Four amino acids define the CO binding pocket of enoyl-CoA carboxylases/reductases.

Carboxylases are biocatalysts that capture and convert carbon dioxide (CO) under mild conditions and atmospheric concentrations at a scale of more than 400 Gt annually. However, how these enzymes bind and control the gaseous CO molecule during catalysis is only poorly understood. One of the most efficient classes of carboxylating enzymes are enoyl-CoA carboxylases/reductases (Ecrs), which outcompete the plant enzyme RuBisCO in catalytic efficiency and fidelity by more than an order of magnitude. Here we investigated the interactions of CO within the active site of Ecr from Combining experimental biochemistry, protein crystallography, and advanced computer simulations we show that 4 amino acids, N81, F170, E171, and H365, are required to create a highly efficient CO-fixing enzyme. Together, these 4 residues anchor and position the CO molecule for the attack by a reactive enolate created during the catalytic cycle. Notably, a highly ordered water molecule plays an important role in an active site that is otherwise carefully shielded from water, which is detrimental to CO fixation. Altogether, our study reveals unprecedented molecular details of selective CO binding and C-C-bond formation during the catalytic cycle of nature's most efficient CO-fixing enzyme. This knowledge provides the basis for the future development of catalytic frameworks for the capture and conversion of CO in biology and chemistry.