Which mutated enzymes act as better drugs than their non-mutated form?
There are mutants of clotting factor VIII that have longer half lives and better activity than natural Factor VIII. Factor VIII is one of the missing proteins commonly involved in hemophilia. Hemophiliacs usually use injections of Factor VIII to prevent bleeding. The mutant form should last longer in the blood, with better activity, resulting in fewer injections.

source: https://encrypted.google.com/patents/US7211559

Since this is a 2007 patent, I don't know if this product is actually being used to treat patients, drug approval takes a long time.