Enzyme reaction stages. Illustration of four stages in the mechanism of an enzyme-based reaction. From left, the substrate molecule (blue) attaches to the active site on the enzyme molecule (red), forming an enzyme-substrate complex (centre left). This complex goes through a transition state where the substrate molecule is strained, resulting in it being broken into two (centre right). The final stage (far right) is the release of the product molecules. An enzyme is a biological catalyst, increasing reaction rates without itself reacting or changing. For this artwork with labels, see image C023/8719.