Metalloproteins are classified on the basis of their function as folloes:
- Respiratory proteins
- Electron transport proteins
- Metal storage proteins
- Catalytic proteins
These are metalloproteins which are involved in gas transport. Example : Hemoglobin, Myoglobin, Hemerythrin and Hemocyanin. Both hemoglobin and myoglobin are heme (prosthetic group) proteins containing Fe2+ . In higher animals hemoglobin transports oxygen from lungs to tissues. Myoglobin stores the oxygen in the tissues. Hemerythrin and hemocyanin are the oxygen carrying proteins of lower animals. Hemerythrin is a nonheme iron containing protein. Hemocyanin is a nonheme copper containing protein.
II . ELECTRON TRANSPORT PROTEINS :
These are metalloproteins which are involved in electron transport. They are the components of electron transport chain (ETC). Example: Cytochromes, iron sulphur protein and blue copper protein. Cytochromes (a, b, c) are the iron containing heheproteins that act as electron carrier in all aerobic forms of life. Iron sulphur proteins (Rubredoxin, ferridoxin) are nonheme proteins involved in electron transfer. Blue copper proteins (stellocyanin, plastocyanin, azurin) are nonheme copper containing proteins involved in electron transfer.
III . METAL STORAGE PROTEINS :
These are metalloproteins which are used to store the surplus metal in nontoxic form. Example: Ferritin, Transferrin (iron storage protein) Ceruloplasmin (copper storage protein).
IV . CATALYTIC PROTEINS :
Enzymes are otherwise known as catalytic proteins. Many enzymes contain metal ions as their structural component. Example: Cytochrome oxidase (iron and copper), Super oxide dismutase (copper and zinc), Nitrogenase (iron and molybdenum). These enzyme are bimetalcomplexes.