X-ray structure determination of a metastable state of carbonmonoxy myoglobin after photodissociation.

Abstract

The x-ray structure of carbon monoxide (CO)-ligated myoglobin illuminated during data collection by a laser diode at the wavelength A = 690 nm has been determined to a resolution of 1.7 A at T 36 K For comparison, we also measured data sets of deoxymyoglobin and CO-ligated myoglobin. In the photon-induced structure the electron density associated with the CO ligand can be described by a tube extending from the iron into the heme pocket over more than 4 A. This density can be interpreted by two discrete positions of the CO molecule. One is close to the heme iron and can be identified to be bound CO. In the second, the CO is dissociated from the heme iron and lies on top of pyrrole ring C. At our experimental conditions the overall structure of myoglobin in the metastable state is close to the structure of a CO-ligated molecule. However, the iron has essentially relaxed into the position of deoxymyoglobin. We compare our results with those of Schlichting et al.

Influence of Distal Residue B10 on CO Dynamics in Myoglobin and Neuroglobin

“…After 1-s illumination of wild-type MbCO at 4 K, all ligands are photolyzed. They are found trapped at the primary docking site B on top of pyrrole C [25,27,30]. Opposite orientations of the ligands in the B site of A 1 molecules are represented by the photoproduct bands at 2,119 and 2,131 cm −1 [21,46].…”

“…Detailed ligand-binding studies on Mb with both spectroscopic [19][20][21][22][23] and crystallographic [24][25][26][27][28][29][30] techniques have disclosed that the amino acids at positions E7 and B10 play important roles in regulating ligand access to the heme iron. No permanent channel exists in the molecular structure of Mb that connects the active site to the outside, but an outward movement of the HisE7 imidazole side chain may transiently open a pathway through which ligands may enter or exit the distal heme cavity [31,32].…”

“…1a). Ligands bound at the heme iron (site A) may dissociate and become transiently trapped at site B [25,27,30]. From there, they can either rebind, escape into the solvent, or migrate to the secondary sites C and D, which are cavities in the protein that were initially found to bind Xe atoms and, therefore, are also denoted as Xe4 and Xe1 cavities [33].…”

Proximal ligand motions in H93G myoglobin

“…1(C) shows a bimodal peak. Isotope data for 1-methyl imidazole (1,(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) N-substituted 1-MeIm) strongly suggest that this band is split by a Fermi resonance [36]. The deoxy and CO photoproduct spectra for H93G(1-MeIm) show two Fermi resonance bands that change significantly in intensity.…”

“…Nonetheless, this shift in the m(Fe-His) Raman band is significant because shifts in absorption bands (the time-dependent Soret band shift and band III shift) have been attributed to iron out-of-plane displacement that should also be coupled to m(Fe-His) [7][8][9][10]. A structural interpretation of these observable phenomena helps to bridge the gap between the extensive X-ray crystallography studies and the thermodynamic and kinetic data available for Mb [7,[11][12][13][14][15][16][17][18][19][20].Histidine-ligated heme enzymes have a surprisingly large range of functions. In peroxidase, a charge relay due to hydrogen bonding of the imidazole ring of histidine permits the formation of high valent iron oxidation states that play a role in the redox function of these enzymes [21][22][23].…”