Ribulose bisphosphate carboxylase/oxygenase (Rubisco) is a key enzyme in photosynthesis catalyzing corbon dioxide fixation. Rubisco is ubiquitus for photosynthetic organisms and is regarded as the most abundant protein on earth. More than 90% of the inorganic carbon that is converted into biomass is fixed by the enzyme RubisCO that catalyzes the carboxylation and cleavage of ribulose-1,5-bisphosphate (RuBP) into two molecules of 3-phosphoglycerate (3PG). RubisCO is found in all three domains of life: bacteria, archaea and eukaryotes. The enzyme makes up 30-50% of the soluble protein in plant leaf.
Structure
Rubisco enzymes are multimeric having two different types of subunits catalytic large (L, 50–55 kDa), and small (S, 12–18 kDa) subunits. Different molecular forms of Rubisco are distinguished by the presence or absence of the small subunit. The most common form (form I) of Rubisco is composed of large and small subunits in a hexadecameric structure, L8S8. This form is present in most chemoautotrophic bacteria, cyanobacteria, red and brown algae, and in all higher plants.
Form I Rubisco consists
of a core of four L2 dimers arranged around a 4-fold axis, capped at each end
by four small subunits. The small subunit is not essential for catalysis,
because the large subunit octamer still retains some carboxylase activity. The form II enzyme is a dimer of
large subunits (L2)n and lacks small subunits.
The form II enzyme was initially discovered in purple non-sulphur bacteria, but
has also been found in several chemoautotropic bacteria. Several non-sulphur
phototropic bacteria, i.e. Rhodobacter sphaeroides, R. capsulatus, and Hydrogenovibrio marinus contain
both form I and form II enzymes. RbcL sequences have
also been identified in archaea and assigned to a separate group, form III.
With respect to quaternary structure, the archaea are diverse and comprise L2,
L8, and L10 enzymes. The crystal structure of Rubisco from Pyrococcus horikoshii consists of an octamer of
large subunits, L8 (PDB codes 2cxe, 2cwx, 2d69). Despite apparent differences in amino acid sequence and
function, the secondary structure of the large (catalytic) subunit is extremely
well conserved throughout different forms of Rubisco. The active site is
located at the intra-dimer interface between the carboxy-terminal domain of one
large subunit and the amino-terminal domain of the second large subunit in the
L2 dimer. In the hexadecameric molecule, the dimers are arranged such that the
eight active sites face the outside solvent. Two loop regions in the
amino-terminal domain of the second large subunit in the dimer contribute
additional residues to the active site. Thus, the functional unit of Rubisco is
an L2 dimer of large subunits containing two active sites. The substrate binds
in an extended conformation across the opening of the α/β-barrel and is
anchored at two distinct phosphate-binding sites at opposite sides of the
α/β-barrel and in the middle at the magnesium-binding site. The small subunit
is more diverse. The function of the small subunit is enigmatic. Its structural
arrangement, covering a substantial area at two opposite ends of the L-subunit
octamer makes it reasonable to assume a structural function of the small subunit. Studies of interspecific hybrid
enzymes have indicated that small subunits are required for maximal catalysis
and, in several cases, contribute to CO2/O2 specificity.
Although small-subunit genetic engineering remains difficult in land plants,
directed mutagenesis of cyanobacterial and green-algal genes has identified
specific structural regions that influence catalytic efficiency and CO2/O2 specificity.
The Rubisco large subunit is encoded by a single
gene in the chloroplast genome and is synthesized by the plastid ribosome. From a nutritional point of view, the large subunit of
Rubisco has an exceptionally ideal composition of essential amino acids among
plant proteins. Therefore, plant Rubisco is expected to be a large source of
food protein in the future. In plants, the
small subunit is coded by a family of closely related nuclear genes and
synthesized in the cytosol. The synthesis and assembly of the Rubisco holoenzyme,
involving the co-ordinated control of chloroplastic and cytosolic processes,
have been shown to require the assistance of ancillary proteins termed
molecular chaperones.
Function
The main function of RubisCO is in
photosynthesis and photorespiration. It catalyses the first step of carbon fixation
in the C3 pathway or Calvin cycle, i.e. carboxylation of RuBP. It results in
the formation of 2 molecules of 3-PGA. RuBisCO also has an affinity for oxygen so it
binds to some amount of O2 in the process known as
photorespiration. It leads to the conversion of RuBP to one molecule each of
phosphoglycerate and phosphoglycolate. Since the affinity of RubisCO is much higher
for CO2 than for O2, photosynthesis is preferred
over photorespiration. RubisCO catalyses the first step of carbon fixation in
the Calvin cycle. Calvin cycle occurs in all plants, i.e. C3, C4 and
CAM. The first step of the Calvin cycle is carboxylation. RuBP is a 5-C
compound. It is carboxylated by utilising CO2 and then C-C bond
cleavage results in the formation of 2 molecules of 3-PGA.
The reaction involves enolisation of RuBP followed by carboxylation, which leads to the formation of an intermediate 3-keto-2′-carboxyarabinitol-1,5-bisphosphate. It is followed by hydration, and then subsequent cleavage of the bond between two carbons to give rise to 2 molecules of 3-phosphoglycerate (3-PGA). The 3-PGA thus formed is utilised in the formation of glucose and other carbohydrates in the subsequent steps. In C3 plants, this process occurs in the mesophyll cells. In the C4 pathway, the Clavin cycle occurs in the bundle sheath cells. The bundle sheath cells are rich in RubisCO. This is an adaptation to reduce photorespiration in C4 plants. RubisCO also has an affinity for oxygen and it oxygenates RuBP in the presence of oxygen. Photorespiration utilises ATP, hence, leads to the wasting of some energy produced in photosynthesis. When RubisCO binds to O2 it converts RuBP to one molecule of phosphoglycerate (3C) and phosphoglycolate (2 Carbon) each. It is a waste process, it neither generates ATP nor sugar.
Reference
Inger Andersson, Anders Backlund, Structure and
function of Rubisco, Plant Physiology and Biochemistry, Volume 46, Issue
3,2008, Pages 275-291, ISSN 0981-9428, https://doi.org/10.1016/j.plaphy.2008.01.001.
Robert J Spreitzer, Role of the small subunit in
ribulose-1,5-bisphosphate carboxylase/oxygenase, Archives of Biochemistry and
Biophysics,Volume 414, Issue 2, 2003, Pages 141-149, ISSN 0003-9861, https://doi.org/10.1016/S0003-9861(03)00171-1.