**Core Concept**
Allosteric enzymes are a type of enzyme that undergoes a conformational change in response to the binding of an effector molecule at a site other than the enzyme's active site. This binding causes a change in the enzyme's activity, either increasing or decreasing its ability to catalyze a reaction.

**Why the Correct Answer is Right**
Allosteric enzymes have two or more binding sites, one of which is the active site where the substrate binds. The binding of an effector molecule at the allosteric site induces a conformational change that affects the enzyme's activity. This can be either cooperative binding, where the binding of one substrate molecule increases the affinity for subsequent substrate molecules, or negative cooperativity, where the binding of one substrate molecule decreases the affinity for subsequent substrate molecules. The Hill coefficient is a measure of the cooperativity of an enzyme.

**Why Each Wrong Option is Incorrect**
**Option A:** Allosteric enzymes are not necessarily substrate-activated. While some allosteric enzymes are activated by substrate binding, others are inhibited by substrate binding.

**Option B:** Allosteric enzymes do not always have a negative allosteric site. While some allosteric enzymes have a negative allosteric site, others have a positive allosteric site that increases the enzyme's activity.

**Option C:** Allosteric enzymes are not always cooperative. While some allosteric enzymes exhibit cooperative binding, others exhibit negative cooperativity.

**Option D:** Allosteric enzymes are not always regulated by pH. While some allosteric enzymes are regulated by pH, others are regulated by other factors such as temperature or ionic strength.

**Clinical Pearl / High-Yield Fact**
Allosteric enzymes are often regulated by feedback inhibition, where the product of the reaction inhibits the enzyme, preventing excessive production of the product. This is a common mechanism of regulation in metabolic pathways.

**Correct Answer: C. Allosteric enzymes do not always have a negative allosteric site. While some allosteric enzymes have a negative allosteric site, others have a positive allosteric site that increases the enzyme's activity.**