title: High order quaternary arrangement confers increased structural stability to Brucella sp. lumazine synthase.
abstract: 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 quaternary divergence found in different species. The protein exists as pentameric and icosahedral forms, built from practically the same structural monomeric unit. The pentameric structure is formed by five 18-kDa monomers, each extensively contacting neighboring monomers. The icosahedrical structure consists of 60 LS monomers arranged as 12 pentamers giving rise to a capsid exhibiting icosahedral 532 symmetry. In all lumazine synthases studied, the topologically equivalent active sites are located at the interfaces between adjacent subunits in the pentameric modules. The Brucella sp. lumazine synthase (BLS) sequence clearly diverges from pentameric and icosahedric enzymes. This unusual divergence prompted us to further investigate its quaternary arrangement. In the present work, we demonstrate by means of solution light scattering and x-ray structural analyses that BLS assembles as a very stable dimer of pentamers, representing a third category of quaternary assembly for lumazine synthases. We also describe by spectroscopic studies the thermodynamic stability of this oligomeric protein and postulate a mechanism for dissociation/unfolding of this macromolecular assembly. The higher molecular order of BLS increases its stability 20 degrees C compared with pentameric lumazine synthases. The decameric arrangement described in this work highlights the importance of quaternary interactions in the stabilization of proteins.
authors.affiliation: Instituto Leloir, Consejo Nacional de Investigaciones Científicas y Técnicas and Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Patricias Argentinas 435 (C1405BWE), Buenos Aires, Argentina.
authors.forename: Vanesa
authors.initials: V
authors.lastname: Zylberman
authors.forename: Patricio O
authors.initials: PO
authors.lastname: Craig
authors.forename: Sebastián
authors.initials: S
authors.lastname: Klinke
authors.forename: Bradford C
authors.initials: BC
authors.lastname: Braden
authors.forename: Ana
authors.initials: A
authors.lastname: Cauerhff
authors.forename: Fernando A
authors.initials: FA
authors.lastname: Goldbaum
mesh_terms: D001665:Binding Sites; D002002:Brucella / Q000201:enzymology*; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D004795:Enzyme Stability; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008027:Light; D008958:Models, Molecular; D015394:Molecular Structure; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000737:chemistry*; D011489:Protein Denaturation; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D012542:Scattering, Radiation; D013050:Spectrometry, Fluorescence; D013329:Structure-Activity Relationship; D013816:Thermodynamics
chemical_list: D009097:Multienzyme Complexes; C044822:6,7-dimethyl-8-ribityllumazine synthase