The stability of acetyl-CoA synthetases (ACS1 and ACS2) from P. blakesleeanus against temperature, urea and trypsin was studied and compared. Thermal inactivation of ACS1 was biphasic, while that of ACS2 was monophasic. The thermodynamic parameters calculated from the inactivation profiles show ACS2 to be a more thermostable enzyme than ACS1. The presence of ATP and Mg(2+) exerted a protective effect on both enzymes, and led to a marked increase in the E(a), DeltaH(not =), DeltaS(not =) and DeltaG(not =) values. ACS2 is also much more stable against denaturation with urea; the estimates of DeltaG(w) (free energy change for protein unfolding at zero denaturant concentration) were 9.4 kJ mol(-1) and 18.1 kJ mol(-1) for ACS1 and ACS2, respectively. Finally, a half-life of 44.5 min for ACS2 versus the 21 min for ACS1 indicates that ACS2 is more stable than ACS1 against digestion by trypsin. These results seem to show that ACS2 is more rigid overall than ACS1, which may be essential for preserving its catalytic activity in the stress situation in which it is expressed.