Which mutated enzymes act as better drugs than their non-mutated form?

Is there an enzyme which is used as a drug (let it be called version $A$) and a mutated version of it ($M$), where the activity of $M$ was found to be higher than $A$?

Any references on such a case would be greatly acknowledged.

-
A possible option: "Site-saturation mutagenesis of Tyr-105 reveals its importance in substrate stabilization and discrimination in TEM-1 β-lactamase" – TMOTTM Feb 28 '13 at 11:45
Another option: "Most Efficient Cocaine Hydrolase Designed by Virtual Screening of Transition States" – TMOTTM Mar 4 '13 at 15:00
A third option: "Why Does the G117H Mutation Considerably Improve the Activity of Human Butyrylcholinesterase against Sarin? Insights from Quantum Mechanical/Molecular Mechanical Free Energy Calculations" – TMOTTM Mar 4 '13 at 15:00
Design of High-Activity Mutants of Human Butyrylcholinesterase against (−)-Cocaine: Structural and Energetic Factors Affecting the Catalytic Efficiency – TMOTTM Mar 4 '13 at 20:17