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Phosphoenolpyruvate carboxykinase (PEPCK) accounts for the bulk of non-photosynthetic carboxylation processes in the marine diatom  Skeletonema costatum  and the kelp  Laminaria setchellii . Activity of the carboxylating enzyme phosphoenolpyruvate carboxylase was undetected in both the diatom and the kelp. The 53-fold purification of diatom PEPCK through gel-filtration and ion-exchange chromatography yielded a 13% recovery of the overall activity. Denaturing gel electrophoresis of diatom PEPCK revealed a single 43 kD polypeptide while the molecular mass of the native PEPCK, determined by gel filtration, was 87 kD indicating that the enzyme exists as a homodimer. These results suggest a structural divergence of diatom PEPCK to the enzyme from yeast and vascular plants. The pH and temperature optimum of PEPCK were identical in both species, however, maximum activity of the enzyme was 7-fold greater in the kelp than in the diatom. The enzyme from both species had an absolute requirement for Mn +2  and ADP, however, the K m  of PEPCK for HCO 3  −  was 9-fold greater in the diatom than in the kelp, indicating a greater CO 2  assimilation efficiency in the macrophyte. These results also indicate an endogenic or regulatory difference among PEPCKs from marine chromophytes.

botanica marina

Phosphoenolpyruvate Carboxykinase from the Marine Diatom  Skeletonema costatum  and the Phaeophyte  Laminaria setchellii . I. Isolation and Biochemical Characterization