PRPP-synthetase subunit aggregation

HUMAN PHOSPHORIBOSYLPYROPHOSPHATE (PRPP) SYNTHETASE REQUIREMENTS FOR SUBUNIT AGGREGATION... 

We have recently examined the responsiveness to Mg + and MgATP of the subunit aggregates from a superactive form of PRPP synthetase with normal substrate and inhibitor binding properties but an increased maximal reaction velocity. The distribution of forms of the mutant enzyme on sucrose gradient after the dilution and incubation procedures described above was indistinguishable from that of normal PRPP synthetase. In several respects, however, the mutant enzyme behaved in an abnormal fashion. First, under conditions of suboptimal Mg2+... 

Figure 1. Effects of preincubation with Mg2+ and MgATP, singly or in coinbination, on the time course of PRPP synthesis catalyzed by the tetrameric form of PRPP synthetase. Highly purified PRPP synthetase was subjected to sucrose density gradient ultracentrifugation in 1 mM Pi, 1 itiM dithiothreitol (pH 7.4) as previously described. Enzyme sedimenting at 7.1s (corresponding to the 4 subunit aggregated form) was preincubated for 30 minutes at 37 in 1 mM Pi, 1 mM dithiothreitol (pH 7.4) with the additions indicated below. PRPP synthesis was then measured at 26° with final concentrations of 32 mM Pi, 1 mM MgATP,...

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