This is the protein myoglobin. Myoglobin is found in muscles and is responsible for the red color of the muscles. It is the oxygen transporting protein that allows muscles not only to circulate oxygen, but also to store it. Myoglobin combines with oxygen released by the erythrocytes (red blood cells), and transports it to the mitochondria (the energy source or cell engine) of our muscle cells. This generates energy by combustion of glucose to CO₂ & H₂0. In humans, the normal blood levels of myoglobin are 0-85 ng/ml. Myoglobin consists of one heme molecule (not shown here, see below) containing one iron molecule attached to a single globin polypeptide chain. This ferrous (containing iron) globin complex is similar to one single subunit of the protein hemoglobin. This molecule is composed of 153 different amino acids.