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. this growth defect results from a mutation in the gene encoding what enzyme? phosphoglycerate kinase phosphoglycerate mutase pyruvate kinase phophoglucose isomerase (phosphohexose isomerase or glucose-6-phosphate isomerase) enolase