The ribulose-1,5-bisphosphate, Commonly abbreviated RuBP, it is a biological molecule that acts as a substrate in the Calvin cycle of photosynthesis, this being the molecule on which CO is fixed.two.
In this process, RuBP can be oxygenated or carboxylated, giving way to the synthesis of hexoses and undergoing various reactions until its own regeneration (recycling). The carboxylation and oxidation of RuBP is carried out by the same enzyme: ribulose-1,5-bisphosphate carboxylase / oxygenase (RuBisCO or Rubisco). In the regeneration of this molecule, the phosphorylation of ribulose-5-phosphate occurs by the enzyme phosphoribulokinase..
Article index
RuBP is a ketopentose-like molecule. These monosaccharides are characterized, as their name indicates, by presenting five carbons with a ketone group, that is, a carbonyl group in one of the central carbons..
As in most ketoses, the carbonyl group is found at C2, while hydroxyl groups are found at C3 and C4 carbons. RuBP is a derivative of ribulose, where the C1 and C5 carbons also have hydroxyl groups. In RuBP these carbons (C1 and C5) are activated by two phosphate groups located at the respective sites.
In the first stage of the Calvin cycle, an enzyme called phosphoribulokinase causes the phosphorylation of ribulose-5-phosphate to generate RuBP. Subsequently, carboxylation occurs, due to the action of the Rubisco enzyme.
In the carboxylation of RuBP, it acts as a CO acceptortwo, joining to said molecule to form two molecules of 3-phosphoglycerate (3PG). During this reaction, an endiolate intermediate is formed by taking up the proton from the C3 carbon of RuBP..
Endiolate generates a nucleophilic attack on COtwo forming a β-oxo acid that is rapidly attacked by HtwoOr on its C3 carbon. The product of this attack undergoes a reaction very similar to an aldol rupture, generating two 3PG molecules, one of which carries the carbon from COtwo.
The Rubisco enzyme that carries out this reaction is a large enzyme, made up of eight equal subunits. This enzyme is considered one of the most abundant proteins on earth, representing approximately 15% of the total proteins within chloroplasts..
As its name indicates (Ribulose bisphosphate carboxylase / oxygenase), Rubisco can catalyze both the carboxylation and oxidation of RuBP, being able to react with both COtwo as with Otwo.
In green plants, photosynthesis produces ATP and NADPH in the light phase. These molecules are used to carry out the reduction of COtwo and form reduced products such as carbohydrates, mostly starch and cellulose.
As mentioned, in the dark phase of photosynthesis, the cleavage of RuBP occurs by the action of Rubisco, with a ratio of two 3PG molecules formed by each RuBP. Upon completion of six rounds of the Calvin cycle, the formation of a hexose (eg glucose) occurs.
In the six rounds of this cycle, six molecules of COtwo they react with six RuBPs to form 12 3PG molecules. These molecules are transformed into 12 BPG (1,3-bisphosphoglycerate) and then into 12 GAP.
Of these 12 GAP molecules, five are isomerized to DHAP of which three react with three more GAP molecules to form three fructose-1,6-bisphosphate. The latter are dephosphorylated to fructose-6-phosphate (F6P) by the action of the enzyme hexosadiphosphatase.
Finally, a glucose phosphate isomerase converts one of the three F6P molecules into glucose-6-phosphate, which is dephosphorylated by its respective phosphatase to glucose, thus completing the path of the formation of a hexose from COtwo.
In the previously described pathway, the GAP molecules formed can be directed towards the formation of a hexose or towards the regeneration of RuBP. For each turn of the dark phase of photosynthesis, a molecule of RuBP reacts with one of COtwo to finally regenerate a RuBP.
As described in the previous section, for every six turns of the Calvin cycle, 12 GAP molecules are formed, of which eight are involved in the formation of a hexose, leaving four available for the regeneration of RuBP..
Two of these four GAPs react with two F6Ps through the action of a transketolase to form two xyluloses and two erythrocytes. The latter bind to two DHAP molecules to produce two seven-carbon carbohydrates, sedoheptulose-1,7-bisphosphate..
The sedoheptulose-1,7-bisphosphate are dephosphorylated and then react with the last two GAPs to form two xyluloses and two ribose-5-phosphate. The latter are isomerized to ribulose-5-phosphate. On the other hand, xyluloses, by the action of an epimerase, are transformed into four more ribuloses.
Finally, the six ribuloses-5-phosphate formed are phosphorylated by phosphoribulokinase to give rise to six RuBP.
Photorespiration is a "light" respiration process that occurs together with photosynthesis, being very active in C3 type plants and almost absent in C4 plants. During this process, the RuBP molecules are not reduced, so hexose biosynthesis does not occur, since the reducing power is diverted towards oxygen reduction..
Rubisco exerts its oxygenase activity in this process. This enzyme has a low affinity for COtwo, in addition to being inhibited by molecular oxygen present in cells.
Because of this, when cellular concentrations of oxygen are higher than those of COtwo, the process of photorespiration can overcome the carboxylation of RuBP by COtwo. In the middle of the 20th century this was demonstrated by observing that illuminated plants fixed Otwo and released COtwo.
In photorespiration, RuBP reacts with Otwo by action of Rubisco, forming an endiolate intermediate that produces 3PG and phosphoglycollate. The latter is hydrolyzed by the action of a phosphatase, originating glycolate that is subsequently oxidized by a series of reactions that occur in peroxisomes and mitochondria, finally yielding COtwo.
Photorespiration is a mechanism that interferes with the photosynthesis process, undoing part of its work, by releasing COtwo and use the necessary substrates for the production of hexoses, thus decreasing the growth rate of the plants.
Some plants have managed to avoid the negative effects of RuBP oxygenation. In C4 plants, for example, prior fixation of CO occurstwo, concentrating it in photosynthetic cells.
In this type of plant the COtwo it is fixed in mesophilic cells that lack Rubisco, by condensation with phosphoenolpyruvate (PEP), producing oxaloacetate that is transformed into malate and passes to the enveloping cells of the bundle, where it releases COtwo that finally enters the Calvin cycle.
CAM plants, on the other hand, separate the fixation of COtwo and the Calvin cycle in time, that is, they carry out the uptake of COtwo at night, through the opening of its stromata, storing it through the metabolism of crassulaceous acid (CAM) through the synthesis of malate.
As in C4 plants, malate passes into the envelope cells of the bundle to release COtwo.
Yet No Comments