Molecular Recognition

The ability of biological and chemical systems to distinguish between molecules and regulate behavior accordingly. How molecules fit together is fundamental in disciplines such as biochemistry, medicinal chemistry, materials science, and separation science. A good deal of effort has been expended in trying to evaluate the underlying intermolecular forces. The weak forces that act over short distances (hydrogen bonds, van der Waals interactions, and aryl stacking) provide most of the selectivity observed in biological chemistry and permit molecular recognition. The recognition event initiates behavior such as replication in nucleic acids, immune response in antibodies, signal transduction in receptors, and regulation in enzymes. Most studies of recognition in organic chemistry have been inspired by these biological phenomena. It has been the task of bioorganic chemistry to develop systems capable of such complex behavior with molecules that are comprehensible and manageable in size, that is, with model systems. See also Enzyme; Hydrogen bond; Intermolecular forces; Nucleic acid.

Metalloproteins and metalloenzymes

These are metal complexes of proteins. In many cases, the metal ion is coordinated directly to functional groups on amino acid residues. In some cases, the protein contains a bound metallo-cofactor such as heme. In metalloproteins with more than one metal-binding site, the metal ions may be found in clusters. Examples include ferredoxins, which contain iron-sulfur clusters (Fe₂S₂ or Fe₄S₄), and nitrogenase, which contains both Fe₄S₄ units and a novel MoFe₇S₈ cluster. See also Protein.

Some metalloproteins are designed for the storage and transport of the metal ions themselves—for example, ferritin and transferrin for iron and metallothionein for zinc. Others, such as the yeast protein Atx1, act as metallochaperones that aid in the insertion of the appropriate metal ion into a metalloenzyme. Still others function as transport agents. Cytochromes and ferredoxins facilitate the transfer of electrons in various metabolic processes.

Low-molecular-weight compounds

A number of coordination compounds found in organisms have relatively low molecular weights. Ionophores, molecules that are able to carry ions across lipid barriers, are polydentate ligands designed to bind alkali and alkaline-earth metal ions; they span membranes and serve to transport such ions across these biological barriers. Molecular receptors known as siderophores are also polydentate ligands; they have a very high affinity for iron. See also Ionophore.

Other low-molecular-weight compounds are metal-containing cofactors that interact with macromolecules to promote important biological processes. Perhaps the most widely studied of the metal ligands found in biochemistry are the porphyrins; iron protoporphyrin IX (see illustration) is an example of the all-important complex in biology known as heme. Chlorophyll and vitamin B₁₂ are chemically related to the porphyrins. Magnesium is the central metal ion in chlorophyll, which is the green pigment in plants used to convert light energy into chemical energy. Cobalt is the central metal ion in vitamin B₁₂; it is converted into coenzyme B₁₂ in cells, where it participates in a variety of enzymatic reactions.

Bioinorganic chemistry

The field at the interface between biochemistry and inorganic chemistry; also known as inorganic biochemistry or metallobiochemistry. This field involves the application of the principles of inorganic chemistry to problems of biology and biochemistry. Because most biological components are organic, that is, they involve the chemistry of carbon compounds, the combination of the prefix bio- and inorganic may appear contradictory. However, organisms require a number of other elements to carry out their basic functions. Many of these elements are present as metal ions that are involved in crucial biological processes such as respiration, metabolism, cell division, muscle contraction, nerve impulse transmission, and gene regulation. The characterization of the interactions between such metal centers and biological components is the heart of bioinorganic chemistry. See also Biochemistry; Inorganic chemistry.

Biochemistry

Field of science concerned with chemical substances and processes that occur in plants, animals, and microorganisms. It involves the quantitative determination and structural analysis of the organic compounds that make up cells (proteins, carbohydrates, and lipids) and of those that play key roles in chemical reactions vital to life (e.g., nucleic acids, vitamins, and hormones). Biochemists study cells' many complex and interrelated chemical changes. Examples include the chemical reactions by which proteins and all their precursors are synthesized, food is converted to energy (see metabolism), hereditary characteristics are transmitted (see heredity), energy is stored and released, and all biological chemical reactions are catalyzed (see catalysis, enzyme). Biochemistry straddles the biological and physical sciences and uses many techniques common in medicine and physiology as well as those of organic, analytical, and physical chemistry.

Supramolecular chemistry

A highly interdisciplinary field covering the chemical, physical, and biological features of complex chemical species held together and organized by means of intermolecular (noncovalent) bonding interactions. See also Chemical bonding; Intermolecular forces.

When a substrate binds to an enzyme or a drug to its target, and when signals propagate between cells, highly selective interactions occur between the partners that control the processes. Supramolecular chemistry is concerned with the study of the basic features of these interactions and with their implementation in biological systems as well as in specially designed nonnatural ones. In addition to biochemistry, its roots extend into organic chemistry and the synthetic procedures for receptor construction, into coordination chemistry and metal ion-ligand complexes, and into physical chemistry and the experimental and theoretical studies of interactions. See also Bioinorganic chemistry; Enzyme; Ligand field theory; Physical organic chemistry; Protein.