Cytochrome c reduction by cysteine plus copper: a pseudosubstrate system for cytochrome c oxidase

Cysteine alone reduces horse heart cytochrome c very slowly (k approximately or equal too 1.0 M-1s-1) with a rate constant virtually identical in high and low ionic strength buffers. Copper catalyzes this reaction increasing the rate by a factor of 10(5) in 50 mM phosphate and by a factor of 10(6) in 10mM Tris buffers. When ferricytochrome c and cysteine are mixed in an oxygen electrode a "burst" of oxygen uptake is seen, the decline in which parallels the reduction of cytochrome c. When cytochrome oxidase is added to such a mixture two routes of electron transfer to oxygen exist: enzymatic and ferricytochrome c dependent nonenzymatic. Both processes are sensitive to cyanide, but azide inhibits only the authentic cytochrome c oxidase catalyzed process and BCS the ferricytochrome c stimulated reaction.