Effect of Phosphate Ion on the Structure of Lumazine Synthase, an Antigen Presentation System From Bacillus anthracis.

Lumazine synthase (LS) is an oligomeric enzyme involved in the biosynthesis of riboflavin in microorganisms, fungi, and plants. LS has become of significant interest to biomedical science because of its critical biological role and attractive structural properties for antigen presentation in vaccines. LS derived from Bacillus anthracis (BaLS) consists of 60 identical subunits forming an icosahedron. Its crystal structure has been solved, but its dynamic conformational properties have not yet been studied. We investigated the conformation of BaLS in response to different stress conditions (e.g., chemical denaturants, pH, and temperature) using a variety of biophysical techniques. The physical basis for these thermal transitions was studied, indicating that a molten globular state was present during chemical unfolding by guanidine HCl. In addition, BaLS showed 2 distinct thermal transitions in phosphate-containing buffers. The first transition was due to the dissociation of phosphate ions from BaLS and the second one came from the dissociation and conformational alteration of its icosahedral structure. A small conformational alteration was induced by the binding/dissociation of phosphate ions to BaLS. This work provides a closer view of the conformational behavior of BaLS and provides important information for the formulation of vaccines which use this protein.