Dissociation and in vitro reconstitution of bovine liver uridine diphosphoglucose dehydrogenase. The paired subunit nature of the enzyme

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Abstract
Uridine diphosphoglucose dehydrogenase (EC 1.1.1.22: UDPglucose dehydrogenase) at pH 5.5-7.8 is a stable homohexamer of 305 .+-. 7 kDa that does not undergo concentration-dependent dissociation at enzyme concentrations > 5 .mu.g/mL. Chemical cross-linking of the native enzyme at varying glutaraldehyde concentrations yields dimers, tetramers, and hexamers; at > 2% (w/v) glutaraldehyde, plateau values of 21% monomers, 16% dimers, 5% tetramers, and 58% hexamers are obtained. Dissociation at acid pH (pH 2.3) or in 4-6 M guanidine hydrochloride leads to inactive monomers (Mr 52000). Denaturation at increasing guanidine hydrochloride concentration reveals separable unfolding steps suggesting the typical domain structure of dehydrogenases holds for the present enzyme. At > 4 M guanidine hydrochloride complete randomization of the polypeptide chains is observed after 10-min denaturation. Reconstitution of the native hexamer afte dissociation/denaturation has been monitored by reactivation and glutaraldehyde fixation. The kinetics may be described in terms of a sequential uni-bimolecular model, governed by rate-determining folding and association steps at the monomer level. Trimeric intermediates do not appear in significant amounts. Reactivation is found to parallel hexamer formation. Structural changes during reconstitution (monitored by circular dichroism) are characterized by complex kinetics, indicating the rapid formation of "structured monomers" (with most of the native secondary structure) followed by slow "reshuffling" prior to subunit association. The final product of reconstitution is indistinguishable from the initial native enzyme.