9.1.3 Chain Reactions

A chain reaction consists of the following sequence:

1. Initiation: formation of an active intermediate
2. Propagation or chain transfer: interaction of an active intermediate with the reactant or product to produce another active intermediate
3. Termination: deactivation of the active intermediate to form products

An example comparing the application of the PSSH with the Polymath solution to the full set of equations is given on the DVD-ROM for the cracking of ethane. Also included is a discussion of Reaction Pathways and the chemistry of smog formation.

9.2 Enzymatic Reaction Fundamentals

An enzyme is a high-molecular-weight protein or protein-like substance that acts on a substrate (reactant molecule) to transform it chemically at a greatly accelerated rate, usually 10³ to 10¹⁷ times faster than the uncatalyzed rate. Without enzymes, essential biological reactions would not take place at a rate necessary to sustain life. Enzymes are usually present in small quantities and are not consumed during the course of the reaction, nor do they affect the chemical reaction equilibrium. Enzymes provide an alternate pathway for the reaction to occur, thereby requiring a lower activation energy. Figure 9-3 shows the reaction coordinate for the uncatalyzed reaction of a reactant molecule, called a substrate (S), to form a product (P)

S → P

Figure 9-3. Reaction coordinate for enzyme catalysis.

The figure also shows the catalyzed reaction pathway that proceeds through an active intermediate (E · S), called the enzyme–substrate complex, that is,

Because enzymatic pathways have lower activation energies, enhancements in reaction rates can be enormous, as in the degradation of urea by urease, where the degradation rate is on the order of 10¹⁴ higher than without the enzyme urease.

An important property of enzymes is that they are specific; that is, one enzyme can usually catalyze only one type of reaction. For example, a protease hydrolyzes only bonds between specific amino acids in proteins, an amylase works on bonds between glucose molecules in starch, and lipase attacks fats, degrading them to fatty acids and glycerol. Consequently, unwanted products are easily controlled in enzyme-catalyzed reactions. Enzymes are produced only by living organisms, and commercial enzymes are generally produced by bacteria. Enzymes usually work (i.e., catalyze reactions) under mild conditions: pH 4 to 9 and temperatures 75°F to 160°F. Most enzymes are named in terms of the reactions they catalyze. It is a customary practice to add the suffix -ase to a major part of the name of the substrate on which the enzyme acts. For example, the enzyme that catalyzes the decomposition of urea is urease and the enzyme that attacks tyrosine is tyrosinase. However, there are exceptions to the naming convention, such as α-amylase. The enzyme α-amylase catalyzes the transformation of starch in the first step in the production of the soft drink (e.g., Red Pop) sweetener high-fructose corn syrup (HFCS) from corn starch, which is a $4 billion per year business.