FH and hyperinsulinemic hypoglycemia, familial, 4: In addition to the abovementioned examples, many other metabolic enzymes in the tricarboxylic acid cycle are also substrates of O-GlcNAcylation, among which fumarate hydratase (FH) is O-GlcNAcylated at the Ser-75 residue, the same site as AMPK-mediated phosphorylation, and the O-GlcNAcylation of this site maintains tumor growth under glucose deficiency in pancreatic cancer patients (30).