STRUCTURAL ELEMENTS OF PROTON TRANSFER IN MURINE CARBONIC ANHYDRASE V. P. Ann Boriack-Sjodin, Dept. Of Chemistry, University of Pennsylvania, 231 S. 34th Street, Philadelphia, PA 19104-6323

Possible pathways for the proton transfer in the mechanism of murine carbonic anhydrase V have been studied using X-ray crystallographic methods. Carbonic anhydrase V is the most recent isozyme in the carbonic anhydrase family whose structure has been solved using crystallographic techniques¹. The proton acceptor of this enzyme has a pK_a of 9.2². The proton shuttle in the prototypical enzyme carbonic anhydrase II is His 64, however, a proton transfer pathway through Tyr 64 in carbonic anhydrase V is compromised due to the adjacent bulky side chain of Phe 65. A variant of murine carbonic anhydrase V which removes the bulky side chain and mimics the proton shuttle of carbonic anhydrase II, Tyr64His Phe65Ala, is more active than the wild-type enzyme, while a single mutation at position 64 (Tyr64His) does not affect proton transfer². Removing the bulky side chain at position 65 allows a second proton transfer pathway to form with a pK_a of 7 while maintaining the original pathway. Structural analysis of the wild-type enzyme reveals other possible proton transfer groups.

Boriack-Sjodin et al., (1995) Proc. Natl. Acad. Sci. USA 92, 10949-10953.

2 Heck et al., (1994) J. Biol. Chem. 269, 24742-24746.