High levels of citric acid inhibit the enzyme phosphofructokinase, a key enzyme in glycolysis. Citric acid binds to the enzyme at a different location from the active site. This is an example ofA) competitive inhibitionB) the specificity of enzymes for their substratesC) positive feed back regulationD) allosteric regulation

D) Allosteric regulation

Explanation:

Allosteric regulation is any form of regulation in which a regulatory molecule (an activator or inhibitor) binds to an enzyme somewhere other than the active site. The place where the regulator connects is called an allosteric site.

In glycolysis, phosphofrutokinase (PFK) is one of the enzymes responsible for catalyzing regulatory reactions. The stimulus for glucose metabolism occurs when energy is low, and when it is high, ATP acts as an allosteric inhibitor.

The inhibition of phosphofrutokinase occurs with the increase in ATP, together with the increase in citric acid (consequently reducing pH). Thus, citric acid acts as an allosteric modulator, participating in the allosteric inhibition of PFK.