Saccharomyces cerevisiae (a yeast species) mutants are available that grow on pyruvate or ethanol, but not sugars or glycerol.
This growth defect results from a mutation in the gene encoding what enzyme?

The enzyme is the pyruvate kinase

Explanation:

The designated enzyme can only grow in pyruvate or in ethanol. They are said to be in anaerobic conditions. If the end product of glycolysis is pyruvate and the enzyme that transforms phosphoenol pyruvate into pyruvate is called pyruvate kinase. In a reaction under anaerobic conditions, the pyruvate is transformed into ethanol. So if the pyruvate kinase enzyme is mutated, pyruvate cannot be transformed into ethanol and Saccharomyces cerevisiae could not survive in that medium.