In many plants that grow in the tropics (and in temperate-zone crop plants native to the tropics, such as maize, sugarcane, and sorghum) a mechanism has evolved to circumvent the problem of wasteful photorespiration. The step in which CO2 is fixed into a three-carbon product, 3-phosphoglycerate, is preceded by several steps, one of which is temporary fixation of CO2 into a four-carbon compound. Plants that use this process are referred to as C4 plants, and the assimilation process as C4 metabolism or the C4 pathway. Plants that use the carbon-assimilation method we have described thus far, in which the first step is reaction of CO2 with ribulose 1,5-bisphosphate to form 3-phosphoglycerate, are called C3 plants.
The C4 plants, which typically grow at high light intensity and high temperatures, have several important characteristics: high photosynthetic rates, high growth rates, low photorespiration rates, low rates of water loss, and a specialized leaf structure. Photosynthesis in the leaves of C4 plants involves two cell types: mesophyll and bundle-sheath cells (Fig. 20-23a). There are three variants of C4 metabolism, worked out in the 1960s by Marshall Hatch and Rodger Slack (Fig. 20-23b).
In plants of tropical origin, the first intermediate into which 14CO2 is fixed is oxaloacetate, a four-carbon compound. This reaction, which occurs in the cytosol of leaf mesophyll cells, is catalyzed by phosphoenolpyru-vate carboxylase, for which the substrate is HCO3, not CO2. The oxaloacetate thus formed is either reduced to malate at the expense of NADPH (as shown in Fig. 20-23b) or converted to aspartate by transamination:
Oxaloacetate + a-amino acid-> L-aspartate + a-keto acid
The malate or aspartate formed in the mesophyll cells then passes into neighboring bundle-sheath cells through plasmodesmata, protein-lined channels that connect two plant cells and provide a path for movement of metabolites and even small proteins between cells. In the bundle-sheath cells, malate is oxidized and decarboxylated to yield pyruvate and CO2 by the action of malic enzyme, reducing NADP+. In plants that use aspartate as the CO2 carrier, aspartate arriving in bundle-sheath cells is transaminated to form oxaloac-etate and reduced to malate, then the CO2 is released by malic enzyme or PEP carboxykinase. As labeling experiments show, the free CO2 released in the bundle-sheath cells is the same CO2 molecule originally fixed into oxaloacetate in the mesophyll cells. This CO2 is now fixed again, this time by rubisco, in exactly the same reaction that occurs in C3 plants: incorporation of CO2 into C-1 of 3-phosphoglycerate.
The pyruvate formed by decarboxylation of malate in bundle-sheath cells is transferred back to the mesophyll cells, where it is converted to PEP by an unusual enzymatic reaction catalyzed by pyruvate phosphate dikinase (Fig. 20-23b). This enzyme is called a dikinase because two different molecules are simultaneously phosphorylated by one molecule of ATP: pyruvate to PEP, and phosphate to pyrophosphate. The pyrophosphate is subsequently hydrolyzed to phosphate, so two high-energy phosphate groups of ATP are used in regenerating PEP. The PEP is now ready to receive another molecule of CO2 in the mesophyll cell.
The PEP carboxylase of mesophyll cells has a high affinity for HCO3 (which is favored relative to CO2 in aqueous solution and can fix CO2 more efficiently than can rubisco). Unlike rubisco, it does not use O2 as an alternative substrate, so there is no competition between CO2 and O2. The PEP carboxylase reaction, then, serves to fix and concentrate CO2 in the form of malate. Release of CO2 from malate in the bundle-sheath cells yields a sufficiently high local concentration of CO2 for rubisco to function near its maximal rate, and for suppression of the enzyme's oxygenase activity.
Once CO2 is fixed into 3-phosphoglycerate in the bundle-sheath cells, the other reactions of the Calvin cycle take place exactly as described earlier. Thus in C4 plants, mesophyll cells carry out CO2 assimilation by the C4 pathway and bundle-sheath cells synthesize starch and sucrose by the C3 pathway.
Three enzymes of the C4 pathway are regulated by light, becoming more active in daylight. Malate dehy-drogenase is activated by the thioredoxin-dependent reduction mechanism shown in Figure 20-19; PEP carboxylase is activated by phosphorylation of a Ser residue; and pyruvate phosphate dikinase is activated by dephosphorylation. In the latter two cases, the details of how light effects phosphorylation or dephos-phorylation are not known.
770 Chapter 20 Carbohydrate Biosynthesis in Plants and Bacteria
Mesophyll cell '
Mesophyll cell '
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