Abstract
In the fourth lecture, we introduce photoenzymes, a class of enzymes that depend on light to function. More specifically, they catalyze a given reaction only in the presence of a continuous stream of photons whose absorption by appropriate cofactors creates, at each catalytic cycle, reaction intermediates responsible for the transformation carried out by the enzyme. For example, DNA photolyases, essential for DNA repair, use flavins or flavin analogues as chromophores. Gifted with the ability to absorb sunlight, these excited flavins possess remarkable reactivities (oxidizing and/or reducing) enabling them to transform molecules such as thymine dimers, modifications frequently formed in DNA under the effect of various stresses, which the enzyme, the photolyase, thus repairs. Another fascinating example is fatty acid decarboxylase, an enzyme that transforms fatty acids into hydrocarbons by decarboxylation, in the presence of light, thanks once again to a flavin-like chromophore. This enzyme is already used in interesting biotechnological applications : biofuel synthesis, separation of unsaturated fatty acids... All these systems are presented from the point of view of their structure and chemical mechanisms.
