X-RAY STRUCTURE ANALYSIS OF APOFLAVODOXIN FROM ANABAENA PCC 7119. A. Romero1, C.G. Genzor² and J. Sancho², ¹Instituto de Quimica Fisica Rocasolano. Departamento de, Cristalografia. CSIC. Serrano, 119 E-28006-Madrid, Spain. ²Departamento de Bioquímica y Biología, Molecular y Celular Facultad de Ciencias, Universidad de Zaragoza, E-50009 Zaragoza, Spain

Many biological reactions are catalysed by flavoproteins, a large group of proteins carrying a flavin cofactor (either FMN or FAD). The redox properties of flavoproteins arise from the interaction between the apoprotein and the redox cofactor. Although the structures of many holoflavoproteins are known, there is no single apoflavoprotein of known three-dimensional structure.

We report here the X-ray structure of apoflavodoxin from Anabaena PCC 7119 at 2.0 Å resolution. Apoflavodoxin is a compact, well-folded protein with the same overall fold as holoflavodoxin (1). The transient cavity formed on removal of the FMN cofactor is filled by the indole ring of Trp57 that penetrates into the depression previously occupied by the pyrimidine portion of the isoalloxazine and by the ribityl. The closure of this aromatic gate disorganises the isoalloxazine and ribityl binding sites but leave the phosphate binding site intact. This suggest that the interaction between the FMN and the apoprotein could start at this place.

(1) Rao., S.T., Shaffie, F., Yu, C., Satyshur, K.A., Stockman, B.J., Markey, J.L. & Sundaralingam, M. Protein Science, 1, 1413-1427 (1992)