Inactive enzyme-homologues find new function in regulatory processes.

Although the catalytic center of an enzyme is usually highly conserved, there have been a few reports of proteins with substitutions at essential catalytic positions, which convert the enzyme into a catalytically inactive form. Here, we report a large-scale analysis of substitutions at enzymes' catalytic sites in order to gain insight into the function and evolution of inactive enzyme-homologues. Our analysis revealed that inactive enzyme-homologues are not an exception only found in single enzyme families, but that they are represented in a large variety of enzyme families and conserved among metazoan species. Even though they have lost their catalytic activity, they have adopted new functions and are now mainly involved in regulatory processes, as shown by several case studies. This modification of existing modules is an efficient mechanism to evolve new functions. The invention of inactive enzyme-homologues in metazoa has thereby led to an enhancement of complexity of regulatory networks.