The machinery of life relies on proteins acting as catalysts for almost all chemical reactions which are necessary to support the living organism. When proteins adopt this role they are called an enzyme. Neutron protein crystallography has been successfully employed to elucidate the mechanism which these enzymes use in order to fulfil their task. To know the atomistic structure of these enzymes often is the key to understand how they function. X-ray protein crystallography has been used to get structural information on the heavier atoms which sometimes led to a good understanding of the enzymatic mechanism. But neutron protein crystallography can also locate hydrogen atoms which give information on the protonation states of relevant side chains of enzymes in a certain step of their mechanistic action. Especially for large enzymes this is not possible to do with any other method. But often the location of one hydrogen atom gives the missing hint on how the enzyme works.