PhysBiochemWeekThree2011PhotorespNitr

PhysBiochemWeekThree2011PhotorespNitr - Rubisco and...

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1 Rubisco and Photorespiration • Calvin cycle synthesizes hexose sugars from carbon dioxide and water. • First step is the fixation of carbon dioxide to 3-PGA. • 3 molecules of CO 2 condense with 3 molecules of ribulose- 1,5 bisphosphate and 3 molecules of water to form an unstable six-carbon compound. • Compound is rapidly hydrolyzed to 6 molecules of 3- phosphoglycerate in C 2 plants. • Enzyme is ribulose 1,5 bisphosphate carboxylase/ oxygenase (Rubisco), which contains a Mg ++ at active site • Rubisco is ~16% of the protein of a green plant cell, and Rubisco is the most prevalent protein on Earth. 1 Rubisco • Enzyme contains 8 large and 8 small subunits. • Large subunits encoded by chloroplast genome, small subunits encoded by nuclear genome. • Has a slow catalytic rate of only 3 CO 2 molecules per second. • Rubisco is the rate-limiting enzyme of the Calvin cycle. • Activity of Rubisco increases markedly upon illumination. 2
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2 Rubisco • Early atmosphere contained little oxygen (reducing atmos). • Rubisco also catalyzes a wasteful oxygenase reaction. • Sometimes reacts with oxygen rather than carbon dioxide. • This reaction causes a deleterious oxygenase reaction. • Photorespiration lowers the efficiency of photosynthesis by removing carbon molecules from the Calvin Cycle. • Products of oxygenase reaction are phosphoglycolate and glyceraldehyde 3-phosphate which reenters C.C. Rubisco carboxylase rate is four times that of the oxygenase reaction at ambient temperature. However, stromal concentration of CO 2 is 10 u M, and stromal concentration of O 2 is 250 u Molar. 3 4 Oxygenase Reaction of Rubisco (C 2 cycle) The reactive intermediate on Rubisco also reacts with molecular oxygen to form a hydroperoxide intermediate, which then proceeds to form one molecule of 3-phosphoglycerate (glyceraldehyde 3-phosphate) and one molecule of 2 - phosphoglycolate . In C 2 cycle, the first product of Rubisco is a three-carbon sugar.
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Phosphoglycolate • 2-Phosphoglycolate is not useful to the cell (lose carbons). • Salvage pathway recovers part of the carbon skeleton of PG. • A special phosphatase converts phosphoglycolate into glycolate , which enters peroxisomes. Glycolate is then oxidized to glyoxylate in peroxisomes by glycolate oxidase. • In peroxisomes, H 2 O 2 is produced, and is cleaved by catalase to water and oxygen. • Glyoxylate can be transaminated to yield the amino acid glycine and then other aa. • Pathway salvages 3 of the 4 carbon atoms to form 2 molecules of glycine. 5 6 2-phosphoglycolate is formed as a product of the oxygenase reaction in chloroplasts. After dephosphorylation, glycolate is transported into peroxisomes and converted into glyoxylate and then glycine. In mitochon- dria, two glycines are converted into serine, after losing a carbon as CO 2 and ammonia. The ammonia is salvaged in chloroplasts. phosphatase
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This note was uploaded on 04/27/2011 for the course PHYS 2202 taught by Professor Mackie during the Spring '11 term at Temple.

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PhysBiochemWeekThree2011PhotorespNitr - Rubisco and...

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