Photosynthetic carbon assimilation in the blue-green alga Coccochloris peniocystis

Abstract. Cells of the blue-green alga Coccochloris peniocystis , grown at air levels of CO₂, were exposed to [^l4C]bicarbonate in the light for periods of 0.5 to 2.0 s followed by exposure to unlabelled bicarbonate for longer periods of time in the light. The kinetics of tracer movement during these pulse-chase experiments demonstrate that the principal mechanism of CO₂ fixation in this alga is the C₃-pathway although an appreciable amount of the C₄ acid aspartate is found as one of the initial products of photosynthesis. Degradation of the labelled aspartate revealed that after 20 s of illumination, over 95% of the radioactivity was located in the β-carboxyl of this C₄ acid. This alga possesses little, if any, capacity for either the enzymatic decarboxylation of C₄ acids or the regeneration of phosphoenolpyruvate (PEP) from pyruvate mediated by the enzyme pyruvate, P_i dikinase. These data further demonstrate the lack of a functional C₄-pathway in this alga.     

Characterization of a malate dehydrogenase in the cyanobacterium Coccochloris peniocystis

A malate dehydrogenase (MDH) was characterized from the cyanobacterium Coccochloris peniocystis. The enzyme was purified approximately 180-fold and had a molecular weight of about 90000. The enzyme had a pH optimum of pH 6.7 to 7.5; a K_m (malate) of 5.6 mM and K_ms for NAD and NADP of 24 M and 178 M, respectively, although similar V_max were obtained with either pyridine nucleotide. Enzyme activity was inhibited by ATP, citrate, oxalacetate, acetyl CoA and CoA. Enzyme assays with uniformly ¹⁴C-labelled malate caused no ¹⁴CO₂ release, indicating this MDH is not a malic enzyme. Electrophoresis and S-200 gel filtration of the partially purified enzyme indicated a single MDH was present in this preparation. A second, less abundant, MDH was present in crude extracts. The presence of MDH in this organism is consistent with the operation of a glyoxylate cycle which, in the absence of a TCA cycle, would provide organic acids required in secondary carbon metabolism. ATP inhibition of MDH may allow for light regulation of MDH activity since, in the light, oxaloacetic acid is generated by phosphoenolpyruvate carboxylase activity.Abbreviations MDH malate dehydrogenase - PEPcase phosphoenolpyruvate carboxylase - MOPS 3-[N-Morpholino] propane sulfonic acid - TRIS Tris(hydroxymethyl)-aminomethane - EDTA Disodium Ethylenadiamine Tetraacetate - MES 2[N-Morpholino]-ethane Sulfonic Acid - EPPS N-2-Hydroxyethylpiperazine Propane - MW Molecular weight - OAA Oxaloacetic acid     

Formation and metabolism of glycolate in the cyanobacterium Coccochloris peniocystis

The formation and metabolism of glycolate in the cyanobacterium Coccochloris peniocystis was investigated and the activities of enzymes of glycolate metabolism assayed. Photosynthetic ¹⁴CO₂ incorporation was O₂ insensitive and no labelled glycolate could be detected in cells incubated at 2 and 21% O₂. Under conditions of 100% O₂ glycolate comprised less than 1% of the acid-stable products indicating ribulose 1,5 bisphosphate (RuBP) oxidation only occurs under conditions of extreme O₂ stress. Metabolism of [1-¹⁴C] glycolate indicated that as much as 62% of ¹⁴C metabolized was released as ¹⁴CO₂ in the dark. Metabolism of labelled glycolate, particularly incorporation of ¹⁴C into glycine, was inhibited by the amino-transferase inhibitor amino-oxyacetate. Metabolism of [2-¹⁴C] glycine was not inhibited by the serine hydroxymethyltransferase inhibitor isonicotinic acid hydrazide and little or no labelled serine was detected as a result of ¹⁴C-glycolate metabolism. These findings indicate that a significant amount of metabolized glycolate is totally oxidized to CO₂ via formate. The remainder is converted to glycine or metabolized via a glyoxylate cycle. The conversion of glycine to serine contributes little to glycolate metabolism and the absence of hydroxypyruvate reductase confirms that the glycolate pathway is incomplete in this cyanobacterium.Abbreviations AAN aminoacetonitrile - AOA aminooxyacetate - DIC dissolved inorganic carbon - INH isonicotinic acid hydrazide - PEP phosphoenolpyruvate - PEPcase phosphoenolpyruvate carboxylase - PG phosphoglycolate - PGA phosphoglyceric acid - PGPase phosphoglycolate phosphatase - PR photorespiration - Rubisco ribulose-1,5-bisphosphate carboxylase oxygenase - TCA trichloroacetic acid - RuBP ribulose-1,5-bisphosphate