Research Article
Galina B Postnikova and Eka
Abstract
We have first shown that oxygen release from MbO2 at near-zero O2 concentrations (p02) only proceeds when interacting the protein with respiring mitochondria. If they are separated from MbO2 solution by a semipermeable membrane, no MbO2 deoxygenation occurs. The rates of O2 uptake by mitochondria from solution in the presence of MbO2 (V1) and MbO2 deoxygenation (V2) completely coincide for different mitochondrial preparations, the native, frozen and uncoupled by FCCP, as both V1 and V2 are determined by respiratory activity of mitochondria. However, V1 and V2 reflect different processes, because they are differently affected by the proteins, like lysozyme, competing with MbO2 for binding to mitochondria. It is found that myoglobin non-specifically interacts with phospholipid sites of the outer mitochondrial membrane, while any specific proteins or protein channels for the binding to myoglobin are lacking. The pronounced ionic strength dependence of the binding implies significant contribution of coulombic electrostatics into the formation of myoglobin–mitochondrial complex. As the total charge of myoglobin molecule does not affect the MbO2 and metMb affinity for the mitochondrial membrane, the ionic strength effect must be due to local electrostatic interactions, most probably between oppositely charged groups of phospholipids (the heads) and polar myoglobin residues in the environment of the heme cavity. The shift of the oxy- / deoxy- equilibrium toward ligand free deoxymyoglobin under anaerobic conditions in the presence of mitochondrial and artificial phospholipid membranes indicates that the myoglobin-membrane interaction results in a decreased myoglobin affinity for oxygen (and increased p50), which facilitates O2 detachment from MbO2 at physiological p02 values. Thus, in the presence of respiring mitochondria, at least two kinds of myoglobin molecules should be present in the cell. Some of them are free, with the high affinity for O2, and respectively, low p50, and the others, associated with mitochondria, with lower affinities and higher p50. For effective O2 transfer from cytoplasm, the exchange between these two kinds of MbO2 molecules must be rather fast. The Km values of MbO2 binding to mitochondria (about 104 M-1 at I = 0,15) and the lifetime of the complex (tens ns) correspond well to this demand.