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

Allosteric site

The correct answer is option 4. Allosteric site.

Here is an explanation of why 'Y' is called the allosteric site and the roles of the other sites listed:

3. Allosteric Site

An allosteric site is a specific location on an enzyme that is distinct from the active site where substrate binding occurs. Molecules binding to the allosteric site can induce a conformational change in the enzyme's structure. When a compound binds to the allosteric site, it can change the shape of the enzyme, including the active site. This can either enhance (allosteric activation) or inhibit (allosteric inhibition) the enzyme's activity. The change in the enzyme’s shape can make it easier or harder for the substrate to bind to the active site, thus regulating the enzyme's function.

Other Sites:

1. Inactive Site

This is not a standard term used in enzymology. The term could imply a site that does not participate in the enzyme's catalytic activity, but it is not specific or widely recognized in the context of enzyme function.

2. Binding Site

A general term that refers to any location on a molecule (including enzymes) where another molecule, such as a substrate, inhibitor, or activator, can bind. This term is non-specific and could refer to the active site, allosteric site, or other regions on the enzyme. It does not specifically imply the regulatory function seen with allosteric sites.

3. Non-Competitive Site

This term is often used to describe a site on the enzyme where non-competitive inhibitors bind. Non-competitive inhibition involves binding at a site other than the active site, which changes the enzyme's activity. While non-competitive inhibitors can bind to allosteric sites, the term "non-competitive site" is less precise than "allosteric site" because it focuses more on the type of inhibition rather than the specific functional site on the enzyme.

Allosteric site: The correct term for a site on an enzyme where a compound can bind and cause a change in the shape of the active site, thereby regulating the enzyme's activity.

Inactive site, binding site, and non-competitive site: These terms either do not specifically refer to the regulatory function described or are less precise.

Thus, in the context of your question, 'Y' is best described as an allosteric site because the binding of a compound there changes the shape of the active site and consequently the enzyme's activity.