Abstract
Acutolysin A isolated from the venom of Agkistrodon acutus is a protein of 22 kDa with marked haemorrhagic and proteolytic activities. The metal ions‐ and pH‐induced conformational changes of acutolysin A have been studied by following fluorescence and activity measurements. Here, we provide evidence for the fact that native holo‐acutolysin A adopts two subtly different conformations, native state a (Na) stable in the weak acidic pH range from 6.0 to 7.0 with low activity and native state b (Nb) stable in the weak alkaline pH range from 7.5 to 9.0 with high activity. Holo‐acutolysin A has an optimum pH of 8.5 for caseinolytic activity, and the protein adopts the most stable conformation with the maximum fluorescence at pH 8.5. The Ca^2+^ and Zn^2+^ ions have significant effects on both the pH‐induced denaturing transition curve and the pH‐dependent activity curve. Addition of 1 m__M__ Ca^2+^ to holo‐acutolysin A shifts both the acid‐induced denaturing transition curve and the end zone of acid‐induced inactivation curve towards lower pH value, and shifts both the alkali‐induced denaturing transition curve and the end zone of alkali‐induced inactivation curve towards higher pH value. Addition of 1 m__M__ Zn^2+^ also shifts both the alkali‐induced denaturing transition curve and the end zone of alkali‐induced inactivation curve towards higher pH value and shifts the acid‐induced denaturing transition curve to lower pH value, but has little effect on the acid‐induced inactivation. Removal of Ca^2+^ and Zn^2+^ from the protein enhances its sensitivity to pH and significantly reduces its overall stability during acid‐induced denaturation. It is also evident from the present work that the free Zn^2+^‐induced inactivation in the pH range from 8.0 to 9.0 should be attributed to the effect of Zn(OH)~2~ precipitation on the protein. © 2006 Wiley Periodicals, Inc. Biopolymers 85: 81–90, 2007.
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