Karteikarten
Lecture 21 PMB
Karten in diesem Stapel (17)
- 1Dark reaction of photosynthesisReaction where ATP and reducing power generated by light reaction are used to reduce CO2 into carbohydrates
- 2Different CO2 reduction cycles1) Calvin Cycle - 99% of CO2 fixation happens by this; aerobic
2) Reductive (reverse) TCA Cycle; anaerobic
3) Reductive acetyl-CoA pathway; anaerobic
4) 3-Hydroxypropionate Cycle; anaerobic - 3Basic principles in Calvin Cycle Experimentt = 0: 14CO2 radioisotope injected into lollipop-shaped growth chamber containing Chlorella
at specific intervals: samples removed from chamber after the addition of 14CO2 via the stopcock at the bottom
Chlorella in samples killed instantly --> look where 14C had gone - 4Detection of first compounds in Calvin experimentobserve compounds present at different intervals using thin layer chromatography and autoradiography (looking for biggest concentration of C14 --> 3 phosphoglycerate (PGA) found to be first stable intermediate of cycle
- 5Basic Steps of the Calvin Cycle1) Carboxylation
2) Reduction
3) Regeneration - 6Carboxylation step of Calvin Cycle1 CO2 added on to RBP by enzyme RuBisCO --> forms PGA --> ATP from light reaction put in to make a phosphoglycerate
- 7Reduction Step of Calvin Cyclephosphoglycerate of first step reduced by NADPH of light reaction to form GAP --> forms fructose that undergoes various pathways (outside cycle) or a glyceraldehyde phosphate (within cycle)
- 8Regeneration Step of Calvin Cycleglyceraldehyde sugar rearranged to another sugar, PRK enzyme uses energy to form RBP again and start cycle over
- 9RuBisCOribulose biphosphate carboxylase
carboxylation step
RBP + ATP + CO2 --> PGA
*once you add the CO2, the intermediate product is really unstable, it actually breaks down and gets hydrolyzed while on the enzyme* - 10PRKphosphoribulose kinase
regeneration step
Ribulose 5-phosphate + ATP --> Ribulose 1,5-biphosphate
*PRK phosphorylates and regenerates reactant to go back to the center* - 11Competing reaction with carboxylationinstead of phosphorylating RBP, can use O2 as competing substrate to produce 1 PGA and 1 phosphoglycolate (toxic)
usually isn't a problem because carboxylation is so fast but is dangerous in high temperatures (aqueous solubility of CO2 decreases below O2) - 12How do photosynthetic organisms achieve high enough [CO2] for RuBisCO to function properly?Use carbon concentrating mechanisms that are performed within the carboxysomes (of cyanobacteria)
- 13carboxysomepolyhedral protein shell compartments containing highly concentrated array of RuBisCO and carbonic anyhdrase (CA)
CO2 can’t dissolve OUT of the protein shell like it can through lipid membranes! meanwhile, O2 doesn't diffuse very well IN - 14carbonic anhydraseconverts HCO3- to CO2
- 15carbon concentrating MECHANISMS1) High-affinity importers of CO2 + HCO3- expressed if CO2 concentrations low
2) HCO3- accumulates in cytoplasm --> diffuses into carboxysome
3) HCO3- --> CO2 by carbonic anhydrase ONLY in the carboxysome
4) CO2 used rapidly by RuBisCO
5) O2 excluded by shell proteins - 16Reverse Citric Acid (TCA) Cycleperformed by green sulfur bacteria
Fdred fixes CO2 (enters at multiple points of cycle UNLIKE Calvin cycle)
ATP drives reaction
enzymes are same EXCEPT for fumarate reductase and ATP citrate lysase (diagnostic)
occurs in chlorosomes - 17Net reaction of Reverse TCA Cycle3CO2 + 12H + 5ATP --> C3H6PO3^2- +3H20