of the Calvin cycle (Boyer 2006). This enzyme was discovered by Melvin Calvin at
Berkeley in the 1940s (Vapaavuori 1986). Substrates such as carbon dioxide (CO 2 ),
Ribulose 1,5 bisphosphate (RuBP), and water are required from the surrounding
environment for the function of Rubisco (Ashida et al. 2003).
The substrates gather at a specific location of the active site of the enzyme
Rubisco, a conformation change takes place in the enzyme and CO 2 reacts with
RubBP. As a result, a highly unstable six-carbon compound intermediate is formed.
The instability of this reaction intermediate, it splits into two molecules of 3phosphoglycerate (3GP) (Vapaavuori 1986). About one sixth of the 3GP is utilized
in the formation of sugars, which is considered as the output of the cycle. The active
site of Rubisco is large and can accept both carbon dioxide and O 2 as substrate. The
affinity of the O 2 molecule for the active site is higher and therefore its rate of
binding is higher than CO 2 (Ashida et al. 2005). Different evolutionary traits in
different species have been encouraged by this disorganization in the molecule
(Alfreider et al. 2003).
Carboxysomes which are small vesicles that have concentrated levels of Rubisco
have been developed in bacteria cells (Pichard and Campbell 1997). To maximize
the exposure of CO 2 to Rubisco, CO 2 is transferred from the outside environment
into the carboxysome. The oxygen is more understood in the higher plant world.
There are four recognized forms of Rubisco, with forms I–III being true
carboxylating Rubisco enzymes. Form IV which is also referred to as Rubisco-like
protein (RLP) is found in many bacteria and archaea. RLPs which are structurally
related to the true Rubiscos do not function as Rubisco enzymes, but instead catalyze
different reactions in sulfur metabolism (Hanson and Tabita 2000). Form I Rubisco
which is found in the cells of most autotrophic bacteria, algae, and terrestrial plants,
is a hexadecamer and is made of eight large and eight small subunits (L 8 S 8 ).
Catalytic function is found only in the large subunit. It is further divided into two
types, “green-like” and “red-like” and the large subunits of these two types differ in
their amino acid composition (Tabita et al. 2007). The green-like Rubiscos are
commonly found in the chloroplasts of terrestrial plants; green algae; cyanobacteria;
and representatives of α, β, and γ proteobacteria. The red-like Rubiscos are present in
most non-green algae and some representatives of α and β proteobacteria. In
Rhodobacter azoto formans, both green-like and red-like enzymes have been
found (Tabita et al. 2008).
Form II Rubisco is found only in bacteria. It has 25–30% amino acid sequence
identity to form I, consists of only large subunits (Ln), from two to eight, depending
on the organism, and is found in photo- and chemoautotrophs. Form III Rubisco
which is only found in archaea, and its metabolic role is unclear. Form IV which is
designated a Rubisco-like protein because its sequence is most closely related to
Rubisco and it is not involved in the fixation of CO 2 in Calvin cycle. Table 11.1
shows the recognized presence, structure, and function of four related Rubisco
enzyme forms in archaea, bacteria, and eukaryotes.
Both CO 2 (in Calvin cycle for carbon fixation) and O 2 (photorespiration) can be
used by Rubisco as substrate. Some Rubisco have been selected for greater CO 2
specificity. Specificity factors are expressions of relative specificities for CO 2 versus
244
R. K. Bharti et al.
Berkeley in the 1940s (Vapaavuori 1986). Substrates such as carbon dioxide (CO 2 ),
Ribulose 1,5 bisphosphate (RuBP), and water are required from the surrounding
environment for the function of Rubisco (Ashida et al. 2003).
The substrates gather at a specific location of the active site of the enzyme
Rubisco, a conformation change takes place in the enzyme and CO 2 reacts with
RubBP. As a result, a highly unstable six-carbon compound intermediate is formed.
The instability of this reaction intermediate, it splits into two molecules of 3phosphoglycerate (3GP) (Vapaavuori 1986). About one sixth of the 3GP is utilized
in the formation of sugars, which is considered as the output of the cycle. The active
site of Rubisco is large and can accept both carbon dioxide and O 2 as substrate. The
affinity of the O 2 molecule for the active site is higher and therefore its rate of
binding is higher than CO 2 (Ashida et al. 2005). Different evolutionary traits in
different species have been encouraged by this disorganization in the molecule
(Alfreider et al. 2003).
Carboxysomes which are small vesicles that have concentrated levels of Rubisco
have been developed in bacteria cells (Pichard and Campbell 1997). To maximize
the exposure of CO 2 to Rubisco, CO 2 is transferred from the outside environment
into the carboxysome. The oxygen is more understood in the higher plant world.
There are four recognized forms of Rubisco, with forms I–III being true
carboxylating Rubisco enzymes. Form IV which is also referred to as Rubisco-like
protein (RLP) is found in many bacteria and archaea. RLPs which are structurally
related to the true Rubiscos do not function as Rubisco enzymes, but instead catalyze
different reactions in sulfur metabolism (Hanson and Tabita 2000). Form I Rubisco
which is found in the cells of most autotrophic bacteria, algae, and terrestrial plants,
is a hexadecamer and is made of eight large and eight small subunits (L 8 S 8 ).
Catalytic function is found only in the large subunit. It is further divided into two
types, “green-like” and “red-like” and the large subunits of these two types differ in
their amino acid composition (Tabita et al. 2007). The green-like Rubiscos are
commonly found in the chloroplasts of terrestrial plants; green algae; cyanobacteria;
and representatives of α, β, and γ proteobacteria. The red-like Rubiscos are present in
most non-green algae and some representatives of α and β proteobacteria. In
Rhodobacter azoto formans, both green-like and red-like enzymes have been
found (Tabita et al. 2008).
Form II Rubisco is found only in bacteria. It has 25–30% amino acid sequence
identity to form I, consists of only large subunits (Ln), from two to eight, depending
on the organism, and is found in photo- and chemoautotrophs. Form III Rubisco
which is only found in archaea, and its metabolic role is unclear. Form IV which is
designated a Rubisco-like protein because its sequence is most closely related to
Rubisco and it is not involved in the fixation of CO 2 in Calvin cycle. Table 11.1
shows the recognized presence, structure, and function of four related Rubisco
enzyme forms in archaea, bacteria, and eukaryotes.
Both CO 2 (in Calvin cycle for carbon fixation) and O 2 (photorespiration) can be
used by Rubisco as substrate. Some Rubisco have been selected for greater CO 2
specificity. Specificity factors are expressions of relative specificities for CO 2 versus
244
R. K. Bharti et al.