enzyme's tertiary and quaternary structures. Its contours limit the number of substrates with which it can react to those substrates that spatially "fit" the enzyme's surface. The part of the enzyme to which the substrate binds is called the active site. It is important to note that an enzyme's active site may undergo conformational changes during bonding that improve the fit and the reactivity of the substrate.
The enzyme and substrate are held together by relatively weak bonds (such as ionic or hydrogen bonds) that can be made and broken easily due to the random collisions caused by ordinary thermal motion. The formation of bonds between an enzyme and its substrate depends on the exposed R groups of the amino acids that form the enzyme's active site. R groups can be polar, non polar, or electrically charged, and the corresponding substrate must be compatible in order to bond. A non polar substrate will not bond with a polar or a charged active site even if otherwise spatially suited; conversely, a polar or a charged substrate will not bond with a non polar active site. In addition, some enzymes require the presence of coenzymes, nonproteinaceous molecules that remain unchanged by the reaction, in order for a substrate to be able to bind to the enzyme's active site.
Enzymes accelerate reactions primarily by stabilizing transition state substrate molecules, thereby lowering the activation energy of the catalyzed reaction. Enzymes do not affect the reaction equilibria. Enzymes can also increase reaction rates by bonding to several reactant molecules simultaneously, thus bringing them closer
