Photorespiration
[1] Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria.
The oxygenation reaction of RuBisCO is a wasteful process because 3-phosphoglycerate is created at a lower rate and higher metabolic cost compared with RuBP carboxylase activity.
The conversion of 2× 2Carbon glycine to 1× C3 serine in the mitochondria by the enzyme glycine-decarboxylase is a key step, which releases CO2, NH3, and reduces NAD to NADH.
In algae (and plants which photosynthesise underwater); gases have to diffuse significant distances through water, which results in a decrease in the availability of CO2 relative to O2.
This oxaloacetate is then converted to malate and is transported into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration.
Here, carbon dioxide is removed from the malate and combined with RuBP by RuBisCO in the usual way, and the Calvin cycle proceeds as normal.
Crassulacean acid metabolism allows plants to conduct most of their gas exchange in the cooler night-time air, sequestering carbon in 4-carbon sugars which can be released to the photosynthesizing cells during the day.
CAM plants usually display other water-saving characteristics, such as thick cuticles, stomata with small apertures, and typically lose around 1/3 of the amount of water per CO2 fixed.
C2 photosynthesis (also called glycine shuttle and photorespiratory CO2 pump) is a CCM that works by making use of – as opposed to avoiding – photorespiration.
It performs carbon refixation by delaying the breakdown of photorespired glycine, so that the molecule is shuttled from the mesophyll into the bundle sheath.
[9] Although C2 photosynthesis is traditionally understood as an intermediate step between C3 and C4, a wide variety of plant lineages do end up in the C2 stage without further evolving, showing that it is an evolutionary steady state of its own.
[10] This type of carbon-concentrating mechanism (CCM) relies on a contained compartment within the cell into which CO2 is shuttled, and where RuBisCO is highly expressed.
There is some debate as to when biophysical CCMs first evolved, but it is likely to have been during a period of low carbon dioxide, after the Great Oxygenation Event (2.4 billion years ago).
[11] In nearly all species of eukaryotic algae (Chloromonas being one notable exception), upon induction of the CCM, ~95% of RuBisCO is densely packed into a single subcellular compartment: the pyrenoid.
The pyrenoid is not a membrane-bound compartment but is found within the chloroplast, often surrounded by a starch sheath (which is not thought to serve a function in the CCM).
[13] Cyanobacterial CCMs are similar in principle to those found in eukaryotic algae and hornworts, but the compartment into which carbon dioxide is concentrated has several structural differences.
The mutants deficient in photorespiratory enzymes are characterized by a high redox level in the cell,[24] impaired stomatal regulation,[25] and accumulation of formate.
