Photorespiration and the C4 and CAM Pathways:-The Salvage of Phosphoglycolate Is Costly
The glycolate pathwayconverts two molecules of 2 phosphoglycolate to a molecule of serine (three carbons) and a molecule of CO2 (Fig. 20–21). In the chloroplast, a phosphatase converts 2-phosphoglycolate to glycolate, which is exported to the peroxisome. There, glycolate is oxidized by molecular oxygen, and the resulting aldehyde (glyoxylate) undergoes transamination to glycine. The hydrogen peroxide formed as a side product of glycolate oxidation is rendered harmless by peroxidases in the peroxisome. Glycine passes from the peroxisome to the mitochondrial matrix, where it undergoes oxidative de carboxylation by the glycine decarboxylase complex, an enzyme similar in structure and mechanism to two mitochondrial complexes we have already encountered: the pyruvate dehydrogenase complex and the -ketoglutarate dehydrogenase complex (Chapter 16). The glycine de carboxylase complex oxidizes glycine to CO2 and NH3, with the concomitant reduction of NAD to NADH and transfer of the remaining carbon from glycine to the cofactor tetrahydrofolate (Fig. 20–22). The one-carbon unit carried on tetrahydrofolate is then transferred to a second glycine by serine hydroxymethyltransferase, producing serine. The net reaction catalyzed by the glycine decarboxylase complex and serine hydroxymethyltransferase is
2 Glycine + NAD++H2O→serine+CO2+NH3+NADH+H+
The serine is converted to hydroxypyruvate, to glycerate, and finally to 3-phosphoglycerate, which is used to regenerate ribulose 1,5-bisphosphate, completing the long, expensive cycle (Fig. 20–21). In bright sunlight, the flux through the glycolate salvage pathway can be very high, producing about five times more CO2 than is typically produced by all the oxidations of the citric acid cycle. To generate this large flux, mitochondria contain prodigious amounts of the glycine decarboxylase complex: the four proteins of the complex make up half of all the protein in the mitochondrial matrix in the leaves of pea and spinach plants! In nonphotosynthetic parts of a plant, such as potato tu bers, mitochondria have very low concentrations of the glycine decarboxylase complex. The combined activity of the rubisco oxygenase and the glycolate salvage pathway consumes O2 and produces CO2—hence the name photorespiration. This pathway is perhaps better called the oxidative photosynthetic carbon cycle or C2 cycle, names that do not invite comparison with respiration in mitochondria. Unlike mitochondrial respiration, “photorespiration” does not conserve energy and may actually inhibit net biomass formation as much as 50%. This inefficiency has led to evolutionary adaptations in the carbon-assimilation processes, particularly in plants that have evolved in warm climates.
FIGURE 20–21 Glycolate pathway .This pathway, which salvages 2 phosphoglycolate (shaded pink) by its conversion to serine and eventually 3-phosphoglycerate, involves three cellular compartments. Glycolate formed by dephosphorylation of 2-phosphoglycolate in chloroplasts is oxidized to glyoxylate in peroxisomes and then transaminated to glycine. In mitochondria, two glycine molecules con dense to form serine and the CO2 released during photorespiration (shaded green). This reaction is catalyzed by glycine decarboxylase, an enzyme present at very high levels in the mitochondria of C3 plants (see text). The serine is converted to hydroxypyruvate and then to glycerate in peroxisomes; glycerate reenters the chloroplasts to be phosphorylated, rejoining the Calvin cycle. Oxygen (shaded blue) is consumed at two steps during photorespiration.
FIGURE 20–22 The glycine decarboxylase system. Glycine decarboxylase in plant mitochondria is a complex of four types of subunits, with the stoichiometry P4H27T9L2. Protein H has a covalently attached lipoic acid residue that can undergo reversible oxidation. Step 1 is formation of a Schiff base between pyridoxal phosphate (PLP) and glycine, catalyzed by protein P (named for its bound PLP). In step 2, protein P catalyzes oxidative decarboxylation of glycine, releasing CO2; the remaining methylamine group is attached to one of the OSH groups of reduced lipoic acid. 3 Protein T (which uses tetrahydrofolate (H4F) as cofactor) now releases NH3 from the methylamine moiety and transfers the remaining one-carbon fragment to tetrahydrofolate, producing N5,N10-methylene tetrahydrofolate. 4 Protein L oxidizes the two OSH groups of lipoic acid to a disulfide, passing electrons through FAD to NAD+ 5, thus completing the cycle. The N5,N10-methylene tetrahydrofolate formed in this process is used by serine hydroxymethyltransferase to convert a molecule of glycine to serine, regenerating the tetrahydrofolate that is essential for the reaction catalyzed by protein T. The L subunit of glycine decarboxylase is identical to the dihydrolipoyl dehydrogenase (E3) of pyruvate dehydrogenase and α-ketoglutarate dehydrogenase (see Fig. 16–6).
