Mechanistic Interrogation of a PQQ and Rare Earth-Dependent Artificial Metalloenzyme

Pyrroloquinoline quinone (PQQ)-binding proteins are found in diverse species and play key roles in the central metabolism of many methylotrophic bacteria, acting as redox-active cofactors in their alcohol dehydrogenase (ADH) enzymes. These enzymes also require a Lewis acidic metal ion to activate PQQ, and the 2011 discovery of lanthanide (Ln³⁺)-dependent ADH enzymes sparked a surge of interest in understanding their functional properties. However, key questions remain regarding the mechanism, metal ion-dependence, and electron transport processes of these enzymes. We report mechanistic, structural, and computational studies on an artificial metalloenzyme (ArM) containing a biomimetic active site that binds Ln³⁺, PQQ, and catalyzes benzyl alcohol dehydrogenation. These studies provide insights into the potential structure-function relationships present in natural MDHs. Examining the relative reactivities of substituted benzyl alcohol substrates revealed a kinetic isotope effect of 2.9 ± 0.4 and a linear free energy relationship consistent with one of the two mechanistic pathways widely proposed to operate in ADHs. Preparing ArMs with metal ions spanning the rare earth series, we observed decreasing reactivity with increasing Lewis acidity, a pattern consistent with that of natural ADH assays. In contrast to patterns observed in natural ADH assays, addition of ammonia had no effect on catalysis. Finally, investigating the role of a conserved active site residue through X-ray diffraction and molecular dynamics simulations, revealed a PQQ/substrate access channel critically regulated by this site. Together, these studies bear new insights into the mechanism, metal ion-dependence, and conformational dynamics associated with PQQ and rare earth-dependent enzymes.