Enzymes are biomolecules that catalyze (i.e., increase the rates
of) chemical reactions. Nearly all known enzymes are proteins.
However, certain RNA molecules can be effective biocatalysts
too. These RNA molecules have come to be known as ribozymes. In
enzymatic reactions, the molecules at the beginning of the
process are called substrates, and the enzyme converts them into
different molecules, called the products. Almost all processes
in a biological cell need enzymes to occur at significant rates.
Since enzymes are selective for their substrates and speed up
only a few reactions from among many possibilities, the set of
enzymes made in a cell determines which metabolic pathways occur
in that cell. Like all catalysts, enzymes work by lowering the
activation energy (Ea or ΔG‡) for a reaction, thus dramatically
increasing the rate of the reaction. Most enzyme reaction rates
are millions of times faster than those of comparable un-
catalyzed reactions. As with all catalysts, enzymes are not
consumed by the react