Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyses the combination of inorganic CO2 into the organic molecules of life. Rubisco is very inactive as a catalyst and its carboxylase activity is cooperated by various side-reactions including oxygenation of its sugar phosphate substrate by atmospheric O2. We put Rubisco as our reference due the enzyme was might have been discovered in plants and green algae that have 8 large and 8 small subunits. Once a functional enzyme has been gathered, catalysis (carboxylation or oxygenation) relies on appropriate activation by formation of a lysyl-carbamate that also plays a crucial role during several phases of catalysis. Rubisco activity is much regulated in response to short-term instabilities in the environment, although such regulation may not be ideally poised for crop productivity.
Key words: Rubisco, regulation, Rubisco activity, catalysis
Ribulosebisphosphate carboxylase-oxygenase (Rubisco) is the basic autotrophic carboxylase in all oxygenic photosynthetic organisms, and > 99.5% of the inorganic carbon (C) assimilated in primary producers (chemolithotrophs and also photolithotrophs) includes Rubisco (Raven, 2009).Rubisco is the enzyme that catalyses the first step in photosynthesis, the change from ribulose-1,5-diphosphate to two molecules of 3- phosphoglyceric acid(Hartman, 1994).
Ellis (1979) indicated that Rubisco was the most internationally abundant protein in land biota, rely on the enzyme from C3 land plants). Rubisco, phosphoenolpyruvate(PEP) carboxylase, and pyruvate orthophosphate dikinase are the most abundant dissolvable proteins in leaves of C4 plants and may share together approximately the same percentage to the soluble leaf proteins as Rubisco alone in leaves of C3 plants (Sugiyama et al., 1984; Sage et al., 1987; Makino et al., 2003).The improvement of Rubiscohad been started maybe in an atmosphere high in carbon dioxide and low in oxygen, and maybe in an aqueous environment (Aard de Jong, 2011).
The significance of the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 18.104.22.168, Rubisco) is not to be overstated easily. Rubiscocatalyses the primary photosynthetic CO2 reduction reaction, the fixation of atmospheric CO2 to ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3PGA), which is then utilized to build the organic molecules of life(Inger Andersson.2008). because of its significance in photosynthetic carbon fixation, Rubiscocan be create in most autotrophic organisms including photosynthetic bacteria, cyanobacteria, algae, and plants. Estimations demonstrated that Rubisco constitutes up to half of the soluble protein in the plant leaf (Ellis, 1979).Rubisco also begins photorespiration by catalyzing the oxygenation of RuBP to form one molecule every about of 2- phosphoglycolate and PGA(Archie R. Portis, 2007).
The significance of the inhibition of photosynthesis in many organisms by oxygen (Warburg, 1920; Ogren, 1984) became clear with the finding of the oxygenation of RuBP and resulting motivation of photorespiration (Bowes et al., 1971; Ogren and Bowes, 1971; Lorimer, 1981). Increased CO2 concentration stopped the inhibitory impact of oxygen on photosynthesis and this also finds demonstration in the properties of Rubisco as a catalyst; the carboxylation and oxygenation reactions are catalysed at the same active site on the enzyme and CO2 and O2 are competitive substrates (Andrews and Lorimer, 1978).This research goals to understanding of the structure and function of Ribulosebisphosphate carboxylase-oxygenase (Rubisco).
Molecular forms of Rubisco
The first crystal structure of Rubiscoappeared from the dimeric form II enzyme from Rhodospirillumrubrum (Schneider et al., 1986, 1990a).There are two diverse forms (Forms I and II) of Rubisco among photosynthetic Bacteria and Eucarya(Fig. 1). Form II Rubiscos are the simplest, consisted of two identical large subunits, and confined to convinced photosynthetic bacteria ( A. J. Parry ,2007). Form I Rubiscos contains a central core, consisting of four large subunit dimers, as well as eight extra small subunits. Form I Rubiscos are found in plants, algae, cyanobacteria, and autotrophic proteobacteria ( A. J. Parry ,2007). Also to Forms I and II, there are twogroups of structurally related proteins that have been designated as Form III and Form IV Rubiscos that consist only of large subunits (Hanson and Tabita2001).
