ABSTRACT
In any cell of an organism, noncovalent interactions account for nearly all communications at the intracellular level (between the components of an individual cell) and intercellular level (between cells or between cells and organs). If we examine how antibodies bind to their antigens, how receptors interact with hormones, and how enzymes catalyze reactions,we ¢nd that nearly all steps in these processes are dominated by noncovalent, reversible interactions. Our genetic information is stored in longDNAchains that exist as double helices that are held together by hydrogen bonding between complementary base pairs. Also, the characteristic shape of a protein is held together by noncovalent interactions between the various side groups of the residues that comprise the chain. An antibody possesses a recognition site that complements the molecular shape and chemical functionality of its antigen These recognition sites frequently contain several interaction points to ensure that the antibody forms a strong, yet highly speci¢c, interaction with the antigen. The 20 di¡erent amino acids that serve as building blocks in natural proteins display a wide variety of molecular interactions. Very di¡erent types of interactions are likely to occur for amino acids that are hydrophobic (e.g., phenylalanine), highly polar (e.g., asparagine), and charged (e.g., glutamic acid or lysine).
