Carboxylation

Carboxylation is the CO2-fixing step that starts the Calvin cycle. The 5-carbon sugar ribulose-1,5-bisphosphate (RuBP) — a pentose carrying two phosphate groups — reacts with a molecule of CO2, catalysed by the enzyme RuBP carboxylase (Rubisco, which makes up about 16% of chloroplast protein by mass), to form an unstable 6-carbon intermediate. This intermediate is so unstable that it splits immediately into two molecules of the 3-carbon compound 3-phosphoglyceric acid (3-PGA) — the na …