Photorespiration: The Futile Cycle?

Abstract

Photorespiration, or C₂ photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C₁ metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Keywords: atmospheric CO2; climate change; crop yield; kinetics; metabolic interactions; metal cofactor; oxygenation; photorespiration; photosynthesis.

Figure 1

Two main reactions of Rubisco: Photosynthesis and photorespiration. Rubisco structure picture credit: Laguna design / science photo library.

Figure 2

The proposed photorespiratory pathway within the context of photosynthetic carbon and nitrogen metabolism. The solid red lines represent reactions of the photorespiratory pathway, the solid blue lines represent reactions of the proposed alternative photorespiratory pathway, the solid purple lines represent reactions of amino acid synthesis, and the dotted lines represent associated transport processes. Numbered reactions are catalyzed by the following enzymes: 1. Rubisco, 2. Malic enzyme, 3. Phosphoglycolate phosphatase, 4. Glycerate kinase, 5. Glycolate oxidase, 6. Glutamate:glyoxylate aminotransferase, 7. Glycine decarboxylase complex, 8. Serine hydroxymethyltransferase-1, 9. Serine:glyoxylate aminotransferase, 10. Hydroxypyruvate reductase-1, 11. Malate dehydrogenase, 12, Nitrate reductase, 13 Nitrite reductase, and 14. Glutamine synthetase. PETC designates photosynthetic electron transport chain and RETC, respiratory electron transport chain. Adapted from ref. [8]. Copyright 2018 Springer Nature Ltd.

Figure 3

Changes in ¹⁴CO₂ concentration that occur upon exposing a leaf in the light to ¹⁴CO₂. Adapted with permission from ref. [55]. Copyright 1971 Canadian Science Publishing.

Figure 4

Simplified INST-MFA workflow to estimate flux. Software, such as INCA and OpenMebius, is used in several steps. Fluxes and pool sizes are initially guessed and then adjusted in each iteration, converging upon a flux map that fits measurements of metabolism during the ILE. Adapted with permission from ref. [92]. Copyright 2018 Elsevier Ltd.

Figure 5

Shoot NO₃^– assimilation as a function of shoot internal CO₂ concentration (C_i) for 9 C₃ species. Adapted with permission from ref. [101]. Copyright 2014 Springer Science Business Media Dordrecht.

Figure 6

An outline of sulfur assimilation and its role in producing sulfur-containing defense compounds. Adapted with permission from ref. [137]. Copyright 2005 Elsevier Ltd.