Enzymes are biological catalysts—mostly proteins—that speed up chemical reactions in living organisms without being permanently altered themselves. Key characteristics include high substrate specificity (they typically bind particular molecules at an active site), the ability to lower activation energy so reactions proceed under mild temperature and pH, and sensitivity to conditions such as temperature, pH, and inhibitors.
Many enzymes also require cofactors (metal ions or organic coenzymes) to function. Since the 1980s, scientists have also identified catalytic RNAs called ribozymes, showing that not all enzymes are proteins.