Fig. 1 Molecular structures of photosynthetic Rubiscos—Forms I (L8S8, 8RUC) and II (L2, 9RUB). The other Rubisco forms (III and IV) also have L2 structures, which in one case are arranged in a pentagonal ring (Kitano et al. 2001). Large subunits (L) are colored green or red and the small subunits (S) are colored blue or yellow. Image created with RasWin.
Form III Rubiscos are found only in Archeaand catalyze the carboxylation of RuBP after its formation via alternative pathways to the Calvin–Benson–Bassham cycle. Form IV Rubiscos are found in Bacteria and Archeaand are also known as Rubisco- similar to proteins, because they lack some key active-site residues essential for the carboxylation of RuBP (Archie R. Portis, 2007). The synthesis and assembly of the subunits is complex and utilizes molecular chaperones and post-translational modification (Fig. 2).
Fig. 2 Rubisco processing and regulation.Redrawn after Houtz and Portis (2003).
The roots of Rubiscoare unknown, branching out to four forms. Forms Iand II are found in aerobic organisms (e.g.cyanobacteria, plants, eukaryotic algae, and purple bacteria), While Form III and Rubisco-like proteins (Form IV) are found in anaerobic prokaryotes (Hanson &Tabita, 2001; Ashidaet al., 2003, 2005).
In a phylogenetic tree (Figure 4) the differences between species are shown clearly. Very clear is the close connection of the form I proteins. The large variety in form IV is perhaps caused by the inactivity of the enzyme, which causes the structural demands to weaken (Tabita, 2008).
Fig.4A phylogenetic tree which shows how far several species of Rubisco are removed genetically from each other. The similarity in the form I Rubisco is striking (Tabita, F. R.; Satagopan, S.; Hanson, T. E.; Kreel, N. E.; Scott, S. S, 2008).
Rubisco is a bifunctional enzyme
Carboxylation, which is the major reaction catalyzed by Rubisco, includes the addition of CO2 to a molecule of a five-carbon sugar substrate, ribulose-1,5- bisphosphate (RuBP) to produce two molecules of 3-phosphoglycerate (3PGA). This reaction can be divided into several partial reactions (Taylor &Andersson, 1997; Clealand et al., 1998) (Fig.5).1)Enolization; abstraction of a proton from C-3 of the substrate results in the formation of the 2,3-enediol intermediate. 2) Carboxylation; the addition of CO2 to the 2,3-enediol produces a 6-carbon intermediate, 2-carboxy-3-keto-arbinitol-1,5-bisphosphate (CKABP). 3) Hydration; the hydration of CKABP yields the gemdiol form of the ketone. 4) Carbon-carbon bond cleavage; deprotonation of the gemdiolresults carbon bond scission and results in formation of one molecule of 3PGA and one molecule of 3PGA in the form of carbanion. 5) Protonation; the carbanion is protonated and the second molecule of 3PGA is shaped.
Fig.5 Reactionpathways for carboxylation and oxygenation catalyzed by Rubisco. 2CABP is an analogue of the gem-diol intermediate prior to bond cleavage (Taylor &Andersson, 1997; Clealand et al., 1998).
Rubisco, however, is a bifunctional enzyme that also possesses an oxygenation activity. In this reaction, the sugar substrate is oxygenated to produce one molecule of 3PGA and one molecule of 2-phosphoglycolate. The latter product is the substrate for photorespiration. The loss of carbon and energy from plant tissues is resulted from Photorespiration. (Bowes et al., 1971; Ogren& Bowes, 1971; Lorimer, 1981). CO2 and O2 compete for the same active site on Rubisco to drive photosynthesis and photorespiration individually. The activation process requires the help of Rubiscoactivase and several effectors molecule (IngerAndersson, 2008).
