Undergraduate level notes

1 – Limitations of RuBisCO Slow enzyme: catalytic turnover of ca.3 molecules s billion years old – evolved before atmosphere was oxygenated, therefore non-specific for CO 2. 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 process by which 2-PG converted to 3-PGA (see next)

2 – Photorespiration 2-PG  2 molecules glycolate in chloroplast glycolate  glyoxylate in peroxisome, accompanied by production of H 2 O 2 (dissociates to O 2 + 2H 2 O, by catalase) glyoxylate  glycine (by incorporation of 2 glutamate and release of 2 2-oxoglutarate) 2 mol glycine  1 mol serine in mitochondria by glycine decarboxylase, releasing NH 4 + which returns to chloroplast, NH glutamate  glutamine (glutamine synthase). (cont.  )

Photorespiration (continued) Serine converted ultimately to glycerate (via hydroxypyruvate) in peroxisome. glycerate  3-PGA in chloroplast. Overall process consumes 2 molecules ATP (glutamine synthase step and glycerate  3-PGA step) and 2 reducing equivalents, per O 2. (Taiz and Zeiger, 2010, fig. 8.8 and table 8.2 for full details)

3 – Unavailability of CO 2 Atmospheric CO 2 : O 2 = 0.04% : 21% Soluble CO 2 concentration = ca.10μM (at ambient temperature and pressure) ≈ Km of RuBisCO for CO2. Therefore RuBisCO can only ever operate at about half V max.

4 – CO 2 in an aquatic environment Diffuses 10,000 times slower than in air. Slow to equilibrate between water and air, resulting in CO 2 depleted waters. Equilibrates with HCO 3 -, on a pH dependent basis – the pH of seawater (7.2) results in relatively low CO 2 availability.

Summary – Why do Plants and Algae need photosynthetic turbochargers? RuBisCO is old, slow and non-specific. Photorespiration (to retrieve the carbon lost through RuBisCO’s oxygenase activity) is wasteful. CO 2 is in low availability both in the atmosphere and in water. CCMs go some way towards overcoming these limitations