Catalytic Properties of Two Pyruvate Kinase Isoforms in Nordic Krill, Meganyctiphanes norvegica, With Respect to Seasonal Temperature Adaptation

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Two isoforms of pyruvate kinase (PK I and PK II) were partly purified and characterized from the Nordic krill Meganyctiphanes norvegica. Both PK variants were present in summer and winter specimens with a tissue specificity in abdominal muscle (PK I) and cephalothorax (PK II). Obvious differences were found in chromatographic and kinetic characteristics. Enzymatic adaptations to low temperatures were found in PK I only, whereas PK II did not contribute to seasonal temperature adaptation. In winter specimens, the activation energy of PK I decreased significantly from 53.2 ± 1.5 to 50.2 ± 1.2 kJ·mol−1. The affinity of PK I to phosphoenol-pyruvate was higher in winter (KM = 0.024 ± 0.002 mmol·l−1) compared to summer (KM = 0.033 ± 0.003 mmol·l−1). Both effects lead to an increased efficiency of this enzyme isoform in the cold. In contrast, KM values of PK II showed no significant differences between summer (KM = 0.181 ± 0.014 mmol·l−1) and winter specimens (KM = 0.193 ± 0.015 mmol·l−1). The effects of cooperativity remained unchanged during the seasons with approximate values of nHill = 1.0 (PK I) and 1.5 (PK II). Fructose-1,6-bis-phosphate affected only PK II by shifting sigmoidal kinetics to hyperbolic curves resulting in a decrease of KM to 0.027 mmol·l−1. Further effectors were tested showing an inhibiting effect of oxalate on both isoforms with a reduction to 20% and 50% in PK I and PK II, respectively. Presumably, the ecophysiological effect of the capacity to regulate muscle PK is related to the necessity to increase motility during vertical migration and phases of feeding activity.