Phosphoenolpyruvate carboxykinase from the marine diatom Skeletonema costatum and the phaeophyte Laminaria setchellii. I. Isolation and biochemical characterization

Phosphoenolpyruvate carboxykinase (PEPCK) accounts for the bulk of non-phot osynthetic carboxylation processes in the marine diatom Skeletonema costatu m and the kelp Laminaria setchellii. Activity of the carboxylating enzyme p hosphoenolpyruvate carboxylase was undetected in both the diatom and the ke lp. The 53-fold purification of diatom PEPCK through gel-filtration and ion -exchange chromatography yielded a 13% recovery of the overall activity. De naturing gel electrophoresis of diatom PEPCK revealed a single 43 kD polype ptide while the molecular mass of the native PEPCK, determined by gel filtr ation, was 87 kD indicating that the enzyme exists as a homodimer. These re sults suggest a structural divergence of diatom PEPCK to the enzyme from ye ast and vascular plants, The pH and temperature optimum of PEPCK were ident ical in both species, however, maximum activity of the enzyme was 7-fold gr eater in the kelp than in the diatom. The enzyme from both species had an a bsolute requirement for Mn+2 and ADP, however, the K-m, of PEPCK for HCO3- was 9-fold greater in the diatom than in the kelp, indicating a greater CO2 assimilation efficiency in the macrophyte. These results also indicate an endogenic or regulatory difference among PEPCKs from marine chromophytes.