CONICET | Buscador de Institutos y Recursos Humanos

The enzyme Lumazine synthase (LS) catalyzes the penultimate step in the biosynthesis of riboflavin (vitamin B2) in plants, fungi and microorganisms. As animals lack the necessary biochemical machinery to synthesize this vitamin, LS and all enzymes involved in the riboflavin pathway are promising targets for the development of new chemotherapeutic agents. To date, crystallographic X-ray structures of 12 different LSs have been solved. According to the species, the particle is found either as single pentamers (~90 kDa), dimers of pentamers (~180 kDa) or dodecamers of pentamers bearing icosahedral 532 symmetry (~1 MDa). Interestingly, the pathogen Brucella and the symbiont Mesorhizobium, two members from the order Rhizobiales in a-proteobacteria, encode two LS isoenzymes named RibH1 (pentameric) and RibH2 (decameric) in their genomes. In this work, we have studied the crystal structure and catalytic properties from RibH1 and RibH2 in both genera. X-ray diffraction data were collected at the protein crystallography beamlines D03B-MX1 at the Laboratório Nacional de Luz Síncrotron (LNLS, Brazil) and X9A at the National Synchrotron Light Source, Brookhaven National Laboratory (NSLS, USA). Key differences observed in the active site architecture that affect substrate recognition have been proposed as the basis for the highly variable enzymatic parameters determined for both isoenzymes, with the RibH2 proteins almost inactive. Additionally, we have extended our analysis to several related genera like Agrobacterium, Mesorhizobium, Sinorhizobium and Rhodopseudomonas by comparing their LS sequences. The question if both isoenzymes are really involved in the biosynthesis of riboflavin or if RibH2 may have evolved to harbor a yet undescribed function is discussed in terms of structural and biochemical approaches.