A. Grant Mauk , Jay R. Winkler  and Harry B. Gray 

^* Magnetic Resonance Center (CERM), University of Florence, 50019 Sesto Fiorentino, Florence, Italy; Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada; and Beckman Institute, California Institute of Technology, Pasadena, California 91125 USA

Correspondence: Address reprint requests to Prof. Ivano Bertini, CERM, University of Florence, Via Luigi Sacconi 6, 50019 Sesto Fiorentino, Florence, Italy. Tel.: 39-055-457-4272; Fax: 39-055-457-4271; E-mail: bertini{at}cerm.unifi.it.

Residual dipolar couplings (RDCs) and pseudocontact shifts are experimentally accessible properties in nuclear magnetic resonance that are related to structural parameters and to the magnetic susceptibility anisotropy. We have determined RDCs due to field-induced orientation of oxidized-K79A and reduced cytochrome c at pH 7.0 and oxidized-K79A cytochrome c at pH 11.1 through measurements of amide ¹⁵N-¹H ¹J couplings at 800 and 500 MHz. The pH 7.0 RDCs for Fe(III)- and Fe(II)-cytochrome c together with available nuclear Overhauser effects were used to recalculate solution structures that were consistent with both sets of constraints. Molecular magnetic susceptibility anisotropy values were calculated for both redox states of the protein. By subtracting the residual dipolar couplings (RDCs) of the reduced form from those of the oxidized form measured at the same magnetic field (800 MHz), we found the RDC contribution of the paramagnetic metal ion in the oxidized protein. The magnetic susceptibility anisotropy, which was calculated from the structure, was found to be the same as that of the paramagnetic metal ion obtained independently from pseudocontact shifts, thereby indicating that the elements of secondary structure either are rigid or display the same mobility in both oxidation states. The residual dipolar coupling values of the alkaline-K79A form are small with respect to those of oxidized native cytochrome, whereas the pseudocontact shifts are essentially of the same magnitude, indicating local mobility. Importantly, this is the first time that mobility has been found through comparison of RDCs with pseudocontact shifts.