The chief objective of this investigation is to clarify the mechanism of action of cytochrome oxidase. The interrelationships of cytochromes a and a3 and copper in the enzyme will be studied in heart muscle and microbial systems through the use of spectrophotometric and electron paramagnetic resonance techniques. The relationship of the subunit structure of the heart muscle oxidase to its activity will be investigated especially with respect to the size and composition of the subunits and to the sites of heme and copper binding. An X-ray diffraction study of a crystalline microbial cytochrome oxidase will be investigated to determine the relative relationship of the two heme prosthetic groups on the protein.