Memory and Imprinting Effects in Multienzyme Complexes

Abstract

Oxidized, free, stable phosphoribuiokinase from Chlamydomonas reinhardtii was almost completely devoid of catalytic activity (0.06 s −1 site). However, when it was bound to glyceraldehyde‐3‐phosphate dehydrogenase from the same organism, it displayed significant activity (3.25 s −1 /site). Moreover, this complex tended to spontaneously dissociate upon dilution; the isolated phosphoribulokinase activity increased up to 56 s −1 site, subsequently decreased, and finally became almost completely inactive. Its intrinsic kinetic properties ( K m and k cat ) changed with the variation of the overall activity. These effects were paralleled by changes of conformation of the enzyme as revealed by fluorescence analysis. A model is proposed that allows quantitative expression of the dynamics of the dissociation of the oxidized bienzyme complex and the effects of either of the two substrates, ATP and ribulose 5‐phosphate, on this dissociation process. Whereas ATP destabilized the complex and promoted its dissociation, ribulose 5‐phosphate tended to stabilize this complex. Inactive, stable, oxidized phosphoribulokinase may form a complex with glyceraldehyde‐3‐phosphate dehydrogenase regaining its catalytic activity. In this case, glyceraldehyde‐3‐phosphate dehydrogenase acts in a manner similar, but not identical to a chaperonin. The information content of the phosphoribulokinase gene, as defined by the sequence of its base pairs, was therefore not sufficient to specify full enzyme activity. It needed the presence of glyceraldehyde‐3‐phosphate dehydrogenase to give the oxidized phosphoribulokinase a conformation competent for its activity.The potential biological significance of these effects remains to be discovered.