Hydrogen-Bond-Assisted\nCatalysis: Hydroxylation of\nPaclitaxel by Human CYP2C8

Paclitaxel (PTX, or Taxol), a chemotherapeutic agent\nwidely employed\nin the treatment of various cancers, undergoes metabolic transformations\nthrough the cytochrome P450 enzymes CYP3A4 and CYP2C8. CYP3A4 catalyzes\nthe aromatic hydroxylation reaction of PTX, whereas CYP2C8 demonstrates\na distinct reactivity pattern, producing 6α-hydroxypaclitaxel\nvia alkane hydroxylation. Despite the significant impact of PTX metabolism\non its anticancer efficacy, the detailed mechanisms underlying these\ntransformations have remained largely unclear. In this study, we employed\nhybrid quantum mechanics and molecular mechanics (QM/MM) calculations\nto elucidate the mechanism of PTX metabolism by human CYP2C8. Our\nQM/MM results reveal that the hydroxylation of PTX by CYP2C8 follows\nan atypical rebound mechanism. Either of the two hydrogen atoms at\nthe C6 position of PTX can be abstracted, leading to a common radical\nintermediate. Although the subsequent rebound barrier is unusually\nhigh, stereochemical scrambling is unlikely, as the rebound barrier\nfor the formation of the 6α-hydroxylated PTXthe actual\nproductis significantly lower than that for the 6β-hydroxylated\nmetabolite. Thus, product selectivity is determined by the non-rate-determining\nrebound step. Furthermore, the hydroxyl group at the C7 position of\nPTX plays a catalytic role by facilitating the hydrogen abstraction\nand rebound steps. Our study also confirms a pronounced stability\nof the transition state in the high-spin sextet spin state, enabled\nby the enzyme’s specific substrate positioning.