MEMORY AND IMPRINTING EFFECTS IN MULTIENZYME COMPLEXES .2. KINETICS OF THE BIENZYME COMPLEX FROM CHLAMYDOMONAS-REINHARDTII AND HYSTERETIC ACTIVATION OF CHLOROPLAST OXIDIZED PHOSPHORIBULOKINASE

Oxidized, free, stable phosphoribulokinase from Chlamydomonas reinhard tii was almost completely devoid of catalytic activity (0.06 s(-1)/sit e). However, when it was bound to glyceraldehyde-3-phosphate dehydroge nase 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 co mpletely inactive. Its intrinsic kinetic properties (K-m and k(cat)) c hanged with the variation of the overall activity. These effects were paralleled by changes of conformation of the enzyme as revealed by flu orescence analysis. A model is proposed that allows quantitative expre ssion of the dynamics of the dissociation of the oxidized bienzyme com plex and the effects of either of the two substrates, ATP and ribulose 5-phosphate, on this dissociation process. Whereas ATP destabilized t he complex and promoted its dissociation, ribulose 5-phosphate tended to stabilize this complex. Inactive, stable, oxidized phosphoribulokin ase may form a complex with glyceraldehyde-3-phosphate dehydrogenase r egaining its catalytic activity. In this case, glyceraldehyde-3-phosph ate dehydrogenase acts in a manner similar, but not identical to a cha peronin. The information content of the phosphoribulokinase gene, as d efined by the sequence of its base pairs, was therefore not sufficient to specify full enzyme activity. It needed the presence of glyceralde hyde-3-phosphate dehydrogenase to give the oxidized phosphoribulokinas e a conformation competent for its activity. The potential biological significance of these effects remains to be discovered.