Why does rubisco have oxygenase activity

While this mechanism reduces the oxygenase activity of rubisco, it has an extra energy cost in the form of another ATP per mole CO2 fixed. If you want to know more, the video below gives a more thorough albeit somewhat slow illustration of this process:.

Photosynthesis and Respiration: mirror images The chemical equations for oxygenic photosynthesis and aerobic respiration are exactly the reverse of each other.

A balance between the global rates of photosynthesis primary production and global rates of respiration is needed to maintain stable atmospheric concentrations of CO2 and O2. In eukaryotes, both photosynthesis and respiration occur in organelles with double membranes and their own circular genomes, that originated as prokaryotic endosymbionts.

Both processes have electron transport chains, chemiosmosis and ATP synthase powered by proton motive force. The powerpoint slides used in the video screencasts are in the Carbon fixation slide set.

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In the open conformation pale green , loop 6 dark blue is retracted and the C-terminal peptide pink is disordered. The RbcL subunits are arranged as a toroid of antiparallel dimers that is capped at both ends by four RbcS subunits Andersson and Backlund, Figure 1B. This process is called carbamylation and serves to position the substrate RuBP for efficient electrophilic attack by the second CO 2 molecule that will be fixed in the CBB cycle Andersson, Upon RuBP binding, the active site is closed via two sequential conformational changes in RbcL: Loop 6 in the C-terminal domain of RbcL extends over the bound RuBP trapping it below; the C-terminal tail of RbcL then stretches across the subunit and pins down loop 6, closing the active site Bracher et al.

Figure 2. Rubisco regulation by Rca. A Regulation of Rubisco activity and inhibition by sugar phosphates. Figure reproduced from reference Bracher et al. B Phylogenetic tree of selected Rubisco RbcL sequences. The RbcL C-terminal sequences and their associated Rca's are indicated.

X represents variable residues. The phylogenetic tree was calculated by multiple sequence alignment using T-Coffee Notredame et al. Form IA prokaryote : M. Form IB eukaryote : Z. Form IB prokaryote : C. PCC, Synechococcus sp. PCC ; F. Form ID eukaryote : D. Form IC prokaryote : X. Moreover, Rubisco is inhibited by so-called misfire by-products, such as xylulose-1,5-bisphosphate XuBP and 2,3-pentodiulose-1,5-bisphosphate PDBP , which are generated at a low frequency during the multistep catalytic reaction Parry et al.

Release of inhibitor from inactive Rubisco at a biologically relevant timescale is made possible through intervention by Rubisco activase Rca Figure 2A. Since the discovery, in the early 's, of the first Rca in a photosynthesis mutant of Arabidopsis thaliana Portis and Salvucci, , Rca enzymes have been identified in many photosynthetic organisms containing either green-type or red-type Rubiscos, from chemoautotrophic bacteria to higher plants Mueller-Cajar et al.

In this review, we will discuss recent advances in understanding the structure and mechanism of Rca's from the red and green lineages of photosynthetic organisms. The diversity of these enzymes provides a fascinating example of convergent evolution, and reflects the constraints under which Rca's and their cognate Rubisco substrates may have co-evolved. Rca has been known since the s Portis and Salvucci, but was assumed to be restricted to plants. The first prokaryotic Rca was only recently discovered in the proteobacterium Rhodobacter sphaeroides , which contains the red-type Rubisco form IC Mueller-Cajar et al.

Inactivation of cbbX in R. The structural and functional analysis of RsRca provided critical insights into the mechanism of Rubisco remodeling.

The two subdomains of the core module are separated by a short flexible linker. Figure 3. The prokaryotic Rca of red-type form IC Rubisco. A Schematic representation of the domain structure of Rca from R. D Model of the putative storage form of prokaryotic Rca Mueller-Cajar et al. In the absence of photosynthetic activity dark period , the concentration of free RuBP is low and Rca populates a helical assembly with no ATPase activity, avoiding unnecessary ATP consumption.

Activation of photosynthesis results in the accumulation of free RuBP, reaching millimolar concentration Von Caemmerer and Edmondson, Free RuBP binds to Rca, inducing its rearrangement to the catalytically competent hexamer. The active Rca hexamer interacts with inhibited Rubisco via its highly conserved top surface and concomitantly transiently pulls the extended C-terminal tail of the RbcL subunit into the central pore CP.

This action is mediated by the ATPase activity of Rca and results in the destabilization of the Rubisco active site, releasing the inhibitory sugar phosphate. Rca is displayed as in C.

The RbcL C-termini are drawn as lines in red. In the absence of RuBP, RsRca forms spiral-shaped high molecular weight assemblies that are largely ATPase inactive and may represent a storage form when the organism is not photosynthetically active Mueller-Cajar et al. Thus, the generation of RuBP during photosynthesis would induce the conversion of this storage form into functional hexamers Figure 3D. Biochemical and mutational analysis showed that remodeling of Rubisco depends on the canonical pore loops and the conserved top surface of the hexamer Mueller-Cajar et al.

