## 3 The Rubisco's Secondary, Tertiary and Quaternary Structure
#### The Rubisco Small Subunit  
As the L subunit guests the enzyme active pocket, the S-subunit (ranging about 12-18 kDa among different Rubisco enzyme) is found only in form I Rubisco and is proposed to play both a structural and a functional role.  
The primary structure for a series of S subunits from different organisms is reported in Figure S2 including the sequence of the secondary structure elements shown in ribbon representation in Figure S4 (orange ribbon) [22]. The figure evidences location of the \(\beta\)A-\(\beta\)B loop while the secondary structure elements of the L subunit functionally related to the S subunit are evidenced in green and yellow color. The \(\beta\)A-\(\beta\)B loop of each S subunit constitute an extension forming a \(\beta\)-hairpin. The \(\beta\)A-\(\beta\)B loops of four S subunits surround the opening of the solvent channel that transverses the L\({}_{8}\) octamer along the axis of four-fold symmetry (Figure 1a).  
Structurally the S-subunit is involved in stabilizing the holoenzyme quaternary structure [23] and possibly modifying the conformation of the enzyme. In this respect, [PERSON] et al., have shown that the S subunit play a decisive role in assembly of the red-type Rubisco from _Rhodobacter__shareoides_[24]. Concerning its functional role, the S-subunit is shown to affect several kinetic properties as the enzymatic catalytic efficiency and S\({}_{\text{C/O}}\) specificity (see Table S3). Moreover, [PERSON] and co-workers have recently shown that overexpression of S subunit from C4 plants (_Pennisetumpurpeum_ and _Megathyrsus maximus_) in rice (a C3 plant) significantly enhances the \(k_{cat}\) value of the C3 Rubisco [25]. Analogously, expression of the small subunit of _Sorghum_ (a C4 plant) in rice produces a Rubisco with higher catalytic rate when compared to rice [26]. Very recently [PERSON] et al., achieved expression of tobacco Rubisco in _E.coli_ assembling enzyme complexes with different small subunits including a small subunit from trichome. The new technique allowed precise evaluation of the effects of different small subunits on the Rubisco kinetic parameters [27]. The Whitney group has produced new tobacco genotypes encoding both large and small subunits in  
Figure 3: **(a)** Ribbon representation showing the quaternary structure of the L\({}_{2}\)S\({}_{2}\) unit of Rubisco from _Galdieria sulphuraria_ viewed down the two-fold axis. The two L subunits are represented in green (L1) and yellow (L2) with the highlighted N-terminal (orange) and C-terminal (red) regions. S1 and S2 subunits are represented in blue; **(b)** close-up of the L1 subunit (heavenly color) evidencing the TIM barrel domain. N- and C-terminal tails are marked in orange and red, respectively. Loop numbering is highlighted. N-terminal of the L2 subunit (L2 N-term small domain, represented in yellow color) is located at the mouth of the TIM barrel of the L1 subunit. Each L subunit guests one active site. In **(b)** the substrate mimic 2-carboxy-D-arabinitol 1,5-bisphosphate (2 CABP) is shown as a bond-stick model and pointed by a black arrow. The CO\({}_{2}\)-Lys201 residue is also pointed by a black arrow. (Figure from ref. [15], adapted and used by permission of PNAS (copyright © 2012).  
the chloroplast genome, producing a bioengineering chassis useful in evaluation of novel Rubisco complexes [28].