The equilibrium constant here has units of liters/mole; for simple binding interactions it is also called the affinity constant or association constant, denoted Ka. The reciprocal of Ka is called the dissociation 1 2 3 dissociation AB A + B dissociation rate = dissociation rate constant ×concentration of AB dissociation rate = koff [AB] association ABA + B association rate = association rate constant ×concentration of A association rate = kon [A] [B] ×concentration of B AT EQUILIBRIUM: association rate = dissociation rate kon [A] [B] = koff [AB] [A][B] koff [AB] kon = = K = equilibrium constant equilibrium constant [A][B] [AB] = K(liters/mole) 1 10 102 103 104 105 106 107 108 109 1010 standard free-energy difference of AB minus free energy of A + B (kJ/mole) 0 –5.9 –11.9 –17.8 –23.7 –29.7 –35.6 –41.5 –47.4 –53.4 –59.4 The relationship between standard free-energy differences (°G°) and equilibrium constants (37°C) (A) (B) constant, Kd (in units of moles/liter).
of a few noncovalent bonds can have a striking effect on a binding interaction, as shown by the example in Figure 3–45. (Note that the equilibrium constant, as defined here, is also known as the association or affinity constant, Ka.)
We have used the case of an antibody binding to its ligand to illustrate the effect of binding strength on the equilibrium state, but the same principles apply to any molecule and its ligand. Many proteins are enzymes, which, as we now discuss, first bind to their ligands and then catalyze the breakage or formation of covalent bonds in these molecules.
Many proteins can perform their function simply by binding to another molecule. An actin molecule, for example, need only associate with other actin molecules to form a filament. There are other proteins, however, for which ligand binding is only a necessary first step in their function. This is the case for the large and very important class of proteins called enzymes. As described in Chapter 2, enzymes are remarkable molecules that cause the chemical transformations that make and break covalent bonds in cells. They bind to one or more ligands, called substrates, and convert them into one or more chemically modified products, doing this over and over again with amazing rapidity. Enzymes speed up reactions, often by a factor of a million or more, without themselves being changed—that is, they act as catalysts that permit cells to make or break covalent bonds in a controlled way. It is the catalysis of organized sets of chemical reactions by enzymes that creates and maintains the cell, making life possible.
We can group enzymes into functional classes that perform similar chemical reactions (Table 3–1). Each type of enzyme within such a class is highly specific,