Nitrogenase superoxide dismutase

Most proteins contain more than one polypeptide chain. The manner in which these chains associate determines quaternary structure. Binding involves the same types of noncovalent forces mentioned for tertiary structure van der Waals forces, hydrophobic and hydrophilic attractions, and hydrogen bonding. However, the interactions are now interchain rather than infrachain (tertiary structure determination). The quaternary structure of hemoglobin (four almost identical subunits) will be discussed in Chapter 4, that of superoxide dismutase (two identical subunits) will be discussed in Chapter 5, and that of nitrogenase (multiple dissimilar subunits) will be discussed in Chapter 6. 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors (Lippard and Berg, 1994), such as the tetrapyrrole porphyrins and corrins, or in metal clusters, such as the Fe-S clusters in Fe-S proteins or the FeMo-cofactor of nitrogenase. Here we discuss briefly how metals are incorporated into porphyrins and corrins to form haem and other metallated tetrapyrroles, how Fe-S clusters are synthesized and how copper is inserted into superoxide dismutase. 

CCS = copper chaperone for superoxide dismutase Moco = molybdenum cofactor MPT = molybdopterin FeMoco = iron-molybdenum cofactor of nitrogenase, SODl = Cu/Zn superoxide dismutase. 

Usually enzymes are classified by a so-called EC number, consisting of 3 or 4 digits separated by dots, to indicate subgroups and sub-subgroups. Well-known metal-containing examples are nitrogenase (EC 1.18.6.1), bovine superoxide dismutase (SOD, EC 1.15.1.1) and urease (EC 3.5.1.5). 

O2 carriers), ferredoxins and cytochromes (redox processes), ferritin (iron storage), acid phosphatase (hydrolysis of phosphates), superoxide dismutases (O2 dismuta-tion) and nitrogenase (nitrogen fixation). A deficiency of iron in the body causes anaemia (see Box 21.6), while an excess causes haemochromatosis. 

---