A catalytic antibody for a multistep Norrish type II photochemical reaction was investigated. Absorption of light energy by alpha-ketoamide substrate Ib produced a high-energy biradical intermediate, that was then directed by the antibody microenvironment to form tetrahydropyrazine 13 with a k(cat) of 1.4 x 10(-3) min(-1) at 280 nm irradiation and an enantiomeric excess of 78%. Antibody-catalyzed reactions performed with radiolabeled substrate indicated that little self-inactivation (6.8 mol % covalent modification after four turnovers per antibody) occurred. The singular product obtained in the antibody-catalyzed reaction was not observed in the uncatalyzed reaction unless the pH was lowered below 4. Studies suggested that the interplay of conformational control and chemical catalysis were responsible for the high specificity. A change in protonation state of the antibody was correlated with the inclusion of a new reaction pathway in the antibody-catalyzed reaction, indicating that general-base catalysis was involved in the rerouting of the Norrish reaction to form 13. An X-ray crystal structure of the substrate was obtained and suggested that the antibody binds the a-ketoamide in a twisted conformation optimal for the first step of the photochemical reaction. The antibody described here is a model for the evolution of light-activated enzymes and can serve as a foundation for the development of light-dependent antibody catalysts for a range of even more complex photochemical reactions.