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 activation of that enzyme. Saccharomyces cerevisiae (a yeast species) mutants are available that grow on pyruvate or ethanol, but not sugars or glycerol. Which of the following glycolytic enzymes is most likely affected?

1. triose phosphate isomerase

2. aldolase -phophoglucose isomerase (phosphohexose isomerase or glucose-6-phosphate isomerase)

3. hexokinase

4. pyruvate kinase

Yeasts convert glycerol and sugars into glyceraldehyde 3-phosphate (G3P) through independent pathways. Then, G3P forms pyruvate and, in some circumstances, pyruvate is converted in ethanol, which can be used as energy sources. If the mutation affects any reaction before G3P formation, it will only affect yeast growing either on sugar or pyruvate but not both.

Pyruvate kinase is the only enzyme on the list acting after G3P is formed and before pyruvate is formed. All other options are enzymes acting only in the formation of G3P from sugars. Meaning that only pyruvate kinase mutants will lack the ability to grow on both sugars and glycerol.