... enzymes after only30 min of incubation, the effect was more pronounced in the case of the L44F mutant. The sharp, singletransition with a Tm of 64.3 °C, in the melting pro-file of unliganded ... the presence of either PRPP or the purine base alone.These melting profiles suggest that the enzyme is desta-bilized not only in the presence of xanthine and PRPP,but also on formation of the ... of the unactivated protein (Fig. 6B). The presence of hypoxanthine and PRPPdestabilizes the protein more than the presence of IMP. Together, these melting profiles clearly indicatethat the thermal...