**Core Concept**
Phosphofructokinase is an enzyme in the glycolytic pathway that catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate. It is an allosteric enzyme, meaning its activity can be modulated by binding of molecules at sites other than the active site. This allosteric regulation allows for fine-tuning of glycolysis in response to cellular energy needs.

**Why the Correct Answer is Right**
Phosphofructokinase is an allosteric enzyme because it has a regulatory site that binds ATP, which acts as an inhibitor, and AMP, which acts as an activator. When cellular energy is high, ATP binds to the regulatory site, reducing phosphofructokinase activity. Conversely, when cellular energy is low, AMP binds, increasing phosphofructokinase activity and promoting glycolysis. This allosteric regulation is crucial for maintaining energy homeostasis in cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Transferrin is a transport protein that binds iron, but it is not an allosteric enzyme. It does not undergo significant conformational changes in response to binding of its ligand.
**Option B:** Ceruloplasmin is an enzyme involved in copper metabolism, but it is not an allosteric enzyme. It does not have a regulatory site that modulates its activity in response to cellular needs.
**Option D:** Haemoglobin is a protein that transports oxygen, but it is not typically classified as an allosteric enzyme. While it does undergo conformational changes in response to oxygen binding, this is not the same as allosteric regulation of enzyme activity.

**Clinical Pearl / High-Yield Fact**
Phosphofructokinase deficiency is a rare genetic disorder that affects muscle and red blood cells, leading to exercise intolerance and other symptoms. Understanding the allosteric regulation of phosphofructokinase is crucial for appreciating the pathophysiology of this disorder.

**Correct Answer:** C. Phosphofructokinase