Gill citrate synthase from an abyssal fish

Abstract

1.1. Citrate synthase, extracted from the gills of an abyssal fish, Antimora rostrata, and purified about 100-fold by ammonium sulfate fractionation and differential absorption to phosphocellulose, displays an unusual polymerization behaviour that encouraged an examination of its catalytic and regulatory properties. Like other vertebrate citrate synthases, the Antimora gill citrate synthase is under feedback inhibitory control by -ketoglutarate, ATP, and GTP.2.2. GTP and ATP are strictly competitive with respect to acetylCoA, but with respect to oxaloacetate, both the Km and the Vmax are reduced. Alpha-ketoglutarate greatly increases the Km for oxaloacetate, but does not affect the Vmax.3.3. AMP, in contrast, is an activator causing a small increase in the maximum velocity and a large increase in enzyme-oxaloacetate affinity.4.4. Although these regulatory properties are unaltered by pressure, the maximum velocity of the enzyme shows a pressure optimum at 200–250 atm, and this pressure activation is most pronounced when at least one of the substrates is at low (Km) levels.5.5. At high substrate levels or at high temperatures, the pressure sensitivity is reduced. In contrast, the Arrhenius activation energy is unaffected by pressure.6.6. These characteristics, coupled with unusually high specific activities, are all consistent with effective in vivo function despite the hostile physical environment of the abyss.