Re: how does sulfuric acid stop the action of the enzyme catalase?

Eugene,

Enzymes typically function over limited range of pH.  In the case of 
catalase, the optimum pH is approximately pH 7.0.  That is, catalase works 
best at a neutral pH.  If the solultion is too acidic (low pH value) or too 
basic (high pH value) the catalase is inactive - no longer functions as an 
enzyme.  When you add the sulfuric acid to your catalase reactions you 
lower the pH below the range where catalase is functional, and the reaction 
- at least the part catalyzed by catalase - stops.

pH affects proteins by changing the state of dissociable amino acid side 
chains, especially aspartic acid, glutamic acid, lysine, arginine, and 
histidine.  Other groups, such as threonine, serine, and tyrosine, may also 
be affected at very high pH values.  The change in charge on these groups 
can directly affect catalysis (enzyme activity) if the groups are actually 
involved in binding the substrate or in the catalytic site of the enzyme.  
Alternatively, the change in charge of one or more sidechain many lead to a 
structural change in the protein - i.e. how the protein is folded, or 
subunits interact, again abolishing activity.