Living organisms carry out thousands of chemical reactions which take place in dilute solution at ordinary temperature and pressure. For example they can use small molecules to assemble complex biopolymers such as proteins and DNA. Organisms can produce molecules that combat bacterial invaders. They can break down large, energy-rich molecules in many steps to extract chemical energy in small portions to drive their many activities.
Most of these reactions are catalyzed by biochemical catalysts called enzymes. Enzymes are proteins with high molar mass ranging from 15,000 to 1,000,000 g/mol. Enzymes are incredibly efficient catalysts. They increase rates by 108 to 1020times. Enzymes are also extremely specific: each reaction is generally catalyzed by a particular enzyme. Urease, for example, catalyses only the hydrolysis of urea and none of the several thousand other enzymes present in the cell catalyses that reaction:
The remarkable specificity of enzymes results from the fact that each enzyme has a specific, active site on its surface. When the reactant molecules, called the substrates of the reaction, bind at the active site, a chemical change is initiated. In most cases, substrates bind to the active site through intermolecular forces: H-bonds, dipole forces and other weak attractions. Two models of enzyme action have been proposed. According to the lock-and-key model, when the ‘key (substrate) fits the ‘lock’ (active site), the chemical change begins. However, modern X-ray crystallographic and spectroscopic methods show that in many cases, the enzyme changes shape when the substrate lands at the active site. This induced-fit model of enzyme action pictures the substrate inducing the active site to adopt a perfect fit, rather than a rigidly shaped lock and key. Therefore, we might picture a hand in a glove, in which the ‘glove’ (active site) does not attain its functional shape until the ‘hand’ (substrate) moves into place.
The kinetic of enzyme catalysis has many features in common with ordinary catalysis. In the enzyme catalyzed reaction, substrate (S) and enzyme (E) form an intermediate enzyme-substrate complex (ES) whose concentration determines the rate of product (P) formation. The steps common to virtually all enzyme catalyzed reactions are:
The rate of enzyme catalyzed reaction changes from first-order to zero-order as the concentration of substrate is increased.