atalytic functions. An estimated 30% of proteins contain metal ions. Examples include the intensely colored vitamin B12, the heme group in hemoglobin, the cytochromes, the chlorin group in chlorophyll, and carboxypeptidase, a hydrolytic enzyme important indigestion. Another complexion enzyme is catalase, which decomposes the cell's waste hydrogen peroxide. Homogeneous catalysis is a major application of coordination compounds for the production of organic substances. Processes include hydrogenation, hydroformylation, oxidation. In one example, a combination of titanium trichloride and triethylaluminium gives rise to Ziegler–Natta catalysts, used for the polymerization of ethylene and propylene to give polymers of great commercial importance as fibers, films, and plastics. Nickel, cobalt, and copper can be extracted using hydrometallurgical processes involving complexions. They are extracted from their ores as ammine complexes. Metals can also be separated using the selective precipitation and solubility of complexions. Cyanide is used chiefly for the extraction of gold and silver from their ores.