Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

How molecules interact with one another
The principle of complementarity

All Cells are built from molecules, which highlights the critical importance of the mechanisms that securely «dock» these molecules together. It is well known that the binding of small molecules to larger ones underlies many biological processes, such as nutrient METABOLISM and hormone action. Interactions between macromolecules are integral to phenomena such as flagellar motility, Muscle contraction, antibiotic action, Nerve Impulse transmission, and many others.

Because intermolecular interactions are weak, molecules can bind to one another securely only if their surfaces match and A large number of atoms are involved in the interaction. To form a stable complex, this fit must be sufficiently precise—meaning the molecular surfaces must be complementary. For instance, if one molecule's surface features a protrusion (such as a —CH₃ group), the complementary surface of the other molecule must have a matching depression; similarly, a positive charge must be positioned opposite a negative one. A proton-donor group can form a Hydrogen bond only if There is a complementary group containing unshared electrons. For Hydrophobic bonds to form, nonpolar (hydrophobic) groups must be positioned directly opposite each other. One of the foundational principles of biochemistry states that two molecules with complementary surfaces tend to interact and bind to one another, whereas molecules lacking complementary surfaces do not interact. Watson termed this THE PRINCIPLE OF selective molecular «stickiness» [1]. It underlies the self-assembly of filaments, tubules, membranes, and polyhedral structures from mutually complementary biological macromolecules. The principle of complementarity is also responsible for the specific base pairing that occurs during DNA Replication.

Surface complementarity is equally vital for the myriad Chemical Reactions taking place within The Cell. These reactions are catalyzed, or «directed», by Enzymes containing reactive chemical groups positioned in precisely defined locations and oriented to interact with other molecules—substrates—and chemically modify them. Selective catalysis is a hallmark of biological systems, and a primary focus of biochemical research. Ultimately, changes in Cell Structure are also driven by chemical reactions, with muscle fiber contraction and cytoplasmic streaming in amoebae serving as prime Examples.