The PBGS quaternary structure equilibrium includes an inactive hexamer (PDB id 1PV8) that does not have subunit interactions necessary for an ordered active site lid. Dissociation to the pro-hexamer dimer can be followed by a conformational change that reorients the two alpha/beta-barrel domains to form the pro-octamer dimer. Association of pro-octamer dimer to octamer (PDB id 1E51) includes formation of subunit interfaces that support order in the active site lid.