Rubisco carboxylase

Rubisco carboxylase (addition of C02) Oxygenase (addition of 02) Storroe, 1983... 

The presence of SSUs have been shown to be essential for catalysis However, their exact function remains questionable Thus the natural occurrence of two distinct SSU populations in the same plant enabled us to study the possible role of SSUs in carboxylation activity Maximal rate of carboxylation (Umax) of Rubisco carboxylase was determined in the presence of increasing concentrations of either of the two substrates (CO2 and RuBP) Table 1 shows that carboxylation activity in enzyme extracts of B-gametophytes is significantly higher than that of R-gametophytes Approximately 3-fold (CO2) and 2-fold (RuBP) higher maximal carboxylation rates... 

The buffering capacity of entire chloroplasts has been shown by titration, to be about one microequivalent per mg chlorophyll per pH unit in the physiological pH range. All of the protons produced by Rubisco carboxylase activity are required for either subsequent starch synthesis within the chloroplast or for sucrose synthesis in the cell cytoplasm (to where equivalent acidity must be transferred from the stroma). So none of the acidity generated by Rubisco would ever be expected to accumulate. 

Rubisco carboxylase activity in the bundle-sheath or Krantz cells. Determining whether this was the distribution therefore provided an experimental test. 

Figure 4.8 The active site in all a/p barrels is in a pocket formed by the loop regions that connect the carboxy ends of the p strands with the adjacent a helices, as shown schematically in (a), where only two such loops are shown, (b) A view from the top of the barrel of the active site of the enzyme RuBisCo (ribulose bisphosphate carboxylase), which is involved in CO2 fixation in plants. A substrate analog (red) binds across the barrel with the two phosphate groups, PI and P2, on opposite sides of the pocket. A number of charged side chains (blue) from different loops as welt as a Mg ion (yellow) form the substrate-binding site and provide catalytic groups. The structure of this 500 kD enzyme was determined to 2.4 A resolution in the laboratory of Carl Branden, in Uppsala, Sweden. (Adapted from an original drawing provided by Bo Furugren.)...

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

The virtual SWISS-PROT entries have a far-reaching effect on TrEMBL. For example, the virtual entry for the Rubisco (ribulose-bisphosphate carboxylase) large chain affects 3300 TrEMBL entries. Therefore a system has been developed to decompose these virtual entries into rules that are stored in a relational database with proper version control features. 

Atmospheric CO2 first moves through the stomata, dissolves into leaf water and enters the outer layer of photosynthetic cells, the mesophyll cell. Mesophyll CO2 is directly converted by the enzyme ribulose biphosphate carboxylase/oxygenase ( Rubisco ) to a six carbon molecule that is then cleaved into two molecules of phosphoglycerate (PGA), each with three carbon atoms (plants using this photosynthetic pathway are therefore called C3 plants). Most PGA is recycled to make ribulose biphosphate, but some is used to make carbohydrates. Free exchange between external and mesophyll CO2 makes the carbon fixation process less efficient, which causes the observed large C-depletions of C3 plants. 

In this reaction, one molecule of ribulose-1,5-bisphosphate (metabolite 1) and one molecule of CO2 (metabolite 2) give rise to two molecules of 3-phosphoglycerate (metabolite 3). The enzyme responsible has the EC number 4.1.1.39. The annotated enzyme list shows that this refers to ribulose bisphosphate carboxylase ( rubisco for short). Rubisco belongs to enzyme class 4 (the lyases) and, within that group, to subclass 4.1 (the car-boxy-lyases). It contains copper as a cofactor ([Cu]). 

Why is ribulose bisphosphate carboxylase (RUBISCO) a poor target for a herbicide ... 

The enzyme responsible for the fixation of CO2, ribulose bisphosphate carboxylase/oxygenase (RUBISCO), also catalyses the oxygenation of ribulose bisphosphate to form one molecule of phosphoglycerate and one molecule of phosphoglycolate. Under normal C02 and 02 concentrations six ribulose bisphosphate molecules react with oxygen for every 15 that react with carbon dioxide. 

The PEP carboxylase of mesophyll cells has a high affinity for HCCU (which is favored relative to C02 in aqueous solution and can fix C02 more efficiently than can rubisco). Unlike rubisco, it does not use 02 as an alternative substrate, so there is no competition between C02 and 02. The PEP carboxylase reaction, then, serves to fix and concentrate C02 in the form of malate. Release of C02 from malate in the bundle-sheath cells yields a sufficiently high local concentration of C02 for rubisco to function near its maximal rate, and for suppression of the enzyme s oxygenase activity. 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

N. E. Tolbert4 has argued that the dual specificity of rubisco for C02 and 02 is not simply a leftover from evolution in a low-oxygen environment. He suggests that the relative activities of the carboxylase and oxygenase activities of rubisco actually have set, and now maintain, the ratio of C02 to 02 in the earth s atmosphere. Discuss the pros and cons of this hypothesis, in molecular terms and in global terms. How does the existence of C4 organisms bear on the hypothesis ... 

In this plant, where would you expect to find (a) PEP carboxylase, (b) rubisco, and (c) starch granules Explain your answers with a model for C02 fixation in these C4 cells. 

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