Chapter 27: Biological Inorganic Chemistry

Biological inorganic chemistry examines the essential roles that metal ions and inorganic compounds play in living systems, bridging coordination chemistry with molecular biology and physiology. The chapter begins by establishing how bulk metal cations maintain fundamental cellular functions: sodium and potassium ions preserve electrochemical gradients across membranes and enable electrical signaling in neurons, magnesium stabilizes the energy currency adenosine triphosphate and protects double-stranded nucleic acids from degradation, and calcium regulates muscle contraction through troponin interactions, initiates blood coagulation cascades, and participates in second-messenger signaling pathways. Transition metal ions including iron, copper, zinc, manganese, cobalt, molybdenum, and nickel function as catalytic or structural components of enzymes and proteins, while toxic heavy metals such as lead, mercury, and cadmium interfere with enzymatic activity and cellular processes. Iron chemistry dominates oxygen transport and storage through heme-containing hemoglobin and myoglobin, which reversibly bind molecular oxygen without oxidizing the iron center, while alternative oxygen carriers in other organisms employ copper-based hemocyanin or iron-dependent hemerythrin, illustrating evolutionary diversity. Electron-transfer proteins including cytochromes, iron-sulfur clusters, and blue copper proteins facilitate the movement of electrons through cellular respiration and photosynthesis. The chapter explores oxygen activation by heme and non-heme iron enzymes such as cytochrome P450, which catalyzes hydroxylation reactions, and catalase and peroxidase enzymes that safely decompose hydrogen peroxide. Diverse metalloenzymes demonstrate specialized reactivity: zinc accelerates proton transfer in carbonic anhydrase and peptide hydrolysis in carboxypeptidase, copper participates in superoxide dismutation and tyrosine oxidation, molybdenum participates in nitrogen reduction through the iron-molybdenum cofactor of nitrogenase, nickel activates nitrogen-containing substrates in urease and catalyzes hydrogen generation in hydrogenase, and cobalt mediates carbon skeleton rearrangements in vitamin B12-dependent reactions. Photosynthetic water oxidation depends on a manganese-calcium-oxygen cluster in photosystem II that accumulates oxidizing equivalents. The chapter extends into medicinal applications including platinum complexes that disrupt cancer cell DNA, gadolinium-based magnetic resonance imaging contrast agents, and radioactive tracers for diagnostic nuclear medicine. Additional topics include biomineralization processes that construct calcium carbonate shells and calcium phosphate bone matrices, and synthetic biomimetic catalysts that replicate natural metalloenzyme function.