TKT and hyperinsulinemic hypoglycemia, familial, 4: Additionally, defects in glycolysis and the PPP, such as triosephosphate isomerase (TPI) deficiency, ribose 5‐phosphate isomerase (RPI) deficiency, and transketolase (TKT) deficiency, predominantly affect the CNS and are associated with poor therapeutic outcomes due to limited treatment options [5].