A novel calcium-dependent bacterial phosphatidylinositol-specific phospholipase C displaying unprecedented magnitudes of thio effect, inverse thio effect, and stereoselectivity.

Understanding the potential range of enzymatic thio effects (kO/kS) is of great value when using sulfur-substituted phosphate analogues to study phosphoryl transfer reactions in enzymes and ribozymes. Herein we report that a newly discovered Ca2+-dependent Streptomyces antibioticus phosphatidylinositol-specific phospholipase C and its mutants display unprecedented magnitudes of thio effect, inverse thio effect, and RP/SP stereoselectivity. We demonstrate that for a single enzyme the bridging thio effect can vary from 0.002 to 20 and the nonbridging thio effect can vary from 1 to 108. These values fall outside the range of those reported for nonenzymatic reactions, emphasizing the need for cautious interpretation when using thio effects to elucidate details of enzyme catalysis.