Crystallographic and enzymatic studies on Lumazine synthases RibH1 and RibH2 from Brucella spp. (Comunicación oral)

SEBASTIÁN KLINKE; VANESA ZYLBERMAN; HERNÁN R. BONOMI; BEATRIZ G. GUIMARAES; BRADFORD C. BRADEN; FERNANDO A.GOLDBAUM

Rosario, Sta. Fé, Argentina

Congreso; XLII Reunión anual de SAIB (Sociedad Argentina de Investigación en Bioquímica y Biología Molecular); 2006

SAIB

Lumazine Synthase (LS) catalyses the penultimate step in the biosynthesis of riboflavin. It is found in plants, fungi and microorganisms showing different quaternary assemblies. The enzyme can exist as free pentamers in solution (~90 kDa), as well as decamers (~180 kDa) and dodecamers of pentamers (~1 MDa). Brucella spp, the bacteria that cause the disease brucellosis, express RibH1 and RibH2, two proteins that bear sequence and structural homology with known LSs. In this work we present the three-dimensional X-ray crystallographic structure of both enzymes bound to a substrate analogue inhibitor, together with their enzymatic parameters. Results lead to the hypothesis that both proteins belong to different LSs families: RibH1 (pentameric) is a member of Type I eubacterial LSs, which are present in all organisms that synthesize riboflavin, whereas RibH2 (decameric) is associated with Type II eubacterial LSs, a group with unusual quaternary structure and a very high Km for one substrate of the reaction. This work is complemented with crystallographic and enzymatic analyses on RibH2 from Mesorhizobium loti, an á2-proteobacterium related to Brucella, as well as the comparison of the 3D-structures of RibH1 from two close Brucella species, namely B. abortus and B. melitensis. Overall, this analysis shows that Type II eubacterial LSs may have evolved for a different yet unknown function.