The activity of Rubisco is regulated by Rca, which facilitates the dissociation of inhibitory sugar phosphates from the active site of Rubisco in an ATP-dependent manner (Spreitzer and Salvucci, 2002). Rubisco is explained by a relatively slow catalytic turnover rate, kcat(e.g.McNevin et al. 2006), consequently, enormous amounts of the enzyme are obligatory to sustain enough photosynthetic rates. The catalytic cycles initiated by Rubisco the carboxylation and oxygenation of ribulose-1,5- bisphosphate (RuBP) are complex and follow a number of steps and transition states (reviewed in Andersson 2008; Tcherkez 2013).The CO2-concentrating mechanisms present in cyanobacteria, algae, C4 and CAM plants capably decrease the oxygenation of RuBP and therefore the proportion of photorespiration in relation to net photosynthesis (Edwards et al. 1985; Nobel 1991; Carmo-Silva et al. 2008; Hagemann et al. 2013; Moroney et al. 2013).
The ratio of carboxylation to oxygenation in the presence of the two gaseous substrates.these limited researches also propose that further useful variation is likely to happen in nature (Parry et al. 2007, 2013). For example, some C3 plants that are native to arid environments have evolved Rubiscos that discriminate more strongly against O2 (i.e. higher SC/O; Galmés et al. 2005). Rubiscos from C4 plants are generally recognized by faster rates of carboxylation with higher sensitivity to O2 (lower SC/O) than Rubiscos from C3 plants (Jordan &Ogren 1983; Seemann et al. 1984; von Caemmerer 2000; Sage 2002; Ghannoum et al. 2005; Kubien et al. 2008; Carmo-Silva et al. 2010). Hence, the challenge is to identify forms of Rubisco described by catalytic properties that maximize carboxylation rates in the chloroplast of the target crop and allow the plant to photosynthesize optimally within its environment (Galmés et al. 2014; Sharwood& Whitney 2014).
The active site of Rubisco assumes a closed conformation with certain phosphorylated ligands regardless of the carbamylation state of Lys-201 (Andersons I. 1996, Taylor TC, Andersson I. 1996). Thisfinding is harmonized with kinetic evidence indicating that RuBP and its epimer,xylulosebisphosphate are bound very firmly to uncarbamylated sites, and even tighter(»103 times) than to carbamylatedsites (Jordan DB, Ogren WL. 1981, Zhu G, Jensen RG. 1991). The delicate balance between RuBP consumption (Rubisco activity) and regeneration (Calvin cycle) (Salvucci 1989; Raines 2003) needs to be put into consideration in attempts to optimize Rubisco function and regulation to enable greater photosynthetic resource use efficiency in current and projected climates (John Wiley & Sons Ltd, 2014).A number of natural sugar phosphates have been illustrated to be binding Rubisco active sites firmly (Table 1).
Table 1.Sugar phosphates that bind the non-carbamylated (E) or carbamylated (ECM) forms of Rubisco, inhibiting its activity.
The substrate, RuBP, has a high affinity for the inactive, noncarbamylated form of Rubisco and acts as an proficient inhibitor of catalytic activity whereas Rubiscocarbamylation is low (Jordan &Chollet 1983; Brooks &Portis 1988; Portis et al. 1995). The inhibitor 2-carboxyarabinitol-1-phosphate (CA1P; Gutteridge et al. 1986; Berry et al. 1987; Moore et al.1992) is not ubiquitous during the plant kingdom, even though inhibits Rubisco in certain species whereas exposed to low light or darkness (Vu et al. 1984; Seemann et al. 1985; Servaites et al. 1986; Holbrook et al. 1992; Sage &Seemann 1993).During the daytime, misfire products of Rubisco catalysis as well lock carbamylated active sites in unproductive forms (Keys et al. 1995; Parry et al. 1997; Pearce & Andrews 2003; Kim &Portis 2004). The best significant of the inhibitors arising from catalytic misfire is likely to be d-glycero-2,3-diulose-1,5-bisphosphate (PDBP; Kane et al. 1998; Andralojc et al. 2012), which can be changed into 2-carboxytetritol- 1,4-bisphosphate (Harpel et al. 1995; Pearce & Andrews 2003). Rubiscocatalyses a wasteful side reaction with oxygen that leads to the release of before fixed CO2, NH3, and energy throughout photorespiration (Fig.6) (Martin A. J. Parry, 2013).
