Phosphoenolpyruvate carboxykinase (PEPCK)
Phosphoenolpyruvate carboxykinase isolated from glycosomes from procyclics (Hunt and Kohler, 1995) is specific for ATP rather than GTP, as are the mammalian homologues. The enzyme is a homodimer with a subunit molecular mass of 59 kDa. Studies on the corresponding gene have shown that the protein has 525 amino acids and ends in a tripeptide (SRL) which is a potential targeting signal for import into the glycosome (Sommer et al., 1994). Like other glycosomal proteins, the enzyme has a high isoelectric point (pH 8.9). It is strictly dependent on adenosine nucleotides for activity, as well as on the presence of Mn2+. Quinolinic acid, a structural analogue of oxaloacetate, completely inhibits the decarboxylation reaction at a 1 mM concentration. Kinetic data suggest that under physiological conditions PEPCK in T. brucei is bidirectional and that its activity is regulated primarily by mass action.