<P> Ribulose - 1, 5 - bisphosphate carboxylase / oxygenase, commonly known by the abbreviations RuBisCO, RuBPCase, or RuBPco, is an enzyme involved in the first major step of carbon fixation, a process by which 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 (also known as RuBP). It is probably the most abundant enzyme on Earth . </P> <P> RuBisCO is important biologically because it catalyzes the primary chemical reaction by which inorganic carbon enters the organic biosphere . 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 smaller contributors to global carbon fixation than that catalyzed by RuBisCO . Phosphoenolpyruvate carboxylase, unlike RuBisCO, only temporarily fixes carbon . Reflecting its importance, RuBisCO is the most abundant protein in leaves, accounting for 50% of soluble leaf protein in C plants (20--30% of total leaf nitrogen) and 30% of soluble leaf protein in C plants (5--9% of total leaf nitrogen). Given its important role in the biosphere, the genetic engineering of RuBisCO in crops is of continuing interest (see below). </P> <P> 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 as shown in Figure 1 (above, right) 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 . </P> <P> Magnesium ions (Mg) are needed for enzymatic activity . Correct positioning of Mg in the active site of the enzyme involves addition of an "activating" carbon dioxide molecule (CO) to a lysine in the active site (forming a carbamate). 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 . </P>

What is the function of the protein rbcl
find me the text answering this question