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Biophysical Journal 85:549-558 (2003)
© 2003 The Biophysical Society

Redox-Induced Structural Dynamics of Fe-Heme Ligand in Myoglobin by X-Ray Absorption Spectroscopy

S. Della Longa * {dagger}, A. Arcovito {ddagger}, M. Benfatto §, A. Congiu-Castellano ¶ {dagger}, M. Girasole || {dagger}, J. L. Hazemann ** and A. Lo Bosco ¶

* Department of Experimental Medicine, Università L'Aquila, L'Aquila, Italy; {dagger} INFM, Università "La Sapienza", Rome, Italy; {ddagger} Department of Biochemical Science "A. Rossi-Fanelli", Università "La Sapienza", Rome, Italy; § Laboratori Nazionali di Frascati, INFN, Frascati, Italy; Department of Physics, Università "La Sapienza", Rome, Italy; || Istituto Struttura della Materia, CNR, Rome, Italy; and ** Laboratoire Crystallografie, CNRS, Grenoble, France

Correspondence: Address reprint requests to S. Della Longa, Dip. Medicina Sperimentale, Università dell'Aquila, Via Vetoio, 67100 loc. Coppito, L'Aquila, Italy. Tel.: 39-86-243-3568; Fax: 39-86-243-3523; E-mail: dlonga{at}caspur.it.

The Fe(III) -> Fe(II) reduction of the heme iron in aquomet-myoglobin, induced by x-rays at cryogenics temperatures, produces a thermally trapped nonequilibrium state in which a water molecule is still bound to the iron. Water dissociates at T > 160 K, when the protein can relax toward its new equilibrium, deoxy form. Synchrotron radiation x-ray absorption spectroscopy provides information on both the redox state and the Fe-heme structure. Owing to the development of a novel method to analyze the low-energy region of x-ray absorption spectroscopy, we obtain structural pictures of this photo-inducible, irreversible process, with 0.02–0.06-Å accuracy, on the protein in solution as well as in crystal. After photo-reduction, the iron-proximal histidine bond is shortened by 0.15 Å, a reinforcement that should destabilize the iron in-plane position favoring water dissociation. Moreover, we are able to get the distance of the water molecule even after dissociation from the iron, with a 0.16-Å statistical error.




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