Video narration
How does Rubisco get ready before it fixes carbon? Think of preparing a reusable lab instrument before a measurement: readiness comes first. In the common plant form, Rubisco has eight large and eight small subunits, arranged around active sites. Together, these subunits form the protein; the active sites are where the reaction starts. At an active site, CO2 binds to a lysine in the protein. That lets Mg2+, a magnesium ion, join and prepare the site. Here's the twist: that first CO2 is not the carbon being fixed. RuBP, the molecule that receives carbon, then enters. A second CO2 is added, forming an unstable six-carbon product that splits into two molecules of 3-PGA, the first stable products of carbon fixation. So one CO2 and Mg2+ activate Rubisco; then it adds a second CO2 to RuBP, yielding two 3-PGA molecules.