C4-dicarboxylic acid pathway

The C4 (dicarboxylic acid) pathway of photosynthetic carbon assimilation may be seen as a biochemical elaboration of the RPP cycle. In this pathway CO2 is transferred via the C-4 carboxyl of C4 acids to the reactions of the RPP cycle. Discovered in sugar cane, the pathway was first thought to be peculiar to tropical grasses but was later found in species of dicotyledons, Amaranthus (Amaranthaceae), and Atriplex (Chenopodiaceae). 

Hatch,M.D., Osmond,C.B. Compartmentation and transport in C4 photosynthesis. In Transport in plants III. Enzyclopedia of plant physiology. New series, Vol. 3. Stocking, C.R., Heber,U. (eds.), pp. 144-184. Berlin, Heidelberg, New York Springer 1976 Hatch, M.D., Slack, C.R. Studies on the mechanism of activation and inactivation of pyruvate, phosphate dikinase. A possible regulatory role for the enzyme in the C4 dicarboxylic acid pathway of photosynthesis. Biochem. J. 112,549-558 (1969)... 

Johnson, H.S., Hatch, M. D. Properties and regulation of leaf nicotinamide-adenine dinucleotide phosphate-malate dehydrogenase and malic enzyme in plants with the C4-dicarboxylic acid pathway of photosynthesis. Biochem. J. 779,173-280 (1970)... 

Laetsch,W.M. Chloroplast specialication on Dicotyledons possessing the C4-dicarboxylic acid pathway of photosynthetic CO2 fixation. Am. J. Bot. 55,875-883 (1968) Laetsch,W.M. The C4 syndrome a structural analysis. Ann. Rev. Plant Physiol. 25, 24-52 (1974)... 

Whelan,T., Sackett,W.M., Benedict, C.R. Enzymatic fractionating of carbon isotopes by phosphoenolpyruvate carboxylase from C4 plants. Plant Physiol. 51, 1051-1054 (1973) Whelan,T.W., Sackett,N., Benedict,C.R. Carbon isotope discrimination in a plant possessing the C4 dicarboxylic acid pathway. Biochem. Biophys. Res. Commun. 41, 1205-1210 (1970)... 

The ecophysiological significance of the C4 dicarboxylic acid pathway is still a matter of discussion. It is striking, though, that it is also found in a large number of halophytes. This has led to the assumption that the C4 dicarboxylic acids formed via the Hatch-Slack pathway might play a role in osmoregulation in these species. However, this is only one of several possibilities. 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

The difference between this route and the Calvin cycle lies in the fact that the CO2 is fixed, not into ribulose-l,5-diphosphate, but into phosphoenol pyruvate with the intermediary formation of C4 dicarboxylic acids. Hence the name, the Q dicarboxylic acid pathway. One of the C4 dicarboxylic acids transmits the CO2 further with the formation, ultimately, of 3-phosphoglyceric acid. 

Gluconeogenesis from Amino Acids. The pathway described above has a general significance beyond the utilization of lactate. We have mentioned previously that many amino acids can be converted to glucose, provided that they give rise to C4-dicarboxylic acids. These acids are members of the citrate cycle and thus can easily produce oxaloacetate and then phosphoenolpyruvate by Utter s reaction. 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

The second pathway is called the C4 cycle because COj is initially converted to the four-carbon dicarboxylic acids, malic or aspartic acids (Fig. 3.3). Phos-phoenolpyruvic acid (I) reacts with one molecule of CO2 to form oxaloacetic acid (II) in the mesophyll of the biomass, and then malic or aspartic acid (III) is formed. The Q acid is transported to the bundle sheath cells, where decarboxylation occurs to regenerate pyruvic acid (IV), which is returned to... 

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