Autotrophic Carbon Dioxide Fixation

Abstract

Carbon dioxide fixation is overwhelmingly carried out by reactions of the Calvin-Benson cycle of plants, green algae, and cyanobacteria. Three other carbon dioxide reduction pathways are known to allow autotrophic growth, but these are mainly limited to anaerobic organisms. In this chapter the anaerobic autotrophic carbon reduction pathways are briefly described followed by a more detailed look at the Calvin-Benson cycle and its regulation. The Calvin-Benson cycle reaction sequence is similar to the non-oxidative branch of the pentose phosphate pathway although the enzyme transaldolase of the pentose phosphate pathway is not used and a novel enzymatic activity, sedoheptulose 1,7-bisphosphatase is substituted. The carbon fixation enzyme of the Calvin-Benson cycle, Rubisco, varies in its properties and is currently the subject of much research aimed at improving the efficiency of photosynthesis. Rubisco exists in three different conformations and there is also a gene coding for a related protein that does not have Rubisco activity but could be the evolutionary progenitor. The Calvin-Benson cycle is often referred to as the dark reactions of photosynthesis but it does not proceed in darkness for several reasons. In addition to providing reducing power and ATP, photosynthetic electron transport causes increased pH and magnesium in the stroma plus reducing power for thioredoxin to activate Calvin-Benson cycle enzymes and inactivate enzymes that would lead to futile cycles when Calvin-Benson cycle enzymes are active. There is a surprising regulation of phosphoglucose isomerase that can shift the regulation of the rate of starch synthesis from ADPglucose pyrophosphorylase to phosphoglucose isomerase. This may control the amount of carbon that leaves the Calvin-Benson cycle, since excessive loss of carbon from the Calvin-Benson cycle intermediates can lead to a collapse of the cycle. Finally, a recently discovered regulation mechanism based on a small protein called CP12 is discussed