A certain compound 'X' occupied a site of an enzyme exactly opposite to that of the active site. This immediately resulted in the change of shape of the active site. What is 'X' called?

non-competitive inhibitor

The correct answer is option 2. non-competitive inhibitor.

Non-competitive inhibitor: This type of inhibitor binds to a site on the enzyme that is not the active site, often referred to as the allosteric site. When a non-competitive inhibitor binds to the allosteric site, it induces a conformational change in the enzyme that alters the shape of the active site. This change can decrease the enzyme's activity by making it less effective at binding to the substrate, even if the substrate concentration is high.

Other Options:

1. Competitive inhibitor:

Function: A competitive inhibitor competes with the substrate for binding to the active site of the enzyme. It does not bind to a different site on the enzyme and does not cause a change in the shape of the active site.

Incorrect for 'X': Because 'X' binds to a site other than the active site and induces a shape change, it does not fit the definition of a competitive inhibitor.

3. Competitive messenger:

Definition: This term is not standard in the context of enzyme inhibition or regulation. It does not describe a known mechanism of enzyme activity modification.

Incorrect for 'X': Because it is not a recognized term for the function described.

4. Receptor:

Function: A receptor is typically a protein molecule that receives and responds to a specific neurotransmitter, hormone, or other substance. While receptors can influence cell behavior, they are not typically described as altering the active site of enzymes directly in this context.

Incorrect for 'X': Because 'X' is specifically altering enzyme activity by changing the shape of the active site, which fits the description of a non-competitive inhibitor rather than a receptor.

'X' is best described as a non-competitive inhibitor because it binds to a site on the enzyme (not the active site) and causes a conformational change that affects the shape and function of the active site, thereby inhibiting the enzyme's activity.