Kinetic, structural, and spectroscopic indentification of geminate states of myoglobin: a ligand binding site on the reaction pathway

Abstract

Elementary steps or geminate states in the reaction of gaseous ligands with transport proteins delineate the trajectory of the ligand and its rebinding to the heme. By use of kinetic studies of the 765-nm optical "conformation" band, three geminate states were identified for temperatures less than approximately 100 K. MbCO, which is accumulated by photolysis between 1.2 and approximately 10 K, was characterized by our previous optical and X-ray absorption studies [Chance, B., Fischetti, R., & Powers, L. (1983) Biochemistry 22, 3820-3829]. Between 10 and approximately 100 K, geminate states that are also identified that have recombination rates of approximately 10(3) s-1 and approximately 10(-5) s-1 (40 K). Thus, it is possible to maintain a steady-state nearly homogeneous population of the slowest recombining geminate state, Mb, by regulated continuous illumination (optical pumping). Both X-ray absorption and resonance Raman studies under similar conditions of optical pumping show that the heme structure around the iron in Mb is similar to that of MbCO. In both geminate states, the iron-proximal histidine distance remains unchanged (+/- 0.02 A) from that of MbCO while the iron to pyrrole nitrogen average distance has not fully relaxed to that of the deoxy state. In MbCO the CO remains close to iron but not bound, and the Fe...CO angle, which is bent in MbCO (127 +/- 4 degrees C), is decreased by approximately 15 degrees [Powers, L., Sessler, J. L., Woolery, G. L., & Chance, B. (1984) Biochemistry 23, 5519-5523]. The CO molecule in Mb, however, has moved approximately 0.7 A further from iron. Computer graphics modeling of the crystal structure of MbCO places the CO in a crevice in the heme pocket that is just large enough for the CO molecule end-on. Above approximately 100 K resonance Raman studies show that this structure relaxes to the deoxy state.

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…In a pioneering experiment, Mills and co-workers probed the reversible photodissociation of a CO group from carboxymioglobin via TR-XANES at the Fe K-edge [140,[234][235][236]. Similar studies on related systems were reported by Clozza et al [237] and by Chance and co-workers [141,[238][239][240]. In addition, Chen et al investigated the lightdriven redox processes in an Fe-Zn diporphyrin integrated in a modified haem protein [241].…”

Spectroscopically and Kinetically Distinct Conformational Populations of Sol-Gel-encapsulated Carbonmonoxy Myoglobin

“…Whereas the time scale for the protein relaxation is highly dependent on solution conditions to the extent that they can be greatly slowed by enhancing the solution viscosity, the rapid motion of the iron appears insensitive to environmental factors. There is Raman evidence that the iron displacement can be inhibited only at liquid helium temperatures (18,20,21,48).…”

Ligand recombination and a hierarchy of solvent slaved dynamics: The origin of kinetic phases in hemeproteins

“…To an extent, the inverse temperature effect is an inverse viscosity effect in which the overall recombination process gets slower with decreasing viscocity The origin of both the onset of the new phases and the related inverse temperature/viscosity effect has been vigorously explored and debated. The central focus was whether these processes were being driven by barrier-raising conformational relaxations (Agmon and Hopfield, 1983;Ahmed et al, 1991;Ansari et al, 1992;Abadan et al, 1995) or by increased access of the dissociated ligand to slow rebinding docking sites (Powers et al, 1987; within the protein matrix by the dissociated CO. Initially, the spectroscopic and theoretical work favored the barrier raising relaxation model.…”