Conversion of an Engineered Potassium-binding Site into a Calcium-selective Site in Cytochrome c Peroxidase*

We have previously shown that the K+ site found in ascorbate peroxidase can be successfully engineered into the closely homologous peroxidase, cytochrome c peroxidase (CCP) (Bonagura, C. A., Sundaramoorthy, M., Pappa, H. S., Patterson, W. R., and Poulos, T. L. (1996) Biochemistry 35, 6107–6115; Bonagura, C. A., Sundaramoorthy, M., Bhaskar, B., and Poulos, T. L. (1999) Biochemistry 38, 5538–5545). All other peroxidases bind Ca2+ rather than K+. Using the K+-binding CCP mutant (CCPK2) as a template protein, together with observations from structural modeling, mutants were designed that should bind Ca2+ selectively. The crystal structure of the first generation mutant, CCPCA1, showed that a smaller cation, perhaps Na+, is bound instead of Ca2+. This is probably because the full eight-ligand coordination sphere did not form owing to a local disordering of one of the essential cation ligands. Based on these observations, a second mutant, CCPCA2, was designed. The crystal structure showed Ca2+ binding in the CCPCA2 mutant and a well ordered cation-binding loop with the full complement of eight protein to cation ligands. Because cation binding to the engineered loop results in diminished CCP activity and destabilization of the essential Trp191 radical as measured by EPR spectroscopy, these measurements can be used as sensitive methods for determining cation-binding selectivity. Both activity and EPR titration studies show that CCPCA2 binds Ca2+ more effectively than K+, demonstrating that an iterative protein engineering-based approach is important in switching protein cation selectivity.