ИСТИНА

Subunit I of cytochrome c oxidase (COX) from mitochondria and many bacteria contains a cation binding site located near heme a, facing the P-phase. Mitochondrial COX binds reversibly Ca2+ or Na+. Recently we have found that Ca2+ binding to COX stabilizes the reduced state of heme a by increasing its midpoint potential by 15-20 mV. Under the same conditions ferrocyanide-induced respiration of bovine COX was reversibly inhibited by Ca2+ but not by Mg2+ ions. The effect was titrated with the apparent Kd value of 10-6M close to that obtained from a Ca2+-induced red shift of heme a absorbance spectra. Similar Ca2+-induced inhibition was observed with a natural electron donor, cytochrome c, when COX was turning over not too fast (less than 10 s-1) and reproduced on mitochondria from different tissues (liver, kidney, heart and skeletal muscle). This is a direct evidence that Ca2+ /Na+ binding to COX affects its activity.