Coordination chemistry is of paramount importance in biochemistry because many biologically important substances possess organic functional groups that form coordinate covalent bonds with transition metal ions. One prominent 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. In the oxygen binding site of hemoglobin and myoglobin, four of six of iron's coordination positions are occupied by bonds to the porphyrin ring, a fifth is involved in a bond to a histidine side chain of the protein leaving the sixth available for the formation of a coordinate covalent bond with oxygen. Heme is also present in the electron carrier cyctochrome c, as well as in cytochrome oxidase, which has two heme groups as well as two copper ions. 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. Coordination chemistry is a major weakness among premedical students, so you do not need much more than a basic orientation to get beyond your competition.