Relationship between the oligomeric state of Brucella spp. Lumazine synthase and its enzymatic activity. (Poster)

VANESA ZYLBERMAN; PATRICIO O. CRAIG; SEBASTIÁN KLINKE; BRADFORD C. BRADEN; ANA CAUERHFF; FERNANDO A.GOLDBAUM

Angra dos Reis, estado de Río de Janeiro, Brasil

Congreso; 1st Latin American Protein Society Meeting (LAPSM); 2004

Protein Society

The penultimate step in the pathway of riboflavin biosynthesis is catalyzed by the enzyme lumazine synthase (LS). One of the most distinctive characteristics of this enzyme is the structural quatemary divergence found in different species. The protein exists as pentameric and icosahedral forms, built from practically the same structural monomeric unit. In all LS studied, the topologically equivalent active sites are located at the interfaces between adjacent subunits in the pentameric modules. The Brucella abortus LS (BLS) sequence clearly diverges from pentameric and icosahedric enzymes. We have already demonstrated by means of solution Light Scattering and x-ray structural analyses that BLS folds as a very stable dimer of pentamers, representing a third category of quatemary assembly for LS. We have also described a mechanism for dissociation/unfolding of this macromolecular assembly. The dissociation of BLS to intermediate folded pentamers in the range of pH 6.0-4.0 is a reversible process that occurs without significant changes in its secondary structure. However the decameric structure of BLS has an increased binding affinity for the substrate analogue 5-nitro-6-ribitylamino-2,4(1H,3H)pyrimidindion, as compared with the pentameric structure. On the other hand, we detected binding of riboflavin to BLS, a competitive inhibitor described for other LS. Similar to substrate analogue behavior, riboflavin binding also decreased in the pentameric state. We are currentIy analyzing the influence of the quatemary arrangement in the enzymatic activity of BLS. We will also study the biological significance of the new quatemary arrangement: Could the decamer act as a transporter and reservoir of 6,7-dimethyl-8-ribityllumazine?