Enzymes are long established as extremely efficient catalysts. Here, we show that enzymes can also be extremely efficient electrocatalysts (catalysts of redox reactions at electrodes). Despite being large and electronically insulating through most of their volume, some enzymes, when attached to an electrode, catalyze electrochemical reactions that are otherwise extremely sluggish (even with the best synthetic catalysts) and require a large overpotential to achieve a useful rate. These enzymes produce high electrocatalytic currents, displayed in single bidirectional voltammetric waves that switch direction (between oxidation and reduction) sharply at the equilibrium potential for the substrate redox couple. Notoriously irreversible processes such as CO(2) reduction are thereby rendered electrochemically reversible--a consequence of molecular evolution responding to stringent biological drivers for thermodynamic efficiency. Enzymes thus set high standards for the catalysts of future energy technologies.