Micronutrient trace minerals such as iron (Fe), manganese (Mn), copper (Cu), cobalt (Co) and zinc (Zn) are usually transition metals that form coordination complexes within proteins. One prominant example is the oxygen transport protein in the blood, hemoglobin, or myoglobin in the muscles, in which iron is bound to the porphyrin ring, the iron and porphyrin together constituting heme. Carbonic anhydrase and carboxypeptidase A contain coordinated zinc, where it performs a catalytic function within the active site. In the enzyme, ATCase, zinc plays a structural roll. Further examples of coordination complexes in biology include vitamin B-12, which is a large complex of cobalt. Additionally, iron sulfur clusters within proteins, in which groups of iron atoms are coordinated with inorganic sulfides are very important in electron transfer reactions in the citric acid cycle, oxidative phosphorylation, and photosynthesis.