Expression and properties of the mitochondrial and cytosolic forms of fumarase in sunflower cotyledons

Fumarase (EC 4.2.1.2) is encoded in sunflower (Helianthus annuus L.) by two genes (FUM1 and FUM2) expressing correspondingly the mitochondrial and the cytosolic form. Both forms have been purified from sunflower cotyledons and characterized. Three quarters of fumarase activity is located in the mitochondrial and one quarter in the cytosolic fraction. The cytosolic form has lower pH optimum than the mitochondrial form, it possesses higher affinity to malate, activated by Mn2+ and less efficiently by Mg2+ while the mitochondrial form is activated only by Mg2+. It is proposed that the mitochondrial form is involved in the respiratory processes linked to the tricarboxylic acid cycle and the cytosolic form participates in the utilization of succinate produced in the glyoxylate cycle providing the flux to gluconeogenesis in germinating sunflower seeds.