| Abstract
| - The globins and peroxidases, while performing completelydifferent chemistry, share featuresof the iron heme active site: a protoporphyrin IX prosthetic group islinked to the protein by the proximalhistidine residue. X-ray absorption spectroscopy provides a methodto determine the local structure ofiron heme active sites in proteins. Our previous studies usingX-ray absorption spectroscopy revealed asignificant difference in the Fe−Nε bond length betweenthe peroxidases and the globins [for a review,see Powers, L. (1994) Molecular Electronics andMolecular Electronic Devices, Vol. 3, p 211CRCPress Inc., Boca Raton, FL]. Globins typically have anFe−Nε distance close to 2.1 Å while theFe−Nεdistance in the peroxidases is closer to 1.9 Å. We have proposed[Sinclair, R., Powers, L., Bumpus, J.,Albo, A., & Brock, B. (1992) Biochemistry31,4892] that strong hydrogen bonding to the proximalhistidine is responsible for the shorter bond length in theperoxidases. Here we use site-specific mutagenesisto eliminate the strong proximal hydrogen bonding in cytochromec peroxidase and to introduce strongproximal hydrogen bonding in myoglobin. Consistent with ourhypothesis, elimination of the Asp235−His175 hydrogen bond in CcP results in elongation ofFe−Nε from ∼1.9 to ∼2.1 Å.Conversely,introduction of a similar strong proximal hydrogen bond in myoglobinshortens Fe−Nε from ∼2.1 to∼1.9 Å. These results correlate well with other biochemicaldata.
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