Photorespiration Flashcards
Photorespiration recycles products of Rubisco’s oxygenation reaction
2-PG is the waste product of Rubisco’s oxygenation reaction
(when Rubisco reacts with O2 instead of CO2)
Photorespiration is a set of reactions that uses energy to convert 2 x 2-PG to 3-PGA, CO2 and NH3
Photorespiration is a multi-organellar process
involves chloroplasts, peroxisomes and mitochondria:
in the chloroplast:
2-PG is dephosphorylated to glycolate and transported to peroxisome
in the peroxisome:
Glycolate is oxidized to glyoxylate, producing H2O2
in the mitochondrion:
Glyoxylate is trans-aminated to glycine, which is transported to the mitochondrion
(consuming oxygen and ATP, generating CO2)
Hence referred to as ‘photorespiration’ similar to respiration process but not the same
Photorespiration releases CO2 and NH3 from organic form
- In the mitochondrion, one glycine is decarboxylated (releasing CO2) and deaminated (releasing NH3) by the Glycine Decarboxylase Complex (GDC).
- The central C of glycine is transferred to a one carbon carrier (tetrahydrofolate) and then transferred to another glycine to produce serine, by serine hydroxymethyltransferase (SHMT)
Photorespiration: Glycine decarboxylase complex disassembles glycine
glycine decarboxylase (GDC) disassembles glycine:
-CO2 is released from glycine
- NH3 is also released
- CH2 transferred to carbon carrier, then to Gly to make Ser
GDC makes up 10% of leaf mitochondrial proteins in the light
- In the chloroplast, glycerate is phosphorylated to 3-PGA which re-enters the Calvin-Benson cycle
- The resulting serine is transported back into the peroxisome where it is reduced to glycerate
Photorespiratory bypasses can minimize energetic costs of photorespiration
bioengineering can introduce various bacterial enzymes into various plant organelles to bypass the cost of photorespiration.
Several strategies to limit the energetic cost of photorespiration have been examined. Here five bacterial enzymes are targeted to the chloroplast where they recycle glycolate to glycerate.
This bypass allows the CO2 to be effectively refixed, and eliminates the need to refix NH3.
why haven’t plants naturally evolved this?
we don’t know - this suggests there are many aspects of the electron transport chain that we do not understand
Carbon-concentrating mechanisms in bacteria, algae and plants
Increasing the effective carbon dioxide concentration is another way to tackle the inefficiency of Rubisco
Specificity for CO2 in Rubisco drops as temperature increases
^So that Rubisco becomes less specific and less efficient
Also CO2 difuses more slowly in water than in air
^ So organisms that live in hot temperatures and/or in water require carbon dioxide concentrating mechanisms
Cyanobacteria concentrate CO2 in protein structures called carboxysomes (here labelled with GFP)
Many algae concentrate CO2 in pyrenoids within their chloroplasts
C4 and CAM plants use phosphoenolpyruvate carboxylase (PEPC) for the primary carboxylation reaction, concentrating or storing CO2 upstream of Rubisco
in water bicarbonate can be used
In air separating light and dark reactions either spatially or temporally can be used
The bacterial carboxysome sequesters Rubisco and concentrates CO2
- Bacteria concentrate CO2 and Rubisco in carboxysomes, a type of bacterial microcompartment. Carboxysomes can concentrate CO2 1000x above ambient
- Inner membrane transporters for HCO3- and thylakoid transporters for CO2 bring inorganic carbon into the cell
-Carboxysomes have an icosahedral (20 sided) protein shell (blue) that resists CO2 efflux, into which Rubisco (green) is packed
Carbonic anhydrase (CA) catalyzes the reversible reaction:
CO2 + H2O HCO3- + H+
Pyrenoids concentrate carbon in some photosynthetic eukaryotes
Pyrenoids are found in some but not all green algae, red algae and diatoms. They are often induced in response to low CO2. Most land plants lack pyrenoids, but some hornworts (bryophytes) have them.
(not in angiosperms, gymnosperms, mosses or liverworts)
Pyrenoids are highly packaged Rubisco, interspersed with thylakoid membranes (see diagram)
An abundant repeat protein, EPYC1, links Rubisco to form pyrenoids
EPYC1 has four conserved repeats that may serve to link Rubisco holoenzymes together to form the pyrenoid
CO2 is concentrated in pyrenoids via transporters and carbonic anhydrase
bicarbonate use in water dwelling photosynthetic organisms
^ More efficient growth than land plants
HCO3- is poorly membrane permeable, but specific transporters move it into chloroplasts and thylakoids
Carbonic anhydrase (CA) converts HCO3- to CO2
CO2 is highly membrane permeable but diffusion is slowed by the pyrenoid’s starch layer
(see diagram)
PEPC-based carboxylation reactions in plants: C4 and CAM
In C4: Carboxylases are spatially separated
^ The advantage of the C4 pathway is that it avoids photorespiration
In CAM: Carboxylases are temporally separated
^CAM conserves water by opening stomata at night, and also avoids photorespiration
C4 photosynthesis separates carbon assimilation and carbon fixation
- Atmospheric CO2 enters mesophyll cells and is converted to bicarbonate. PEPC carboxylates PEP to produce OAA, a four-carbon compound.
- OAA (or a derivative) is transported to a bundle sheath cell and decarboxylated, releasing CO2 at Rubisco, which initiates the Calvin-Benson cycle. A three-carbon compound returns to the mesophyll cell.
Two chloroplast populations within a single-cell suffice for C4 in some species
- Single-cell C4 takes place in two subcelluar compartments containing two types of chloroplasts within the same cell of Bienertia sinuspersici, a eudicot angiosperm
- PEPC accumulates in the cytosol where primary CO2 fixation takes place. Rubisco is confined to the central compartment chloroplasts (CCC)
