Please note that the content of this book primarily consists of articles
available from Wikipedia or other free sources online. Enzymes are
proteins that catalyze (i.e., increase the rates of) chemical reactions.
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.
