Amino acid stimulation of carbon dioxide fixation in cell-free extracts of Chlamydomonas reinhardtii.

In Chlamydomonas reinhardtii the synthesis of glutamate and aspartate during photosynthesis has been shown to be closely associated with the Krebs cycle (2). The addition of P-enolpyruvate but not pyruvate was found to enhance greatly the incorporation of "4CO2 in cell-free preparations of C. reinhardtii (3), suggesting that P-enolpyruvate carboxylase was important in CO2 fixation in this alga in replenishing Krebs cycle intermediates. One of the functions of P-enolpyruvate carboxylase is to maintain an operative Krebs cycle by maintaining adequate levels of oxaloacetate for the continued generation of the cycle at times when aspartate and glutamate are being synthesized (5). Such an anaplerotic mechanism has been reported in bacteria where it was also found that acetyl-CoA stimulated P-enolpyruvate carboxylase activity but that aspartate inhibited the enzyme (7, 8). Little