This coordination results in an unstable complex, but produces a favorable environment for the binding of Mg 2+. Mg 2+ 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. Mg 2+ is first enabled to bind to the active site by the rotation of His335 to an alternate conformation. The close proximity allows for the formation of a covalent bond, resulting in the carbamate. Mg 2+ 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. Correct positioning of Mg 2+ 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). Magnesium ions ( Mg 2+) are needed for enzymatic activity. In some Pseudomonadota and dinoflagellates, enzymes consisting of only large subunits have been found. A total of eight large chains (= four dimers) and eight small chains assemble into a larger complex of about 540,000 Da. 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. 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 large-chain gene ( rbcL) is encoded by the chloroplast DNA in plants. In plants, algae, cyanobacteria, and phototrophic and chemoautotrophic Pseudomonadota (formerly 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). rbcL is one of the 21 protein-coding genes involved in photosynthesis (green boxes). Location of the rbcL gene in the chloroplast genome of Arabidopsis thaliana (positions ca. All other residues are placed in grayscale. Mg 2+ ion (green sphere) is shown coordinated to CO 2, and is followed by three water molecules (red spheres). Distances of the hydrogen bonding interactions are shown in angstroms. Structure Active site of RuBisCO of Galdieria sulphuraria with CO 2: Residues involved in both the active site and stabilizing CO 2 for enzyme catalysis are shown in color and labeled. Given its important role in the biosphere, the genetic engineering of RuBisCO in crops is of continuing interest (see below). Reflecting its importance, RuBisCO is the most abundant protein in leaves, accounting for 50% of soluble leaf protein in C 3 plants (20–30% of total leaf nitrogen) and 30% of soluble leaf protein in C 4 plants (5–9% of total leaf nitrogen). Phosphoenolpyruvate carboxylase, unlike RuBisCO, only temporarily fixes carbon. While many autotrophic bacteria and archaea fix carbon via the reductive acetyl CoA pathway, the 3-hydroxypropionate cycle, or the reverse Krebs cycle, these pathways are relatively small contributors to global carbon fixation compared to that catalyzed by RuBisCO. RuBisCO is important biologically because it catalyzes the primary chemical reaction by which inorganic carbon enters the biosphere. In chemical terms, it catalyzes the carboxylation of ribulose-1,5-bisphosphate (also known as RuBP). It is probably the most abundant enzyme on Earth. It emerged approximately four billion years ago in primordial metabolism prior to the presence of oxygen on earth. Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known by the abbreviations RuBisCo, rubisco, RuBPCase, or RuBPco, is an enzyme ( EC 4.1.1.39) involved in light-independent (or "dark") part of photosynthesis, including the carbon fixation by which atmospheric carbon dioxide is converted by plants and other photosynthetic organisms to energy-rich molecules such as glucose. The active site Lys175 residues are marked in pink, and a close-up of the residue is provided to the right for one of the monomers composing the enzyme. A 3d depiction of the activated RuBisCO from spinach in open form with active site accessible.
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