why is it important that enzymes are not changed by the reactions they control?

Answer 1

Enzymes do not control reactions. They participate in chemical reactions as catalysts, lowering the energy required to allow a specific chemical reaction to take place.

As a catalyst, it is important that the enzyme is not changed (ever all) into something else or it would no longer be effective.

Enzymes which react irreversibly with substrates are said to be poisoned. They no longer function. A similar topic on an inorganic catalyst is the "poisoning" of the platinum in a catalytic converter by leaded gasoline. After a while, it is no longer effective.

Unlike inorganic catalysts, enzymes can be biologically renewed. An example is the amylase found in human saliva. This helps conver starch into simple sugars and is the reason why the longer you chew a piece of bread or a cracker, the sweeter it tastes. Once you swallow, the amylase no longer functions in the acid environment of the stomach. The enzyme changes and, in fact, is digested. The digested components are used to make more amylase for future use in the saliva.

Answer 2

So they are not consumed, otherwise it is highly energy inefficient to make an enzyme every time you needed a single reaction done.

Answer 3

if that happens it would be called a suicide enzyime or
suicide substrate

there are many such reactions

the reaction actually stops after that