Lumazine synthase catalyzes the penultimate step
in the synthesis of riboflavin in plants, fungi, and microorganisms.
The enzyme displays two quaternary structures, the pentameric
forms in yeast and fungi and the 60-meric icosahedral capsids
in plants and bacteria. To elucidate the structural features
that might be responsible for differences in assembly,
we have determined the crystal structures of lumazine synthase,
complexed with the inhibitor 5-nitroso-6-ribitylamino-2,4-pyrimidinedione,
from spinach and the fungus Magnaporthe grisea
to 3.3 and 3.1 Å resolution, respectively. The overall
structure of the subunit and the mode of inhibitor binding
are very similar in these enzyme species. The core of the
subunit consists of a four-stranded parallel β-sheet
sandwiched between two helices on one side and three helices
on the other. The packing of the five subunits in the pentameric
M. grisea lumazine synthase is very similar to
the packing in the pentameric substructures in the icosahedral
capsid of the plant enzyme. Two structural features can
be correlated to the differences in assembly. In the plant
enzyme, the N-terminal β-strand interacts with the
β-sheet of the adjacent subunit, thus extending the
sheet from four to five strands. In fungal lumazine synthase,
an insertion of two residues after strand β1 results
in a completely different orientation of this part of the
polypeptide chain and this conformational difference prevents
proper packing of the subunits at the trimer interface
in the icosahedron. In the spinach enzyme, the β-hairpin
connecting helices α4 and α5 participates in the
packing at the trimer interface of the icosahedron. Another
insertion of two residues at this position of the polypeptide
chain in the fungal enzyme disrupts the hydrogen bonding
in the hairpin, and the resulting change in conformation
of this loop also interferes with proper intrasubunit contacts
at the trimer interface.