RuBisCO

• UniProt: O03042

• CAS Number: 9027-23-0

The enzyme ribulose -1 ,5 -bisphosphate carboxylase / oxygenase - ( RuBisCO ) is responsible for ensuring that all photosynthetic plants and bacteria can absorb carbon dioxide, so it is probably the most abundant water-soluble protein of the earth. As a preliminary step in the Calvin cycle, fused RuBisCO is a molecule of carbon dioxide ( CO2) of ribulose -1 ,5- bisphosphate. The resulting compound is divided into two phosphoglycerate molecules, which are further structured to carbohydrates. The energy for these reactions comes in the form of ATP from the photosynthesis, therefore the sunlight, or, as in the case of some bacteria chemolithotropic from the reactions of chemosynthesis.

In addition to the CO2 fixation RuBisCO catalyses a side reaction and the incorporation of oxygen (O2). In the further utilization of the resulting product energy and a carbon atom as CO2 is lost, so this process is called photorespiration. In all organisms engaged in a oxygenic photosynthesis, both reactions occur simultaneously, with the incorporation of CO2 predominates. Through spatial ( C4 plants ) or temporal separation ( CAM plants ) of metabolic pathways, some terrestrial plants are able to increase the efficiency of RubisCO. Most algae and hornworts form pyrenoids in which carbon dioxide is also locally enriched. The activity of RuBisCO is dependent on light. RuBisCO must be activated before the enzymatic activity by a light-dependent Activase.

Construction

RuBisCO exists in plants, algae and cyanobacteria from 16 subunits. Here are so-called eight large subunits ( LSU or "L", about 51,000 to 58,000 Da) and as eight small subunits (SSU or "S", about 18,000 Da) as a hexadecamer before. RuBisCO is assembled in plants in the chloroplast of the cell. A special feature is that the eight identical large subunits in the chloroplast genome and eight identical small subunits of the enzyme are encoded in the nuclear genome. In the thale cress ( Arabidopsis thaliana ), the gene for the long chain is 1,440 base pairs long and results after transcription, translation and post-translational modification in the protein containing amino acids 477. The short chains are made from 126 amino acids in Arabidopsis.

The quaternary structure of the most common form of the enzyme (type I, see ' orthologs forms ') (L2 ) 4 ( S4 ) 2, where the catalytic site is formed by one pair of large subunits; each of which binds one of the product molecules 3 -phosphoglycerate (3- PG; cf. Calvin cycle). The small subunits hold together the complex, but are dispensable for the catalytic function. Presumably, they also increase the specificity of the cylindrical holoenzyme.

Catalyzed reactions

As a cofactor of RuBisCO acts magnesium.

In the dark reaction ( Calvin-Benson cycle) a molecule ribulose -1 ,5 -bisphosphate reacted with carbon dioxide to two molecules of D- 3- phosphoglycerate. Thus, the carbon atom of the carbon dioxide is now within the plant metabolism ( carbon fixation ).

In the fixation of oxygen by RuBisCO instead falls to one molecule of 2- phosphoglycolate, the toxic effect in larger quantities and therefore must be disposed of photorespiration.

Terrestrial plants fix an estimated 120 gigatons (109 t = 1 billion tons ) of carbon from CO2. This is about one-sixth of the total atmospheric CO2 and approximately corresponds to the 17 - to 20-fold of the released annually by human activities into the atmosphere CO2 amount. Of these, currently about 1-2 gigatons of carbon per year net remain stored in terrestrial ecosystems due to accumulation of biomass and soil organic matter. The rest is delivered by autotrophic and heterotrophic respiration back into the atmosphere. For fixation are 0.2% of the total protein occurring on Earth required (accounting for every earthling, evenly distributed, 10 kg RuBisCO ).

With an exchange rate of 17 / s ( living in the cell: 3 / s ) and the lossy side reaction of photorespiration the RuBisCO appears paradoxically as one of the worst optimized (or mostly misunderstood ) enzymes. Therefore, it has been no lack of attempts to modify its properties in the way of genetic engineering to achieve theoretical yield increases of up to 100 %.

However, these experiments soon showed that any increase in the turnover number at the expense of specificity: The enzyme was able to distinguish between poor oxygen and carbon dioxide, which photorespiration favored. Conversely, improved specificity resulted in a lower turnover number, and thus a lower productivity. It appears that Rubisco a particular species is almost completely optimized in spite of its above-mentioned disadvantages of the present environmental conditions ( concentration of O2 and CO2, temperature).

Regulation

RuBisCO is regulated by a Carbamatylierung an L - lysine. This lysine is located at position 201 of the large subunit. A Kohlenstoffdioxidmolekül this reacts with the ε - amino group of lysine to a carbamate (see middle picture ). RuBisCO is but only become active when that lysine is present as carbamate and also a magnesium ion binds to this Carbambat (see image right). This causes a conformational change (which stabilizes magnesium ), in consequence of the large subunit may be enzymatically active.

The Kohlenstoffdioxidmolekül, which reacts with the ε - amino group of lysine to a carbamate, has nothing to do with the CO2 molecule ( see above ) is converted enzymatically into the Carboxylasereaktion.

Orthologs forms

Four different RuBisCO forms have been identified in nature, which have different tertiary structures and kinetic characteristics:

• All green plants, algae and cyanobacteria possess a RuBisCO type I
• In some photosynthetic proteobacteria, chemoautotrophic bacteria and dinoflagellates form a II -type RuBisCO was discovered. Methanococcoides burtonii Also, an archaeon, has a type II RuBisCO. She has no small subunits and forms a dimer. In addition, it has a higher turnover number than type I RuBisCO, but is less specific with respect to the incorporation of oxygen.
• The RuBisCO type III are found in archaea. Your tertiary structure largely corresponds to type I or type II, but it also features were identified. So has the RuBisCO from Thermococcus kodakaraensis a novel, ring-shaped structure consists of five major subunits. Type III enzymes are often adapted to high temperatures and have high turnover numbers up. For a long time it was a mystery why those archaea encode Although a type III RuBisCO enzyme, but no ribulose -5-phosphate kinase. The latter catalyzes the formation of Ru -1 ,5- bP and is therefore a key enzyme of the Calvin cycle. However, it is postulated that other routes may archaea Ru -1 ,5 -BP synthesis, for example 5-phosphoribosyl - 1-pyrophosphate.
• There is a so-called " RuBisCO - like enzyme ", but that does not catalyze the fixation of carbon dioxide and thus represents no bona fide RuBisCO. It is involved in methionine metabolism (methionine salvage pathway). For example, the Archaeoglobus fulgidus hyperthermophilic archaeon a " RuBisCO - like enzyme ".