METAMORPHOSIS OF AN ENZYME

The protein shells of the bifunctional Lumazine/Riboflavin synthase complex found in bacteria, archaea and plants show some similarity to the assembly of small spherical viruses. Sixty lumazine synthase subunits form a T = 1 icosahedral capsid, which instead of nucleic acids in the central core, contains a trimer of riboflavin synthase. Lumazine synthases from fungi, yeasts and some bacteria, however, exist only in pentameric form. Capsid formation in icosahedral lumazine synthases is dependent on the presence of certain substrate-analogous ligands, on pH and phosphate concentration. The experimental background from X-ray crystallography, X-ray small angle scattering and electron microscopy will be discussed. Different active assemblies of the enzyme are observed in vivo and in vitro. There is experimental evidence for the formation of large capsids, obtained spontaneously or after certain mutations to the sequence of the lumazine synthase subunit. Those presumably metastable T = 3 capsids can be reassembled into T = 1 capsids by ligand-driven reassembly in vitro. The active site of lumazine synthase is relatively resilient to point mutations. One lethal mutation to the binding site for the phosphate-substrate, however, has a strong influence on both, capsid stability and enzymatic activity.