Why is cyanide poisonous?
Cyanide binds cytochrome oxidase so as to prevent the binding of oxygen. Electron transport is reduced to zero, and none of the oxygen breathed in can be used. Cytochrome oxidase is one of a superfamily of proteins which act as the terminal enzymes of respiratory chains, so if the cyanide is binding to the cytochrome then it inhibits the process and making it dangerous. Another example I would give rotenone which binds competitively.