Scientists can use mutants to study metabolic pathways. These organisms have a mutation in a gene encoding a metabolic pathway enzyme that results in the inactivation of that enzyme. Saccharomyces cerevisiae (a yeast species) mutants are available that grow on fructose, but not glucose. Which of the following glycolytic enzymes is mostly likely affected?

(A) phosphoglycerate kinase
(B) enolase
(C) pyruvate kinase
(D) phostphoglycerate mutase
(E) Phosphoglucose isomerase (phosphohexose isomerase or glucose-6-phosphate isomerase)

(E) Phosphoglucose isomerase (phosphohexose isomerase or glucose-6-phosphate isomerase)

Explanation:

Glyocolysis is break down of glucose into pyruvate.

Isomerization:The second step of glycolysis involves the conversion of glucose-6-phosphate to fructose-6-phosphate (F6P). This reaction occurs with the help of the enzyme phosphoglucose isomerase (PI). As the name of the enzyme suggests, this reaction involves an isomerization reaction.

Yeast mutant lacking phopshoglucose isomerase unable to performed this step and that's why it unable to grow in media containing glucose while in fructose break down this step is not required so it is able to grow in media containing fructose.

Deletion of  phosphoglucose isomerase, prevents growth on glucose; therefore, phosphoglucose isomerase mutant is able to  grown in a fructose medium.