Photorespiration

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the entry-point enzyme of carbon fixation, but its name reveals a duality: it is both a carboxylase and an oxygenase. Both reactions begin identically — Rubisco activates ribulose-1,5-bisphosphate (RuBP) into a reactive enediol intermediate at its active site. From there, the enediol can attack either CO₂ or O₂, because both molecules can approach the same activated carbon. If CO₂ wins, the six-carbon intermediate splits into two molecules of 3-phosphoglycerate (3-PGA), feeding the Calvin cycle. If O₂ wins, the product is one molecule of 3-PGA plus one molecule of 2-phosphoglycolate (2-PG), a two-carbon compound that is not a Calvin-cycle intermediate and is mildly toxic if allowed to accumulate — it inhibits enzymes of the Calvin cycle, including triose phosphate isomerase. Because 2-PG cannot be used directly, the cell must salvage its carbon skeleton. This salvage route is the photorespiratory pathway, sometimes called the oxidative photosynthetic carbon cycle or the C2 cycle (named for the two-carbon glycolate backbone that is processed). The pathway recovers three-quarters of the carbon in 2-PG as 3-PGA, returning it to the Calvin cycle, while releasing the remaining quarter as CO₂. It is called "photo"-respiration because, like mitochondrial respiration, it consumes O₂ and releases CO₂ — but unlike normal respiration, it happens only in the light, depends on products of the light reactions (ATP, NADPH, and reducing power), and yields no usable energy. It is a tax on photosynthesis, not a source of energy. The key quantitative fact that governs how much photorespiration occurs is Rubisco's specificity factor (Sc/o), the ratio describing how much more readily the enzyme reacts with CO₂ than with O₂ at equal concentrations. In most C3 plants, Sc/o is roughly 80–100, meaning Rubisco strongly prefers CO₂ — but because atmospheric O₂ (~21%, ~478 μM dissolved at 25 °C) vastly outnumbers CO₂ (~0.04%, ~11 μM dissolved), a substantial fraction of catalytic events are still oxygenations. Rising temperature makes the problem worse: warming decreases CO₂ solubility relative to O₂ solubility and lowers Sc/o itself, so the oxygenase reaction becomes proportionally more frequent on hot days — exactly when transpiration also forces stomata to close and internal CO₂ to drop further.

Step 1 — Oxygenation at Rubisco (chloroplast stroma). Rubisco's active site, embedded in the large subunit of the hexadecameric L₈S₈ enzyme, first carbamylates a stromal lysine residue with a non-substrate CO₂ molecule and Mg²⁺, activating the enzyme. RuBP then binds and is deprotonated to form a 2,3-enediol intermediate. This intermediate is the fork in the road. If molecular O₂ diffuses into the site and reacts, it forms a peroxide intermediate that cleaves the five-carbon RuBP into:

• one molecule of 3-phosphoglycerate (3-PGA, 3 carbons) — enters the Calvin cycle normally
• one molecule of 2-phosphoglycolate (2-PG, 2 carbons) — must be salvaged

Step 2 — Dephosphorylation (chloroplast). 2-PG is rapidly dephosphorylated by chloroplastic phosphoglycolate phosphatase to glycolate, releasing inorganic phosphate (important because it prevents phosphate from being locked away from the Calvin cycle). Glycolate is exported from the chloroplast to the peroxisome via a dedicated plastid membrane transporter (a glycolate/glycerate antiporter).

Step 3 — Oxidation in the peroxisome. Glycolate oxidase converts glycolate to glyoxylate, generating hydrogen peroxide (H₂O₂) as a byproduct. This H₂O₂ is immediately broken down by peroxisomal catalase into water and O₂, preventing oxidative damage — one reason photorespiration is compartmentalized in peroxisomes, which are rich in catalase. Glyoxylate is then transaminated (using glutamate as the amino donor) to form the amino acid glycine.

Step 4 — Decarboxylation in the mitochondrion. Glycine is exported to the mitochondrion, where two molecules of glycine are converted into one molecule of serine by the glycine decarboxylase complex (GDC, also called the glycine cleavage system) together with serine hydroxymethyltransferase. This step is the pathway's single CO₂-releasing reaction: 2 Glycine + NAD⁺ + H₂O → Serine + CO₂ + NH₃ + NADH This is the CO₂ that is "lost" during photorespiration — it is fixed carbon that Rubisco worked to capture, now released back to the atmosphere (or partially refixed by nearby chloroplasts). The NH₃ released must also be reassimilated, at additional ATP cost, via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle — in fact, photorespiratory nitrogen flux through GS/GOGAT can be many times larger than primary nitrogen assimilation from soil nitrate, making ammonia recapture a major hidden cost of the pathway.

Step 5 — Return to the peroxisome and chloroplast. Serine returns to the peroxisome, where it is transaminated to hydroxypyruvate and then reduced (using NADH) to glycerate. Glycerate is transported back into the chloroplast stroma, where glycerate kinase phosphorylates it (consuming one ATP) to regenerate 3-PGA, which finally rejoins the Calvin cycle.

