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The interaction of cyanide with the oxidised and reduced forms of cytochrome‐c oxidase has been investigated by kinetic and equilibrium measurements at 20 °C and pH 7.4. The inhibition by cyanide of the oxidation of cytochrome c has also been studied under different conditions. When the oxidised form of cytochrome oxidase is mixed with cyanide, the heme‐absorption bands are changed extremely slowly in a process whose rate is independent of the concentrations of cyanide and protein. Thus, the primary binding site does not appear to be one of the cytochrome components of the oxidase. With reduced cytochrome oxidase, the interaction can be described as a simple secondorder process involving the a32+ form and HCN. The stability constant for the complex is 1.8×103 M−1 and the rate constant for its formation 1.3X102M−1sec−1. The inhibition of the enzyme may occur in two ways. One involves the reduction of the enzyme by cytochrome c and a subsequent reaction between a32+ and the inhibitor. However, if the oxidase is preincubated with cyanide, inhibition has been reported at much lower concentrations of cyanide. This cannot be related to the much slower changes in the heme‐absorption bands observed in the reaction of cyanide with the oxidised form of the enzyme, and it is suggested that it involveds binding of cyanide to one of the copper ions in the oxidase.

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Cu,Zn yeast superoxide dismutase was crystallized from polyethylene glycol solutions. The crystals belong to the P2~212 space group, with cell dimensions a = 105.3, b = 143.0, c = 62-1 ]k; two dimers of Mr = 32000 each are contained in the asymmetric unit. Diffraction data at 2.5A resolution were collected with the image-plate system at the EMBL synchrotron radiation facility in Hamburg. The structure was determined by molecular replacement using as a search model the 'blue-green' dimer of the bovine Cu,Zn superoxide dismutase. The crystallographic refinement of the molecular replacement solution was performed by means of molecular dynamics techniques and resulted in an R factor of 0.268 for the data between 6-0 and 2.5 A. The model was subsequently subjected to conventional restrained crystallographic refinement of the coordi-* To whom correspondence should be addressed.0108-7681/91/060918-10503.00 nates and temperature factors. The current R value for the data between 6.0 and 2.5 A is 0.220. Owing to the large radius of convergence of the molecular dynamics-crystallographic refinement, the convergence of the refinement process was reached after 18.1 ps of simulation time. The geometry of the active site of the enzyme appears essentially preserved compared with the bovine superoxide dismutase. The fl-barrel structure in the yeast enzyme is closed at the upper part by an efficient hydrogen-bonding scheme.