Although the catalytic center of an enyzme is usually highly conserved, there have been a few reports of proteins with substitutions at essential catalytic positions, which convert the enyzme into a catalytically inactive form.  In order to gain insight into the function and evolution of inactive enzyme-homologues a large-scale analysis of substitutions at enzymes' catalytic sites was performed.  The 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, which I will discuss in detail using the protein tyrosine phosphatase family.
  The 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.


References:
Pils B, Schultz J. Inactive Enzyme-homologues Find New Function in Regulatory Processes. J Mol Biol. 2004 Jul 9;340(3):399-404.
Pils B, Schultz J. Evolution of the multifunctional protein tyrosine phosphatase family. Mol Biol Evol. 2004 Apr;21(4):625-31.