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Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions. Enzymes are biochemical catalysts. In these reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts these into different molecules, the products. Almost all processes in the cell need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines the biochemistry and signal transduction that occur in a cell.