Fig.6Biotechnological strategies for improving photosynthetic carbon assimilation in crops.
In addition to directly improving Rubisco catalysis, other strategies purpose to improve CO2 levels around Rubisco to minimize photorespiratory expenses related with recycling of Rubisco’soxygenase product, 2-phosphoglycolate (2PG, see dashed line). These strategies include: introducing assimilation characteristics from C4-physiology into C3 cells (Covshoff and Hibberd, 2012); cyanobacteria inorganic carbon (Ci) pumps into chloroplast membranes (Price et al., 2011); and novel catabolic by-pass pathways (Kebeish et al., 2007). Other strategies include enhancing Calvin cycle RuBP regeneration by increasing sedoheptulose-1,7-bisphosphatase (SBPase) activity (Rosenthal et al., 2011) and increasing the thermotolerance of Rubiscoactivase to sustain Rubisco activity under reasonably elevated temperatures (Kureck et al., 2007; Kumar et al., 2009).
Mechanism of Rubisco:
The mechanisms involved in Rubisco regulation are summarized in Fig. 7. Rubisco (E) activity in vivo is modulated also by the carbamylation of an essential lysine residue at the catalytic site and subsequent stabilization of the resulting carbamate by a Mg2+ ion, resulting a catalytically active ternary complex (E.CO2.Mg2+); or through the tight binding of low molecular weight inhibitors (I) (Martin A. J. Parry, 2008). Note that the CO2 involved in active site carbamylation is diverse from CO2 reacting with the acceptor molecule, RuBP, throughout catalysis. Inhibitors bind either before (E.I) or after carbamylation (E.CO2.Mg2+.I) and block the active site of the enzyme, preventing carbamylation and/or substrate binding. The removal of tightly bound inhibitors from the catalytic site of the carbamylated and decarbamylated forms of Rubisco requires Rubiscoactivase and the hydrolysis of ATP. In this way Rubisco activase ensures that the Rubisco active site is not blocked by inhibitors and so free also to become carbamylated or to partake directly in catalysis (Martin A. J. Parry, 2008).
Fig.7 (A) Principles of regulation of Rubisco catalytic activity.
For full details, see text. E, unmodified enzyme (‘decarbamylated’ Rubisco); E.I, decarbamylated enzyme with substrate (RuBP) or misfire product (XuBP) bound at active sites—in this background both compounds are inhibitors (I); E.CO2.Mg2+, ternary complex with catalytically competent active site geometry; E.CO2.Mg2+ I, carbamylated enzyme with catalytic site occupied by tight binding inhibitor (CA1P, PDBP and possibly KABP). (B) Reversible inhibition of carbamylated Rubisco by CA1P, showing the light-dependent taking away and dephosphorylation of CA1P, mediated by Rubiscoactivase and CA1P phosphatase, respectively. CA can be rephosphorylated to CA1P in a subsequent period of darkness (Martin A. J. Parry, Alfred J. Keys , Pippa J. Madgwick , 2008).
Rubisco is the enzyme that catalyses to begin with phase in photosynthesis, the transformation from ribulose-1, 5-diphosphate to 2 molecules of 3-phosphoglyceric acid. The name Rubisco is utilized for a very large set of different enzymes relying on their place in the phylogenetic tree. Rubiscoisthe enzyme found in plants and green algae that have 8 large and 8 small subunits. RuBisCO exists in two forms, one a dimer (Form II) and the other a hexadecamer (Form I). The majority abundant form of Rubisco (form I) is a hexadecameric complex consisting ofeight large and eight small subunits and the Form I enzyme take place in other bacteria (including cyanobacteria) and in all green plants and non green ”algae”. The Form II enzyme happens in some eubacteria and is made up of two large subunits (LSU).