Hammer-and-anvil Proteins Catalyze 2-Step Reactions (IMAGE) Washington University in St. Louis Caption All of the GH3 enzymes examined so far have the same hammer-and-anvil structure. Reactants (green) are clamped into an active site in the ”anvil” (blue), and the “hammer“ (purple) swivels over the active site. The enzymes catalyze a two-step reaction: The first step takes place when the active site is open and the second when it is closed. Credit Jez Lab/WUSTL Usage Restrictions Credit required License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.