1. Why Do Plants and Algae have Carbon Concentrating Mechanisms (CCMs)?
Undergraduate Level Notes

2. Limitations of RuBisCO
Slow enzyme: catalytic turnover of ca.3 molecules s-1.
3.8 billion years old – evolved before atmosphere was oxygenated, therefore non-specific for CO2.
Oxygenase activity competes with carboxylase activity (increasingly at high temperature); generates 1 molecule 3-phosphoglycerate (3-PGA) ( CBB cycle) and 1 molecule of 2-phosphoglycolate (2-PG). Photorespiration is the process by which 2-PG converted to 3-PGA (see next slide).

3. CHLOROPLAST
MITOCHONDRIA
PEROXISOME
This process mainly involves three organelles, shown above.

4. 2-PG
2 glycolate
NH4+
Serine
Glycine
Glycolate
Glyoxylate
2 2-oxoglutarate
H202
2 glutamate
RuBisCO’s oxygenase activity produces one 2-PG molecule per O2 molecule in the stroma of the chloroplast.
This gets converted to 2 glycolate molecules. The glycolate is translocated to the peroxisome.
There it is broken down into glyoxylate and H202, which is detoxified to H20 & 02 by catalase.
Glyoxylate +2 glutamate => glycine oxoglutarate. The glycine is translocated to the mitochondria and converted to serine by glycine decarboxylase, releasing NH4+, which is translocated to the chloroplast.

5. glutamate
NH4+
glutamine
Serine
3-PGA
Glycerate
In the chloroplast glutamine synthase catalyses NH4+ + glutamate => glutamine.
Serine is converted to glycerate in the peroxisome.
The glycerate is converted to 3-PGA in the chloroplast. An ATP molecule is used in this reaction and in the glutamine synthase step.

6. Unavailability of CO2 Atmospheric CO2 : O2 = 0.04% : 21%
Soluble CO2 concentration = ca.10μM (at ambient temperature and pressure) ≈ Km of RuBisCO for CO2.
Therefore RuBisCO can only
ever operate at about half
Vmax.

7. CO2 in an Aquatic Environment
Diffuses 10,000 times slower than in air.
Slow to equilibrate between water and air, resulting in CO2 depleted waters.
Equilibrates with HCO3-, on a pH dependent
basis – the pH of seawater (7.2) results in
relatively low CO2 availability.

8. Summary – Why are Photosynthetic Turbochargers necessary?
RuBisCO is old, slow and non-specific.
Photorespiration (to retrieve the carbon lost through RuBisCO’s oxygenase activity) is wasteful.
CO2 is in low availability both in the atmosphere and in water.
CCMs go some way towards overcoming these limitations

9. Copyright
Slide 1: RuBisCO structure courtesy of PDB displayed using PyMol.
Slide 6: Graph taken from Olusola Solomon Amodu, Tunde Victor Ojumu and Seteno Karabo Obed Ntwampe (2013). Bioavailability of High Molecular Weight Polycyclic Aromatic Hydrocarbons Using Renewable Resources, Environmental Biotechnology - New Approaches and Prospective Applications, Prof. Marian Petre (Ed.), ISBN: , InTech, DOI: / Available from:
Chlamydomonas image used throughout: Copyright of Ninghui Shi