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Science 6 January 1989:
Vol. 243. no. 4887, pp. 69 - 72
DOI: 10.1126/science.2563171
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Articles

Science, Vol 243, Issue 4887, 69-72
Copyright © 1989 by American Association for the Advancement of Science

articles

Effects of buried ionizable amino acids on the reduction potential of recombinant myoglobin

R Varadarajan, TE Zewert, HB Gray, and SG Boxer

Department of Chemistry, Stanford University, CA 94305.

The temperature dependences of the reduction potentials (E degrees') of wild-type human myoglobin (Mb) and three site-directed mutants have been measured by the use of thin-layer spectroelectrochemistry. Residue Val68, which is in van der Waals contact with the heme in Mb, has been replaced by Glu, Asp, and Asn. The changes in E degrees' and the standard entropy (delta S degrees') and enthalpy (delta H degrees') of reduction in the mutant proteins were determined relative to values for wild type; the change in E degrees' at 25 degrees C was about -200 millivolts for the Glu and Asp mutants, and about -80 millivolts for the Asn mutant. At pH 7.0, reduction of Fe(III) to Fe(II) in the Glu and Asp mutants is accompanied by uptake of a proton by the protein. These studies demonstrate that Mb can tolerate substitution of a buried hydrophobic group by potentially charged and polar residues and that such amino acid replacements can lead to substantial changes in the redox thermodynamics of the protein.


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