Moreover, reactivation of R. These findings suggest that RsRca docks onto Rubisco with its top surface and the pore loops transiently pull the C-terminal tail of RbcL into the central pore, to facilitate opening of the active site pocket and release the inhibitory sugar phosphate Figure 3E. This mechanism resembles the threading of ssrA-tagged proteins through the central pore of the bacterial ClpX for degradation by the ClpP protease Olivares et al.

Interestingly, the red alga Cyanidioschyzon merolae , containing Rubisco form ID Figure 2B , has two cbbX genes, one nuclear-encoded and one plastid-encoded Loganathan et al. It was recently shown that the functional CmRca is a hetero-hexamer between nuclear- and plastid-encoded subunits Loganathan et al. Structural and biochemical characterization showed that these proteins function as bipartite complexes consisting of the hexameric CbbQ activase AfRcaI; HnRca with CbbO as a co-factor Sutter et al.

This suggests that a two-step conformational change in the activase hexamer leads to optimal ATPase activity for Rubisco reactivation. Figure 4. The prokaryotic Rca of the green-type form IA Rubisco. A Schematic representation of the domain structure of Rca from H. Alternating subunits shown in two shades of blue. The hexameric HnRca is displayed as in C. The RbcL C-termini are represented by blue lines.

This suggests that the interaction of AfRcaI with the RbcL C-terminus is functionally critical, similar to the mechanism of red-type Rca described above.

However, AfRcaI and HnRca do not have the canonical pore loop residues known to be involved in threading of flexible sequences into the central pore Hanson and Whiteheart, ; Olivares et al. Accordingly, mutating these residues did not result in loss of function Tsai et al.

In the current model, CbbO acts as an adapter between the activase and Rubisco. Whether and how a pulling force is involved in remodeling remains to be investigated. Interestingly, A. The form II Rubisco of A. Almost three decades after the discovery of Rca in A. The sequences of these activases are longer than those of the Rca enzymes described above.

The N-domain is required for targeting Rca to Rubisco Esau et al. The C-terminal extension is critical for the constitutive ATPase activity and mutation of tyrosine results in loss of the ATPase and activase function Stotz et al.

Higher plants, including A. The isoforms are either expressed from separate genes or result from alternate splicing. Under oxidizing conditions, generally at night in the absence of photosynthesis, disulphide bond formation in the C-terminal extension inhibits ATP binding and thus Rubisco activation Shen and Ogren, ; Zhang and Portis, ; Zhang et al.

Figure 5. The eukaryotic Rca of the green-type form IB Rubisco. A Schematic representation of the domain structure of Rca from N. The unfilled electron density at the top of the hexamer probably represents the N-domains. Alternating subunits are shown in two shades of green and the specificity helix H9 in purple.

The Rca hexamer interacts with inhibited Rubisco via the N-domain and H9 recognizes the exposed basic residue Arg89 dark green on the RbcL subunit. The hexameric NtRca is displayed as in C. The RbcL C-termini are shown as green lines. Plant Rca enzymes have been reported to populate a range of dynamic oligomeric states in vitro , but are active as hexamers, as shown for the Rca enzymes of N. Analysis of the NtRca by electron microscopy revealed the position of the N-domains at the top of the hexamer Stotz et al.

In the crystal structure of AtRca the N-domain was disordered Hasse et al. Stable hexamers of NtRca were generated by mutation of arginine to valine at the interface between adjacent subunits. NtRca and AtRca do not contain the canonical pore loop motif aromatic-hydrophobic-glycine. Instead, three conserved loop segments face the central solvent channel and mutational analysis of NtRca implicates all of them in Rubisco remodeling Stotz et al.

Based on the currently available structural and biochemical data, NtRca recognizes the inhibited Rubisco via the N-domain, with species specificity being imparted by helix H9. We were able to synthesise a synthetic scaffold protein and use interacting sticky domains successfully to attach Rubisco and CA to the scaffold.

The second step was to assemble this synthetic complex in a cyanobacterium a photosynthetic bacterium using genetic transformation. This has been achieved and future work will determine if the scaffolded complex is able to improve photosynthesis. Sectors Agriculture, Food and Drink. Results and Impact Talk on synthetic biology approaches to improve photosynthesis.

Prompted discussion amongst colleagues on this topic. This workshop was aimed at students with an interest in pursuing a career in biological sciences, who were largely from non-traditional University attending backgrounds. None no actual impacts realised to date Year s Of Engagement Activity Nicholas Smirnoff Principal Investigator. Nicholas James Harmer Co-Investigator. Steven Lee Porter Co-Investigator.

Ron Yang Co-Investigator. Our aim was to carry out a feasibility study for a synthetic biology approach to improving photosynthesis. Agriculture, Food and Drink. Talk on synthetic biology approaches to improve photosynthesis. A one-hour workshop was held for a group of approximately 30 year 9 students on synthetic biology at the University of Exeter.

None no actual impacts realised to date.