While some enzymes are made of RNA (ribozymes), the vast majority of enzymes are proteins. Proteins are large, complex molecules made up of chains of amino acids. The specific sequence of amino acids determines the unique three-dimensional shape of a protein, which in turn dictates its function. Enzymes act as catalysts, speeding up biochemical reactions without being consumed in the process. Their complex structures allow them to bind to specific molecules (substrates) and facilitate chemical transformations.