Peroxisomal Glycolate Metabolism and the C₂ Oxidative Photosynthetic Carbon Cycle

Abstract

In plants, pathways for the oxidative photosynthetic carbon (C₂) cycle, fatty acid β oxidation, and the glyoxylate cycle are associated with peroxisomes. Level of enzyme activity for these pathways may change with the stage of tissue development. A new example is increased β oxidation during leaf senescence. Several changes in peroxisomal metabolism are related to evolutionary development. In unicellular algae, enzymes of the C₂ cycle are located in the mitochondria, whereas in multicellular algae and plants most of these enzymes are peroxisomal. It is possible that glycolate metabolism shifted to peroxisomes as a result of low CO₂ and high O₂ in the atmosphere. Lactate metabolisms also shifted from the D-isomer in most bacteria and algae to the L-isomer in plants with peroxisomes. Using a mutant of Chlamydomonas deficient in cytochrome oxidase, we have shown that the glycolate or D-lactate dehydrogenase is linked to mitochondrial electron transport and the alternative pathway of respiration. The C₂ cycle in plants may also be associated with alternative respiration for NADH oxidation during glycine oxidation. This NADH can also be shuttled from mitochondria to peroxisomes via a malate shuttle. Peroxisomal activity is limited by substrate availability. A new example is the countertransport of two intermediates of peroxisomal metabolism, glycolate and glycerate, across the chloroplast envelope. Additionally, a major research effort in plants and algae deals with C0₂ concentrating mechanisms which regulate the synthesis of glycolate, and therefore, the availability of substrate for peroxisomal C₂ metabolism.