Enzymes and Stereospecific Active Sites

Enzymes are proteins that act as biological catalysts. They speed up specific reactions in living cells by providing an alternative pathway with a lower activation energy, and they are not used up.Each enzyme has an active site: a small region, often a pocket or cleft, formed by the tertiary structure of the protein. The substrate binds to the active site through interactions with the side chains there, such as hydrogen bonds, ionic interactions and van der Waals forces. Bonds in the substrate are then weakened, and the reaction takes place.Active sites are stereospecific. The active site is built from chiral amino acids arranged in a fixed three-dimensional shape, so it only fits a substrate with the complementary shape. For a chiral substrate, only one enantiomer can bind with all of its groups lined up with the correct binding points. The other enantiomer is a mirror image, so it cannot make the same set of interactions, and it does not react.Worked example. An enzyme converts one enantiomer of lactic acid (2-hydroxypropanoic acid) but not the other. Explain. The two enantiomers have the same groups but arranged differently in space. The active site has binding points in a fixed 3D arrangement, and only one enantiomer can place its -OH, -COOH and -CH₃ groups on the right points at the same time, so only that one binds and reacts.Since the active site depends on the tertiary structure, anything that changes that shape, such as high temperature or an extreme pH, can stop the enzyme working.