section_path: Biological role/Metalloproteins and inorganic cofactors
section_name: Metalloproteins and inorganic cofactors
paragraph_text: Metalloproteins and inorganic cofactors
  
  Metalloproteins—in which the active site is a transition metal ion (or metal-sulfide cluster) often coordinated by sulfur atoms of cysteine residues —are essential components of enzymes involved in electron transfer processes. Examples include plastocyanin (Cu) and nitrous oxide reductase (Cu–S). The function of these enzymes is dependent on the fact that the transition metal ion can undergo redox reactions. Other examples include many zinc proteins, as well as iron–sulfur clusters. Most pervasive are the ferrodoxins, which serve as electron shuttles in cells. In bacteria, the important nitrogenase enzymes contain an Fe–Mo–S cluster and is a catalyst that performs the important function of nitrogen fixation, converting atmospheric nitrogen to ammonia that can be used by microorganisms and plants to make proteins, DNA, RNA, alkaloids, and the other organic nitrogen compounds necessary for life.
  
  Sulfur is also present in molybdenum cofactor.