Question

An allosteric substance influences the activity by
A. Competing for catalytic site
B. Changing the specificity of the enzyme for substance
C. Changing the conformation of the enzyme by binding to site other than catalytic site
D. All of the above.

Answer 1

Hint: The competitive inhibitors are the inhibitors that compete with the substrate for the active sites of enzyme. The non-competitive inhibitors are the inhibitors that bind on the other specific site rather than active sites in enzymes.

Complete answer: The allosteric substances are the examples of regulatory enzymes that increased or decreased the enzymatic activity in response to specific signals. These substances inhibit the enzyme activity through binds upon the specific site at enzymes.
The inhibitors can be termed as modifiers or modulators and the sites at which these allosteric substances are binds, known as allosteric sites. The modulators can be of two types- negative (inhibitors) and positive (activators).
The binding of allosteric substance to the specific site at the enzyme causes conformational changes in the enzymes, that results into the prevention of substrate to the active site of enzymes. It stops the reaction. This process of inhibiting the enzyme action by the binding of allosteric substrate is known as allosteric inhibition.
The common example of inhibition of enzyme action by allosteric substances is hexokinase that converts glucose to glucose 6-phosphate. The glucose 6-phosphate causes the allosteric inhibition of hexokinase when it is produced in excess. This is called feedback allosteric inhibition.

${\text{Glucose }} + {\text{ ATP }}\xrightarrow{{Hexokinase}}\;Glucose\;6 - phosphate + \;ADP$

So, the correct answer is option C, changing the conformation of the enzyme by binding to a site other than catalytic site.

Note: When the modulator is a substance other than substrate, then enzyme is known as heterotopic. The allosteric substances are larger and complex than non-allosteric enzymes with two or more subunits.