Allosteric enzymes are enzymes that have allosteric sites, which are specific regions on the enzyme molecule that are distinct from the active site. These allosteric sites play a crucial role in regulating the activity of the enzyme. The correct answer to the question is option 'D', which states that allosteric enzymes have allosteric sites for both activation and inhibition. Let's explore this in more detail.

Explanation:
- Allosteric sites and their significance:
- Allosteric sites are regions on the enzyme molecule that can bind to specific molecules called allosteric modulators.
- These allosteric modulators can be either activators or inhibitors, and their binding to the allosteric site can regulate the enzyme's activity.
- The binding of an activator to the allosteric site can enhance the enzyme's activity, while the binding of an inhibitor can decrease or completely inhibit the enzyme's activity.
- This regulation allows the cell to fine-tune enzyme activity in response to changing metabolic needs.

- Allosteric activation:
- Allosteric activation occurs when an activator molecule binds to the allosteric site of an enzyme.
- The binding of the activator induces a conformational change in the enzyme, resulting in an increase in the enzyme's catalytic activity.
- This conformational change can enhance the enzyme's ability to bind to its substrate or increase the efficiency of the catalytic reaction.
- Allosteric activation is an important regulatory mechanism that allows the cell to increase enzyme activity when needed.

- Allosteric inhibition:
- Allosteric inhibition occurs when an inhibitor molecule binds to the allosteric site of an enzyme.
- The binding of the inhibitor induces a conformational change in the enzyme, leading to a decrease in the enzyme's catalytic activity.
- This conformational change can decrease the enzyme's affinity for its substrate or inhibit the catalytic reaction.
- Allosteric inhibition is another crucial regulatory mechanism that allows the cell to decrease enzyme activity when necessary.

- Dual role of allosteric sites:
- Allosteric enzymes have allosteric sites that can accommodate both activators and inhibitors.
- This means that the same allosteric site can bind to different molecules, depending on the metabolic needs of the cell.
- The binding of an activator or inhibitor to the allosteric site induces a specific conformational change in the enzyme, resulting in either activation or inhibition of the enzyme's activity.
- The presence of allosteric sites allows allosteric enzymes to respond to changes in the cellular environment and maintain optimal enzyme activity.

In conclusion, allosteric enzymes have allosteric sites that can bind to both activators and inhibitors. This dual role of allosteric sites allows the enzymes to be regulated and fine-tuned in response to changing metabolic needs. The binding of an activator to the allosteric site enhances the enzyme's activity, whereas the binding of an inhibitor decreases or inhibits the enzyme's activity. Allosteric regulation is an essential mechanism that ensures proper enzymatic activity in cells.