Acetyl-coenzyme a carboxylase from the marine prymnesiophyte isochrysis galbana

Acetyl-CoA carboxylase [Acetyl-CoA carbon dioxide ligase (ADP forming), EC 6.4.1.2] was purified to an apparent homogeneity from the marine prymnesiophyte Isochrysis galbana. The purification steps included ammonium sulphate precipitation sucrose density gradient and monomeric avidin affinity chromatography. The enzyme recovery was less than 10% with 300 fold purification. The molecular mass of the native enzyme was estimated at 700 kDa by elution from a calibrated Sepharose 6B column. Polyacrylamide gel electrophoresis containing sodium dedocyl sulphate revealed a single subunit of 160 kDa that contained biotin. Investigation of the kinetic properties of the purified enzyme indicated K_m for acetyl CoA, NaHCO₃ and ATP of about 310, 910 and 51 μM respectively. Malonyl CoA and palmitoyl CoA were found as highly potent inhibiting metabolites. These metabolites competitively inhibited the enzyme with respect to acetyl CoA. Polyclonal antibodies prepared against the subunit of the purified enzyme effectively inhibited the enzyme activity in an in vitro system. The prepared antibodies used in an immunoblot system specifically bound to acetyl CoA carboxylase subunit of I. galbana. These antibodies failed to react with a corresponding subunit of the enzyme from a diatom, indicating a poor immunological conservation of that protein among algal classes.