Subunit structure of aldolase: Chemical and crystallographic evidence

Detailed studies of the reactivity of the sulphydryl groups of aldolase have resulted in evidence for a tetrameric structure of the enzyme. Several distinct classes of sulphydryl groups were found which differed (i) in their reactivities, (ii) in their involvement in the active form of the enzyme and (iii) in that some are protected by substrates while others are not. These studies of the sulphydryl groups also resulted in the preparation of a monoclinic crystalline derivative which contained four equivalents of p-chloromecuribenzoic acid per mole of protein. X-ray diffraction studies on these crystals and on hexagonal bipyramid crystals obtained with native aldolase have provided information concerning the molecular symmetry of the aldolase molecule. In the hexagonal crystal form the molecule is situated on a crystallographic dyad axis which implies that the subunits are at least identical in pairs. In the monoclinic crystals non-crystallographic symmetry has been detected by Patterson and rotation function methods and shows the molecule to be a tetramer with pseudo-222 symmetry. These data provide unambiguous evidence for a four subunit structure of aldolase.