Enzymes are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and therefore the enzyme converts the substrates into different molecules referred to as products. Almost all metabolic processes within the cell need enzyme catalysis so as to occur at rates fast enough to sustain life. Metabolic pathways depend on enzymes to catalyze individual steps. The study of enzymes is named enzymology and a replacement field of pseudoenzyme analysis has recently grown up, recognising that in evolution, some enzymes have lost the power to carry out biological catalysis, which is usually reflected in their aminoalkanoic acid sequences and weird 'pseudocatalytic' properties. Enzymes are known to catalyze quite 5,000 biochemical reaction types. Other biocatalysts are catalytic RNA molecules, called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures.