Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle

Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle 

In addition to serving as a substrate, C02 activates ribulose-bisphosphate carboxylase by binding to the e-amino group of a lysyl residue in the large subunit to form a carbamate (lysine—NH—C02 ). This process requires Mg2+, which binds to the carboxyl group of the carbamate. The Mg2+ in turn forms part of the binding site for a second molecule of C02, which acts as the substrate in the carboxylase reaction. The reactive molecule of C02 probably adds to the enolate of ribulose-1,5-bisphosphate to form 2-carboxy-3-ketoarabinitol-l,5-bisphosphate as an intermediate, as shown in figure 15.26. 

The same active site of ribulose bisphosphate carboxylase also catalyzes a competing reaction in which 02 replaces C02 as a substrate. The products of this oxygenase reaction are 3-phosphoglycerate and a two-carbon acid, 2-phosphoglycolate (fig. 15.27). Phosphoglycolate is oxidized to C02 by 02 in additional reactions involving enzymes in the cytosol, mitochondria, and another organelle, the peroxisome. This photorespiration is not coupled to oxidative phosphorylation, and it appears to constitute a severe 

Photorespiration results from the oxygenation reaction catalyzed by ribulose bisphosphate carboxylase/ oxygenase. 2-Phosphoglycolate generated by the reaction moves from the chloroplast to the cytosol, where other enzymes break it down to C02, H20, and Pj. The oxygenation reaction, like carboxylation, does not require light directly. It occurs mainly during illumination, however, because the formation of the substrate, ribulose-1,5-bisphosphate, requires ATP and NADPH (see fig. 15.25). 

The outcome of the competition between the carboxylase and oxygenase reactions depends on the concentrations of C02 and 02. The Km of the activated enzyme for C02 is about 20 /xm, and that for 02 is about 200 /xM, so C02 is the preferred substrate. In illuminated chloroplasts, however, the concentration of 02 is elevated as a result of the photosynthetic oxidation of H20. The concentrations of the two gases are frequently on the order of the Km values, making 02 a serious competitor. Plants that live in dry climates face the additional dilemma that opening the stomata in the leaves to improve the exchange of C02 and 02 with the atmosphere can result in dehydration. 

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Flow of Electrons from H20 to NADP+ Drives Proton Transport into the Thylakoid Lumen Protons Return to the Stroma through an ATP-Synthase Carbon Fixation Utilizes the Reductive Pentose Cycle Ribulose-Bisphosphate Carboxylase-Oxygenase Photorespiration and the C-4 Cycle... 

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