Electron transfer between cytochrome b(5) and some oxidised haemoglobins: the role of ionic strength

"Bind and Crawl" Association Mechanism of Leishmania major Peroxidase and Cytochrome c Revealed by Brownian and Molecular Dynamics Simulations

Leishmania major, the parasitic causative agent of leishmaniasis, produces a heme peroxidase (LmP), which catalyzes the peroxidation of mitochondrial cytochrome c (LmCytc) for protection from reactive oxygen species produced by the host. The association of LmP and LmCytc, which is known from kinetics measurements to be very fast (∼10(8) M(-1) s(-1)), does not involve major conformational changes and has been suggested to be dominated by electrostatic interactions. We used Brownian dynamics simulations to investigate the mechanism of formation of the LmP-LmCytc complex. Our simulations confirm the importance of electrostatic interactions involving the negatively charged D211 residue at the LmP active site, and reveal a previously unrecognized role in complex formation for negatively charged residues in helix A of LmP. The crystal structure of the D211N mutant of LmP reported herein is essentially identical to that of wild-type LmP, reinforcing the notion that it is the loss of charge at the active site, and not a change in structure, that reduces the association rate of the D211N variant of LmP. The Brownian dynamics simulations further show that complex formation occurs via a "bind and crawl" mechanism, in which LmCytc first docks to a location on helix A that is far from the active site, forming an initial encounter complex, and then moves along helix A to the active site. An atomistic molecular dynamics simulation confirms the helix A binding site, and steady state activity assays and stopped-flow kinetics measurements confirm the role of helix A charges in the association mechanism.

The effect of ionic strength on the electron-transfer rate of surface immobilized cytochrome C

Horse heart cytochrome c was immobilized on four different self-assembled monolayer (SAM) films. The electron tunneling kinetics were studied in the different assemblies as a function of the ionic strength of the buffer solution using cyclic voltammetry. When cytochrome c is electrostatically immobilized, the standard electron exchange rate constant k0 decreases with the increase of the solution's ionic strength. In contrast, the protein covalently attached or ligated has a rate constant independent of the ionic strength. The inhomogeneity of electrostatically immobilized cytochrome c increases with the increase of the solution's ionic strength whereas that of the covalently attached protein is independent of the ionic strength. A comparison of these different electron-transfer behaviors suggests that the thermodynamically stable geometry of cytochrome c in the electrostatic assemblies is also an electron transfer favorable one. It suggests that the surface charges of cytochrome c are capable of guiding it into geometries in which its front surface faces the electron-transfer partner. The inhomogeneity observed in this study indicates that a distribution of cytochrome c orientations and thus a distribution of electron transfer rate constants exists.

The reaction of neuroglobin with potential redox protein partners cytochromeb5and cytochromec

Previously identified, potentially neuroprotective reactions of neuroglobin require the existence of yet unknown redox partners. We show here that the reduction of ferric neuroglobin by cytochrome b(5) is relatively slow (k=6 x 10(2)M(-1)s(-1) at pH 7.0) and thus is unlikely to be of physiological significance. In contrast, the reaction between ferrous neuroglobin and ferric cytochrome c is very rapid (k=2 x 10(7)M(-1)s(-1)) with an apparent overall equilibrium constant of 1 microM. Based on this data we propose that ferrous neuroglobin may well play a role in preventing apoptosis.

Molecular aspects of embryonic hemoglobin function

In order to provide the appropriate level of oxygen transport to respiring tissues, we need to produce a molecular oxygen transporting system to supplement oxygen diffusion and solubility. This supplementation is provided by hemoglobin. The role of hemoglobin in providing oxygen transport from lung to tissues in the adult is well-documented and functional characteristics of the fetal hemoglobin, which provide placental oxygen exchange, are also well understood. However the characteristics of the three embryonic hemoglobins, which provide oxygen transport during the first three months of gestation, are not well recognized. This review seeks to describe the state of our understanding of the temporal control of the expression of these proteins and the oxygen binding characteristics of the individual protein molecules. The modulation of the oxygen binding properties of these proteins, by the various allosteric effectors, is described and the structural origins of these characteristics are probed.