We identify that the antiviral protein kinase PKR alters glycolysis. This response is found to be independent of the kinase’s established control of gene expression that is executed through phosphorylation of the Eukaryotic Translation Initiation Factor 2. Instead, a novel substrate is recognised within the pentose phosphate pathway. The kinase is shown to regulate the activity of an isomerase that produces ribose 5-phosphate, which is an essential precursor for a number of macromolecules, including Coenzyme A, FAD, ATP, and nucleotides.