Ground-state destabilization by electrostatic repulsion is not a driving force in orotidine-5′-monophosphate decarboxylase catalysis

The origins of enzyme catalysis have been attributed to both transition-state stabilization as well as ground-state destabilization of the substrate. For the latter paradigm, the enzyme orotidine-5′-monophosphate decarboxylase (OMPDC) serves as a reference system as it contains a negatively charged residue at the active site that is thought to facilitate catalysis by exerting an electrostatic stress on the substrate carboxylate leaving group. Snapshots of how the substrate binds to the active site and interacts with the negative charge have remained elusive. Here we present crystallographic snapshots of human OMPDC in complex with the substrate, substrate analogues, transition-state analogues and product that defy the proposed ground-state destabilization by revealing that the substrate carboxylate is protonated and forms a favourable low-barrier hydrogen bond with a negatively charged residue. The catalytic prowess of OMPDC almost entirely results from the transition-state stabilization by electrostatic interactions of the enzyme with charges spread over the substrate. Our findings bear relevance for the design of (de)carboxylase catalysts.