The Primary Acceptor of CO2

Let us ask the question that puzzled the early scientists studying the 'dark reaction': how many carbon atoms would a molecule need so that, after accepting (fixing) CO2, it would have three carbons (as in PGA)? A simple guess would be a 2-carbon compound (2 + 1 = 3), and indeed the scientists believed for a long time that, since the first product PGA is a C3 acid, the primary acceptor must be a 2-carbon compound. They spent many years trying to identify such a 2-carbon molecule.

The studies, however, showed something quite unexpected:

• The actual primary CO2 acceptor is a 5-carbon ketose sugar — ribulose 1,5-bisphosphate (RuBP).
• It took a long time and many experiments to reach this conclusion, correcting the earlier assumption of a 2-carbon acceptor. …