Once a substrate has bound to its active site, an enzyme can cause bonds in the substrate to stretch, putting it in an unstable transition state (Figure 8.11B). For example, lysozyme is a protective enzyme abundant in tears and saliva that destroys invading bacteria by cleaving peptidoglycans in their cell walls. Lysozyme’s active site “stretches” the bonds between the glycan monomers, rendering the bonds unstable and more reactive to lysozyme’s other substrate, water.