TEACHING TOOLS IN PLANT BIOLOGYTM: LECTURE NOTES Carbon-Fixing Reactions of Photosynthesis

Photosynthesis in plants converts the energy of sunlight into chemical energy. Although photosynthesis involves many proteins and catalytic processes, it often is described as two sets of reactions: the light-dependent reactions and the carbonfixing reactions. Plants do not have a monopoly on photosynthesis, which also occurs in other photosynthetic eukaryotes and several species of bacteria. In a separate Teaching Tool, The Light-Dependent Reactions of Photosynthesis, we describe how the energy of sunlight is converted to chemical energy and ultimately stored as ATP and NADPH. Here, we describe how these energy-storing compounds are used in the reduction of carbondioxide (CO2) to formsugars, startingwith the key enzyme of carbon fixation and one of the most intriguing of all enzymes, ribulose-1,5-bis-phosphate carboxylase/oxygenase, alsoknown as Rubisco. Rubisco is the key enzyme of the photosynthetic carbon fixing reactions and is responsible for the fixation into organic form of ;100 Gt (1 Gigaton [Gt] 5 1 petagram [Pg] 5 1015 g) of carbon annually. This annual turnover represents approximately oneseventh of the total amount of atmospheric CO2. In spite of this impressive figure, Rubisco is a notoriously inefficient enzyme. Rubisco’s lowefficiencyand thegreatdemand forphotosynthetic carbon fixationmean that Rubisco is themost (or nearly themost) abundant protein in the world. Rubisco’s inefficiency is due to both its slow reaction rate and its inability to fully discriminate between CO2 and O2. Rubisco’s oxygenation reaction forms an inhibitoryproduct thatmustbe removedand recycled through the process of photorespiration. To minimize this, many organisms concentrate CO2 upstream of Rubisco, through structural or biochemical processes. About 10% of terrestrial plants use the enzyme phosphoenolpyruvate carboxylase (PEPC) for the primary carboxylation reaction, upstream of Rubisco. Two similar PEPC-based carboxylation strategies have evolved,C4 andCAM (Crassulacean acid metabolism). This lesson introduces the carbon-fixing reactions and their variations and explains how a greater understanding of Rubisco, photorespiration, C4, and CAM can contribute to yield enhancements of crop and biofuel plants. It also addresses how and why plants are affected by rising atmospheric CO2 levels.