## 6 Structure of the Rubisco \"Catalytic Active Site\"  
The inspection of the list of Rubisco's structure published by [PERSON] and [PERSON] in 2008 [13] evidences that a majority of the structural studies have dealt with Rubisco in complex with RuBP and Ca\({}^{2+}\)[33] or in complex with 2 CABP or XuRB (Scheme 2) and Mg\({}^{2+}\). Only one crystal structure of Rubisco in complex with RuBP and the Mg\({}^{2+}\) ion has been solved, probably because of an inherent instability of the RuBP-Mg\({}^{2+}\)-protein complex [34]. By contrast, 2-carboxy-D-arabinitol-1,5-bisphosphate (2 CABP) forms with the Mg\({}^{2+}\) ion an exchange-resistant complex which turned out to be useful in crystallographic studies [35]. The structure of 2-carboxy-D-arabinitol-1,5-bisphosphate (2 CABP) shown in Scheme 2 closely resembles the six-carbon intermediate 2-carboxy-3-keto-D-arabinitol-1,5-bisphosphate (3-keto-2-CABP) assumed to be the transient intermediate in the carboxylation reaction. Therefore, for a long time it was considered that the interaction of 2 CABP with Mg\({}^{2+}\) mimics the transition state of the metal-bound \(\beta\)-keto acid intermediate. Two capstone publications reporting the structure of Rubisco from spinach in complex with 2-carboxyarabinitol-bisphosphate (2 CABP) are those of [PERSON] et al., in 1990 [36] and [PERSON] et al., in 1996 (PDB code number 8 ruc) [35]. The two structures were resolved at 2.4 and 1.6 A, respectively. As amino acids taking part in catalysis are often directly involved in substrate interactions, a distance <4.0 A between amino acid residues and 2 CABP was used to identify amino acids as catalytically competent. As shown in Table 1[36], amino acids contributing to the catalytic pocket of Rubisco are located at the loops following the C-terminal ends of the \(\beta\)-strands (from \(\beta\)1 to \(\beta\)8 of the C-terminal domain) and at the N-terminal domain of the antiparallel L subunit. Amino acid residues represented in bold are strongly conserved among Rubiscos from diverse species. \begin{table}
\begin{tabular}{c c c c} \hline
**Loop** & **Amino Acid** & **Loop** & **Amino Acid** \\ \hline  & Thr173 & Loop 7 (C-terminal domain) & **Ser379** \\ Loop 1 (C-terminal domain) & **Lys175** & Loop 7 (C-terminal domain) & Gly380 \\ Loop 1 (C-terminal domain) & **Lys177** & Loop 7 (C-terminal domain) & Gly381 \\ Loop 2 (C-terminal domain) & **Lys201** & Loop 8 (C-terminal domain) & Gly403 \\ Loop 2 (C-terminal domain) & **Asp203** & Loop 8 (C-terminal domain) & Gly404 \\ Loop 5 (C-terminal domain) & **His294** & LoopA (N-terminal domain) & **Glu60** \\ Loop 5 (C-terminal domain) & **Arg295** & (N-terminal domain) & Thr65 \\ Loop 6 (C-terminal domain) & **His327** & (N-terminal domain) & Trp66 \\ Loop 6 (C-terminal domain) & Lys334 & LoopB(N-terminal domain) & **Asn123** \\ Loop 6 (C-terminal domain) & Leu335 & & \\ \hline \end{tabular}
\end{table} 
Table 1: A distance <4.0 Å between amino acid residues and 2 CABP was used to identify amino acids which were catalytically competent. The list denotes amino acids located at the C-terminal end of the \(\alpha\beta\)-barrel of L1 subunit and at the N-terminal end of the antiparallel L2 subunit. Amino acid residues represented in bold are strongly conserved among Rubisco proteins (Table from ref. [36], used by permission of Elsevier (copyright © 1990)). Figure 7 shows the structure of the active site of activated 2 CABP-bound Rubisco as proposed by [PERSON] et al., in 2008 ([PERSON] et al., 2008) (the structure has been plotted from the file 8 RUC deposited at the Protein Data Bank, Anderson, 1996 ([PERSON] et al., 2008)). According to this model, carbamylated Lys201 (KCX201), Asp203, Glu204, 2-carboxy-D-arabinitol-1,5-bisphosphate (2 CABP) and H\({}_{2}\)O complete the octahedral distorted geometry around Mg\({}^{2+}\). Moreover, Figure 7 evidences Asn123, Glu60 and Tyr20 residues (marked with an asterisk), located at loopA and loopB of the N-terminal domain of the L-partner subunit, interacting with the ligands of the first coordination sphere.