Enzymes associated with carbohydrate metabolism of scyphistomae of Aurelia aurita and Chrysaora quinquecirrha (Scyphozoa: semaeostomae)

1. 1. A functional tricarboxylic acid cycle in the scyphistomae of Aurelia aurita and Chrysaora quinquecirrha is indicated by the presence of NADP⁺-linked isocitrate dehydrogenase (IDH, E.C. 1.1.1.42), succinate dehydrogenase (SDH, E.C. 1.3.99.1), and malate dehydrogenase (MDH,E.C. 1.1.1.37). 2. 2. These scyphistomae are potentially capable of survival under low oxygen conditions by utilizing phosphoenol pyruvate carboxykinase (PEPCK, E.C. 4.1.1.32), MDH, and SDH in the absence of lactate dehydrogenase. 3. 3. The Michaelis constants (K_m), energies of activation (E_a), and temperature dependence are reported for several enzymes. 4. 4. The scyphistomae cultured at lower temperature always have a higher substrate enzyme affinity (lower K_m) and lower E_a than organisms maintained at room temperature. The reduction in velocity of enzymatic reaction with decreased temperature may be partially, or, sometimes fully, offset by a decrease of K_m and E_a. 5. 5. The pH dependence of pyruvate kinase (PK, E.C. 2.7.1.40), PEPCK, and MDH is discussed in relation to possible pH control at the branch point of phosphoenol pyruvate.