Cyanide is in love with a protein called the cytochrome oxidase. The cytochromes in the membrane of the mitochondria are involved in a process called oxidative phosphorylation. Oxidative phosphorylation is when the cell makes ATP energy for the cell. The cell needs proteins called cytochromes in order to complete the process of producing ATP by oxidative phosphorylation. So, in normal circumstances, ATP is produced by the mitochondria via oxidative phosphorylation. This requires oxygen. When the process is working, oxygen is reduced to water. When cyanide is present (remember that cytochrome oxidase and cyanide are in love with each other) the cyanide binds to the cytochrome oxidase and prevent oxygen from binding. Cyanide does not let go, therefore, oxygen cannot bind nor can it be reduced to water. Because of this, the mitochondria cannot make ATP. When a cell cannot make ATP, the cell dies. That is why it is toxic !