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Carbon metabolism. Rubisco. Carboxylation. Glycerolaldehyde 3-phosphate. 3-phosphoglycerate. Rubisco. C 3 photosynthesis: Rubisco capacity - at or below normal ambient [CO 2 ] RuBP regeneration - at high [CO 2 ]. Regeneration of RuBP.
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Glycerolaldehyde 3-phosphate 3-phosphoglycerate Rubisco C3 photosynthesis: Rubisco capacity - at or below normal ambient [CO2] RuBP regeneration - at high [CO2]
Regeneration of RuBP One out of six G3P produced is exported to synthesize sugar. The other five G3P are saved for regeneration of RuBP.
1. Carbon fixation CO2 Ribulose 1,5-bisphosphate 3-phospho-glycerate 3-phospho-glycerate Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase The most abundant enzyme on the earth (50% of stromal protein)
Rubisco: 8 large subunits + 8 small subunits Large subunit is the catalytic subunit.
Rubisco is a multisubunit enzyme consisting of eight identical catalytic subunits at the core surrounded by eight smaller subunits that function to stabilize the complex and presumably enhance enzyme activity. Considering that rubisco plays a central role in all photosynthetic autotrophic organisms on earth, of which ~85% are photosynthetic plants and microorganisms that inhabit the oceans, rubisco is the most abundant enzyme on this planet.
Ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco, tradi-tionally with a capital R) is a remarkable enzyme • 1. It catalyzes the actual CO2 fixation step in the pathway • 2. It has a remarkably poor turnover number, approximately 3 sec-1 • 3. It makes up about 50% of the soluble protein in plant leaves, seemingly to make up for its lousy catalytic efficiency; the concen-tration of active sites in the cell has been estimated to be as high as 4 mM. • 4. Rubisco is a highly regulated enzyme that essentially determines the rate of CO2 assimilation in plants.
a) It is inactive in the dark (so much for the “dark reaction,” huh?), inhibited both its substrate, ribulose 1,5-bisphosphate and by 2-carboxyarabinitol 1-phosphate • b) Rubisco is activated by the rise in stromal pH and Mg2+ concentration associated with the light reaction, and by some sort of ATP-driven activase; in the light 2-carboxyara-binitol 1-phosphate concentrations fall, and ribulose 1,5-bisphosphate binds in a less-inhibitory way to the new conformation.
Activation A. Carbamylation at the active site – light activation using different CO2 from the substrate CO2 increase in both pH and [Mg2+] B. Binding substrate ribulose 1,5-bisphosphate should follow the carbamylation removed by the action of rubisco activase D. Inhibition by a natural sugar phosphate 2-carboxyarabinitol 1-phosphate the level is dependent on the species, high concentration in leaves of legumes bind to rubisco at night, and removed by the action of rubisco activase, longer isoform is regulated through a thioredoxin dep. reduction phosphatase action
Ribulose bisphosphate carboxylase/oxygenase (rubisco) * L8S8: 560 kDa (55/14 kDa, each), 8 nucleus-encoded small subunits and 8 plastid-encoded large subunits a kind of anterograde *50% or more of the total protein in plant leaves *its concentration within chloroplasts is extremely high (ca. 0.2 g/ml) Redis small subunit Blueandgreenis large subunit
Nocturnal inhibitor also regulate photosynthesis Some plants synthesize 2-carboxyarabinitol 1-phosphate in the dark, which is a potent inhibitor of carbamolyated rubisco. It will be break down by rubisco activase or by light.
Phosphoglycolate is produced from oxidation of RuBP by Rubisco.
Phosphoglycolate is dephosphorylated to glycolate in chloroplast.
Glycolate is then oxidized to glycoxylate and hydrogen peroxide in peroxisome.
Peroxide is broken down by catalase and glyoxylate is transaminated to form glycine.
Ammonia is reassilimated in chloroplast by GS (glutamate synthetase)-GOGAT (glutamate synthase) cycle Two glycines are converted to one serine and one CO2 in mitochondria, releasing one NH3.
Glycerate returned to chloroplast and is phosphorylated into phosphoglycerate.
Photorespiration and Rubisco Rubisco also catalyzes a oxygenase reaction that combines RuBP with O2 to generate one molecule of 3-phosphoglycerate (C3) and one molecule of 2-phosphoglycolate (C2). It is thought that this "wasteful" reaction belies the ancient history of the rubisco enzyme which has been around since before O2 levels in the atmosphere were as high as they are today.
Photorespiration and Rubisco In order to salvage the carbon “wasted” on 2-phosphoglycolate, it must first be converted to glycolate, which is exported to peroxisomes to make glyoxylate and glycine which is then exported to mitochondria where two molecules of glycine are converted to one molecule of serine.
C4 and C3 leaf anatomy Anatomía Kranz (células de la vaina del haz con cloroplastos) Anatomía C3 (células de la vaina del haz sin cloroplastos)
C3 vs. C4 Leaf Morphology C3 Monocot: Barley, Rice C4 Monocot: Sugarcane
C4 Leaf Morphology C4 Dicot: Flaveria australasica Australian Yellow Weed (Asteraceae)
C4 photosynthetic pathway CO2 concentrates in bundle sheath cells
Mesophyll cell chloroplast: Multiple layered grana ( PSI and PSII) Bundle sheath chloroplasts no or feww PSII ( grana is one to few layers) Chloroplasts of mesophyll and Bundle sheath cells
Light: Stomata closed How do plants regulate this?
Photorespiration Rubisco catalyzes the fixation of both CO2 and O2
Oxygenation of RuBP • Rubisco is a bifunctional enzyme, hence the name carboxylase-oxygenase. • The oxygenase activity of Rubisco will convert RuBP into 3-PGA and phosphoglycolate.
Photorespiration – C2 glycolate cycle • Phosphoglycolate must be recycled and the recycling process involved three different organelles. • Because oxygen is consumed and CO2 is evolved during the recycling process, the C2 glycolate cycle is commonly known as photorespiration.
Why photorespiration? • Photorespiration consumes a lot of energy from plants. • The oxygenase activity of Rubisco is probably an intrinsic activity because even Rubisco from anaerobic photosynthetic microorganism exhibits oxygenase activity. • The presence of O2 can prevent plant from photooxidation when plants are illuminated under condition without CO2, so photorespiration could also be an extra mechanism to dissipate excess excitation energy.