Net carbon accounting. Two molecules of glycolate (4 carbons total) enter the pathway; one CO₂ (1 carbon) is released in the mitochondrion, and one molecule of 3-PGA (3 carbons) is returned to the chloroplast. Three of every four carbons are recovered — a 75% carbon-salvage efficiency — but the pathway still nets a loss of fixed carbon and a real energetic cost.

Energetic cost per photorespiratory turnover (per 2 oxygenations, producing 2 glycolate → 1 CO₂ + 1 3-PGA):

Compared with the Calvin cycle's cost of about 3 ATP and 2 NADPH per CO₂ fixed, photorespiration returns far less carbon for a similar or greater energy outlay — this is why, under conditions that favor oxygenation, net photosynthesis (CO₂ assimilation measurable by gas exchange) can fall by 20–50% relative to a hypothetical Rubisco that only carboxylated.

Otto Warburg's discovery of O₂ inhibition of photosynthesis (1920s). Long before the pathway was understood, Otto Warburg observed that raising O₂ concentration around illuminated Chlorella algae suppressed the rate of photosynthetic O₂ evolution and CO₂ fixation — a phenomenon that came to be called the Warburg effect in photosynthesis (not to be confused with the unrelated Warburg effect in cancer metabolism). This was the first quantitative clue that oxygen actively interferes with carbon fixation, decades before anyone knew Rubisco could act as an oxygenase.

Isotope tracing establishes glycolate as the entry metabolite (1950s–1960s). Using ¹⁴CO₂ pulse-labeling of illuminated leaves and algae, researchers including Warburg's contemporaries and later Israel Zelitch and Nathan Tolbert traced newly fixed carbon and found that a substantial fraction rapidly appeared in glycolate, a two-carbon acid, especially when O₂ was high and CO₂ was low. Tolbert's laboratory in the 1960s went further, isolating the peroxisome (then newly described as a distinct organelle by Christian de Duve) and showing biochemically that it contained glycolate oxidase and catalase — establishing that glycolate metabolism, and the H₂O₂ it generates, occurs in this specific compartment rather than diffusely in the cytosol.

George Bowes, William Ogren, and Richard Hageman's identification of Rubisco as the oxygenase (1971). The decisive mechanistic experiment came when Bowes, Ogren, and Hageman showed directly that purified Rubisco (then called RuBP carboxylase) itself catalyzes a reaction between RuBP and O₂, producing 3-PGA and phosphoglycolate in a ratio and stoichiometry matching what whole-leaf physiology predicted. This proved that photorespiration is not a separate side-enzyme's mistake, but an intrinsic dual activity of the very enzyme that fixes CO₂ — a single active site serving two competing substrates. This single finding reframed photorespiration from a mysterious leaf phenomenon into a predictable consequence of enzyme chemistry, explained by competitive kinetics (a Michaelis-Menten-type competition between CO₂ and O₂ for the same active site, later formalized in the Farquhar-von Caemmerer-Berry model of C3 photosynthesis).

Genetic and mutant evidence (1970s–1980s). Ogren's group and others isolated barley and Arabidopsis mutants deficient in specific photorespiratory enzymes (e.g., glycine decarboxylase, glycolate oxidase, phosphoglycolate phosphatase). These mutants grow normally in high-CO₂, low-O₂ air — where photorespiration is negligible — but die or show severe growth defects and glycine/glycolate accumulation when moved to normal air, because they cannot process the 2-PG that Rubisco's oxygenase activity inevitably produces. This "conditional lethal" phenotype is powerful genetic proof that the C2 pathway is not optional bookkeeping but an essential detoxification route required whenever Rubisco oxygenates RuBP — and it also demonstrated, by process of elimination, the order of intermediates in the pathway, since blocking each step caused a specific, predictable metabolite to accumulate.

Gas-exchange quantification (Ogren, Laing, and others, 1970s–1990s). By comparing net CO₂ assimilation of C3 leaves in normal air (21% O₂) versus low-O₂ air (1–2% O₂, which nearly abolishes oxygenation), physiologists directly measured the "photorespiratory burden": net photosynthesis typically rises by 30–50% when O₂ is dropped to near-zero, holding CO₂ constant. This comparison, combined with measurements of the CO₂ compensation point (the CO₂ concentration at which net assimilation is zero — nonzero in C3 plants, essentially zero in C4 plants), remains the standard physiological signature used to detect and quantify photorespiration in any species.

