Redox-induced structural dynamics of Fe-heme ligand in myoglobin by X-ray absorption spectroscopy.
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-A accuracy, on the protein in solution as well as in crystal. After photo-reduction, the iron-proximal histidine bond is shortened by 0.15 A, 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-A statistical error.
Della Longa
S
S
Department of Experimental Medicine, Università L'Aquila, L'Aquila, Italy. dlonga@caspur.it
Arcovito
A
A
Benfatto
M
M
Congiu-Castellano
A
A
Girasole
M
M
Hazemann
J L
JL
Lo Bosco
A
A
D015502:Absorptiometry, Photon / Q000379:methods*; D003198:Computer Simulation; D003461:Crystallography / Q000379:methods; D004735:Energy Transfer; D006418:Heme / Q000737:chemistry* / Q000528:radiation effects; D007501:Iron / Q000737:chemistry* / Q000528:radiation effects; D008024:Ligands; D008958:Models, Molecular; D009211:Myoglobin / Q000737:chemistry* / Q000528:radiation effects*; D010084:Oxidation-Reduction; D011487:Protein Conformation; D013052:Spectrometry, X-Ray Emission / Q000379:methods*; D014867:Water / Q000737:chemistry*
D008024:Ligands; D009211:Myoglobin; D014867:Water; D006418:Heme; D007501:Iron