Structural and kinetic properties of Lumazine synthase isoenzymes in the order Rhizobiales

KLINKE, S., ZYLBERMAN, V., BONOMI, H. R., HAASE, I., GUIMARÃES, B. G., BRADEN, B. C., BACHER, A., FISCHER, M. AND GOLDBAUM, F. A.

JOURNAL OF MOLECULAR BIOLOGY

ACADEMIC PRESS LTD-ELSEVIER SCIENCE LTD

Año: 2007 vol. 373 p. 664 - 680

0022-2836

6,7-Dimethyl-8-ribityllumazine synthase (lumazine synthase; LS) catalyzesthe penultimate step in the biosynthesis of riboflavin in plants andmicroorganisms. This protein is known to exhibit different quaternaryassemblies between species, existing as free pentamers, decamers (dimers ofpentamers) and icosahedrally arranged dodecamers of pentamers. Aphylogenetic analysis on eubacterial, fungal and plant LSs allowed us toclassify them into two categories: Type I LSs (pentameric or icosahedral)and Type II LSs (decameric).The Rhizobiales represent an order of α-proteobacteria that includes,among others, the genera Mesorhizobium, Agrobacterium and Brucella. Here,we present structural and kinetic studies on several LSs from Rhizobiales.Interestingly, Mesorhizobium and Brucella encode both a Type-I LS and aType-II LS called RibH1 and RibH2, respectively. We show that Type II LSsappear to be almost inactive, whereas Type I LSs present a highly variablecatalytic activity according to the genus. Additionally, we have solved fourRibH1/RibH2 crystallographic structures from the genera Mesorhizobiumand Brucella. The relationship between the active-site architecture andcatalytic properties in these isoenzymes is discussed, and a model thatdescribes the enzymatic behavior is proposed. Furthermore, sequencealignment studies allowed us to extend our results to the genusAgrobacterium. Our results suggest that the selective pressure controllingthe riboflavin pathway favored the evolution of catalysts with low reactionrates, since the excess of flavins in the intracellular pool in Rhizobiales couldact as a negative factor when these bacteria are exposed to oxidative ornitrosative stress.