Example 1 — Estimating oxygenation:carboxylation ratio from Rubisco kinetics. Suppose a C3 leaf has a Rubisco specificity factor Sc/o = 90 (dimensionless, defined so that the ratio of carboxylation to oxygenation velocities equals Sc/o × [CO₂]/[O₂]). At 25 °C in mesophyll cells, dissolved CO₂ is 8 μM and dissolved O₂ is 250 μM (lower than ambient due to stomatal limitation on a warm day). Ratio of carboxylation (Vc) to oxygenation (Vo): Vc/Vo = Sc/o × [CO₂]/[O₂] = 90 × (8/250) = 90 × 0.032 = 2.88 So for every 2.88 carboxylations, there is 1 oxygenation — meaning roughly 1 in 3.88 catalytic events (≈ 26%) is an oxygenation. Now raise leaf temperature to 38 °C on a heat-stressed day: Sc/o typically falls to about 70, and warming also lowers CO₂ solubility more than O₂ solubility, dropping dissolved CO₂ to 6 μM while O₂ stays near 240 μM. Vc/Vo = 70 × (6/240) = 70 × 0.025 = 1.75 Now only 1.75 carboxylations occur per oxygenation — the fraction of oxygenation events rises from ≈26% to ≈36%. This directly illustrates why photorespiration accelerates disproportionately under heat: both the enzyme's intrinsic preference and the gas concentrations shift against CO₂ simultaneously. A student should be able to reconstruct this calculation and explain, in words, why a heat wave combined with midday stomatal closure is the worst-case scenario for a C3 crop.

Example 2 — Carbon and energy balance for a leaf assimilating at a known photorespiratory rate. A soybean leaf under gas exchange measurement shows a gross carboxylation rate (Vc) of 20 μmol CO₂ m⁻² s⁻¹ and an oxygenation rate (Vo) of 8 μmol O₂ m⁻² s⁻¹ (a Vc/Vo of 2.5, consistent with a warm, partly stressed leaf). Photorespiratory CO₂ release: one CO₂ is released for every two oxygenation events (from the mitochondrial glycine decarboxylase step), so: CO₂ released by photorespiration = Vo / 2 = 8 / 2 = 4 μmol CO₂ m⁻² s⁻¹ Net CO₂ assimilation (what a gas-exchange instrument, an infrared gas analyzer, actually measures) is gross carboxylation minus photorespiratory release minus "dark" mitochondrial respiration (Rd, say 1 μmol m⁻² s⁻¹): A_net = Vc − (Vo/2) − Rd = 20 − 4 − 1 = 15 μmol CO₂ m⁻² s⁻¹ Without any photorespiration (Vo = 0), net assimilation would have been 20 − 0 − 1 = 19 μmol m⁻² s⁻¹. The percentage loss to photorespiration alone: (19 − 15) / 19 × 100% ≈ 21% This single leaf, on this single afternoon, is losing about a fifth of its potential carbon gain to photorespiration — consistent with the commonly cited 20–50% range, with the exact figure depending on temperature, CO₂, and O₂ at the time of measurement. This calculation is exactly the logic behind why researchers report photosynthesis measurements alongside temperature and stomatal conductance: the same leaf's photorespiratory loss will look very different at 20 °C versus 38 °C.

Bowes trained as a plant physiologist with an interest in how environmental gases shape photosynthetic efficiency. Working in Ogren's lab, which specialized in the biochemistry of Rubisco purification and kinetics, Bowes designed assays exposing purified, activated Rubisco to controlled O₂ atmospheres in the presence of RuBP, then measured the reaction products directly. The clean demonstration that the purified enzyme alone, with no other cellular components, produced phosphoglycolate in an O₂-dependent manner closed a decades-old mystery and reoriented photorespiration research away from "which organelle does what" toward the enzyme mechanism itself — namely, that CO₂ and O₂ are truly competing substrates at a single catalytic site, a fact later formalized quantitatively as the specificity factor (Sc/o) used throughout plant physiology to compare Rubisco across species, including the notably higher Sc/o values found in some algae and C4 plants.

Crop improvement and food security. Because photorespiration can reduce yield-relevant carbon gain in wheat, rice, and soybean by a substantial margin, engineering more efficient photorespiratory bypasses is a major biotechnology target. Field trials (notably from the RIPE — Realizing Increased Photosynthetic Efficiency — project) have introduced synthetic shortcut pathways that convert glycolate to glycerate directly inside the chloroplast, reducing the energetic cost and CO₂ release, and reporting measurable increases in field biomass and yield in tobacco and soybean test lines.

Understanding C4 and CAM evolution. Photorespiration is the selective pressure that drove the independent evolution of C4 photosynthesis in over 60 plant lineages and CAM photosynthesis in many succulents — both are, at their core, biochemical or temporal CO₂ pumps that raise CO₂ concentration around Rubisco and suppress its oxygenase activity. Understanding the C2 pathway is therefore the necessary foundation for understanding why C4 and CAM anatomy and biochemistry look the way they do.

Climate change physiology. As atmospheric CO₂ rises, the CO₂:O₂ ratio available to Rubisco increases, which is predicted to modestly suppress photorespiration and boost C3 photosynthesis (a component of the "CO₂ fertilization effect"); but rising global temperatures simultaneously lower Rubisco's specificity factor and CO₂ solubility, partially counteracting that benefit — making photorespiration a key variable in crop and ecosystem climate models.

Herbicide and enzyme engineering. Because photorespiratory enzymes (e.g., glycolate oxidase, phosphoglycolate phosphatase) are essential only under ambient O₂, they are studied both as potential herbicide targets and as engineering targets for improving Rubisco's CO₂ specificity directly through directed evolution and structural biology approaches.

Ask the tutor: How does the Farquhar-von Caemmerer-Berry model mathematically incorporate photorespiration into predictions of leaf-level photosynthesis?