The Plant-Like C2 Glycolate Cycle and the Bacterial-Like Glycerate Pathway Cooperate in Phosphoglycolate Metabolism in Cyanobacteria

Abstract

The occurrence of a photorespiratory 2-phosphoglycolate metabolism in cyanobacteria is not clear. In the genome of the cyanobacterium Synechocystis sp. strain PCC 6803, we have identified open reading frames encoding enzymes homologous to those forming the plant-like C2 cycle and the bacterial-type glycerate pathway. To study the route and importance of 2-phosphoglycolate metabolism, the identified genes were systematically inactivated by mutagenesis. With a few exceptions, most of these genes could be inactivated without leading to a high-CO 2 -requiring phenotype. Biochemical characterization of recombinant proteins verified that Synechocystis harbors an active serine hydroxymethyltransferase, and, contrary to higher plants, expresses a glycolate dehydrogenase instead of an oxidase to convert glycolate to glyoxylate. The mutation of this enzymatic step, located prior to the branching of phosphoglycolate metabolism into the plant-like C2 cycle and the bacteriallike glycerate pathway, resulted in glycolate accumulation and a growth depression already at high CO 2 . Similar growth inhibitions were found for a single mutant in the plant-type C2 cycle and more pronounced for a double mutant affected in both the C2 cycle and the glycerate pathway after cultivation at low CO 2 . These results suggested that cyanobacteria metabolize phosphoglycolate by the cooperative action of the C2 cycle and the glycerate pathway. When exposed to low CO 2 , glycine decarboxylase knockout mutants accumulated far more glycine and lysine than wild-type cells or mutants with inactivated glycerate pathway. This finding and the growth data imply a dominant, although not exclusive, role of the C2 route in cyanobacterial phosphoglycolate metabolism.

The Metabolic Network of Synechocystis sp. PCC 6803: Systemic Properties of Autotrophic Growth

“…Although it was suggested that cyanobacteria are capable of synthesizing Ser directly from 3-phosphoglycerate via 3-phosphohydroxy pyruvate and phosphoserine (Colman and Norman, 1997), no candidate genes for such a conversion are currently known for Synechocystis. The assumption that glyoxylate is an essential precursor for Ser synthesis is further supported by the fact that Eisenhut et al (2006) were unable to obtain a completely segregated mutant with an inactivated Ser hydroxymethyltransferase (sll1931). The Ser hydroxymethyltransferase catalyzes interconversion of Ser and Gly and would be dispensable if an alternative synthesis pathway for Ser exists.…”

“…Remarkably, our results indicate that the seemingly wasteful oxygenation of ribulose-1,5-bisphosphate (RuBP) is required to achieve an optimal flux state. This unintuitive result is discussed in the light of ongoing work to elucidate and understand photorespiratory metabolism (Eisenhut et al, 2006(Eisenhut et al, , 2008.…”

Algal evolution in relation to atmospheric CO 2 : carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

“…The other pathway from glycolate to 3-phosphoglycerate involves tartronic semialdehyde, and is called the tartronic semialdehyde pathway by phycologists; bacteriologists call it the glycerate pathway, even though glycerate is also an intermediate of the PCOC [85,[88][89][90]. Parts of the PCOC (glycerate dehydrogenase, serine transminases and serine hydroxymethyltransferase) could have been recruited from core metabolism synthesizing serine and glycine from glycolytic intermediates, while others (glycolate dehydrogenase/oxidase, glycine decarboxylase and glycerate kinase) have no known roles other than in the metabolism of glycolate to PCRC intermediates [93,94].…”

Mitochondrial Protein Lipoylation Does Not Exclusively Depend on the mtKAS Pathway of de Novo Fatty Acid Synthesis in Arabidopsis

“…After centrifugation, the supernatants were dried by lyophilization and redissolved in 8 mM Na 2 PO 4 (pH 6.8) and 0.4% (v/v) tetrahydrofurane. Individual amino acids were quantified after derivatization with o-phthaldialdehyde as described elsewhere (Eisenhut et al, 2006).…”