Phosphofructokinase from the overwintering gall fly larva, Eurosta solidaginis: Control of cryoprotant polyol synthesis
Kinetic and regulatory properties of phosphofructokinase (PFK) from the larvae of the gall fly, Eurosta solidaginis, are strongly affected by temperature. Arrhenius plots show an activation energy of 19,804 ± 1330 cal/mole and a very high Q10 of 3.64 (between 10 and 0°C). Enzyme affinity for ATP and Mg2+ showed a slight positive modulation with decreasing assay temperature but enzyme affinity for fructose-6-P was strongly negatively modulated, S0.5 for fructose-6-P rising from 4.4 ± 0.5 mM at 25°C to 7.7 ± 0.7 mM at 5°C. Metabolite modulators of PFK showed differential effects on enzyme affinity for fructose-6-P at high vs low temperature. AMP (0.05 mM) activation produced an 8.2 fold decrease in S0.5 for fructose-6-P at 25°C but only a 3.3 fold decrease in S0.5 at 5°C. Two enzyme inhibitors, glycerol-3-P and sorbitol, however, showed stronger effects at 5°C compared to 25°C. At physiological levels of both inhibitors, 4mM glycerol-3-P and 250 mM sorbitol, S0.5 for fructose-6-P was increased to 14.1 mM at 25°C and to 22.3 mM at 5°C; AMP only partially reversed these inhibitory effects. Control at the PFK locus appears to be responsible for the differential, temperature dependent synthesis of glycerol and sorbitol as cryoprotectants in E. solidaginis. Low temperature inactivation of PFK by direct temperature effects on enzyme fructose-6-P affinity and regulation by modulators as well as increased levels of inhibitors, glycerol-3-P and sorbitol, in vivo at low temperatures could result in a severe restriction of flux through the PFK locus at low temperature blocking low temperature glycerol synthesis and potentiating sorbitol production.
|Keywords||Eurosta solidaginis, low temperature enzyme regulation, overwintering, Phosphofructokinase, polyol synthesis|
Storey, K. (1982). Phosphofructokinase from the overwintering gall fly larva, Eurosta solidaginis: Control of cryoprotant polyol synthesis. Insect Biochemistry, 12(5), 501–505. doi:10.1016/0020-1790(82)90018-X