Metamorphosis of an Enzyme

ABSTRACT

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. Cryo electron microscopy of the IDEA mutant of Aquifex aeolicus LS surprisingly showed large icosahedral 180 subunit capsids (T = 3) with a diameter of ∼ 290 Å. The pentamers in this structure assumed an expanded conformation including a widened central channel. This feature led us to suggest a model for assembly-controlled catalysis, which relates the LS/RS complex, on a microscopic scale, to form and function of a biochemical reactor.

Keywords:

• lumazine synthase
• riboflavin synthase
• protein dissociation and association
• vitamin biosynthesis
• enzyme catalysis
• X-ray crystallography
• protein assembly