page_title: RuBisCO
section_title: Structure
hierarchical_section_title: RuBisCO / Structure
caption_attribution_description: English: Active site of RuBisCO of Galdieria sulphuraria with CO2. Residues involved in either the active site or stabilizing CO2 for enzyme catalysis are shown in color and labeled. Distances of the hydrogen bonding interactions are shown in Angstroms. Mg2+ ion (green sphere) is shown coordinated to CO2, and is followed by three water molecules (red spheres). All other residues are placed in grayscale.
context_page_description: Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations Rubisco, rubisco, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which the atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose. In chemical terms, it catalyzes the carboxylation of ribulose-1,5-bisphosphate. It is probably the most abundant enzyme on Earth.
context_section_description: In plants, algae, cyanobacteria, and phototrophic and chemoautotrophic proteobacteria, the enzyme usually consists of two types of protein subunit, called the large chain (L, about 55,000 Da) and the small chain (S, about 13,000 Da). The large-chain gene (rbcL) is encoded by the chloroplast DNA in plants. There are typically several related small-chain genes in the nucleus of plant cells, and the small chains are imported to the stromal compartment of chloroplasts from the cytosol by crossing the outer chloroplast membrane. The enzymatically active substrate (ribulose 1,5-bisphosphate) binding sites are located in the large chains that form dimers in which amino acids from each large chain contribute to the binding sites. A total of eight large-chains (= 4 dimers) and eight small chains assemble into a larger complex of about 540,000 Da. In some proteobacteria and dinoflagellates, enzymes consisting of only large subunits have been found.
  Magnesium ions (Mg²⁺
  ) are needed for enzymatic activity. Correct positioning of Mg²⁺
   in the active site of the enzyme involves addition of an "activating" carbon dioxide molecule (CO
  2) to a lysine in the active site (forming a carbamate). Mg²⁺ operates by driving deprotonation of the Lys210 residue, causing the Lys residue to rotate by 120 degrees to the trans conformer, decreasing the distance between the nitrogen of Lys and the carbon of CO
  2. The close proximity allows for the formation of a covalent bond, resulting in the carbamate.  Mg²⁺ is first enabled to bind to the active site by the rotation of His335 to an alternate conformation. Mg²⁺ is then coordinated by the His residues of the active site (His300, His302, His335), and is partially neutralized by the coordination of three water molecules and their conversion to ⁻OH. This coordination results in an unstable complex, but produces a favorable environment for the binding of Mg²⁺. Formation of the carbamate is favored by an alkaline pH. The pH and the concentration of magnesium ions in the fluid compartment (in plants, the stroma of the chloroplast) increases in the light. The role of changing pH and magnesium ion levels in the regulation of RuBisCO enzyme activity is discussed below. Once the carbamate is formed, His335 finalizes the activation by returning to its initial position through thermal fluctuation.
caption: Active site of RuBisCO of Galdieria sulphuraria with CO2. Residues involved in both the active site and stabilizing CO2 for enzyme catalysis are shown in color and labeled. Distances of the hydrogen bonding interactions are shown in Angstroms. Mg2+ ion (green sphere) is shown coordinated to CO2, and is followed by three water molecules (red spheres). All other residues are placed in grayscale.
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