C4 pathway

CAM (Crassulacean Acid Metabolism) photosynthetic pathway A variant of the C4 pathway phosphoenolpyruvate fixes C02 in C4 compounds at night, and then, the fixed C02 is transferred to the ribulose bisphosphate of the Calvin cycle within the same cell during the day. Characteristic of most succulent plants, such as cacti. 

The loss of a halide from 2 (step C4) was shown to be a reversible reaction, which was more shifted toward the monohalide cation for X = Br than for X = Cl. The electrochemical steps E2 and E4 were studied using the Mo(V) complex [MoOCl(dtc)2]. The reduction of 2 follows the E1-C1-E2-C2 pathway and the oxidation of [MoO(dtc)2] the E3-C3-E4-C4 pathway [23]. 

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 C02 is fixed into a three-carbon product, 3-phosphoglycerate, is preceded by several steps, one of which is temporary fixation of C02 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 C02 with ribulose 1,5-bisphosphate to form 3-phosphoglycerate, are called C3 plants. 

Once C02 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 C02 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 dehydrogenase 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 dephosphorylation are not known. 

Holtum Winter, 1982) and during C4 development (e.g. Nelson Langdale, 1989). It is to be hoped that in addition to more work on gene structure and expression, additional work will be focused on the biochemistry and structure of the enzymes. Interest in the developmental aspects of the establishment of the C4 pathway, in plants such as Flaveria (Adams et al., 1986) that contain interfertile C3, C3/C4 intermediate, and C4 species, and in CAM as a water-conserving pathway, should be sufficient reason to do this. 

Izui, K., Ishijima, S., Yamaguchi, Y., Katagiri, F., Murata, T., Shi-gesada, K., Sugiyama, T. Katsuki, H. (1986). Cloning and sequence analysis of cDNA encoding active phosphoeno/pyruvate carboxylase of the C4-pathway from maize. Nucleic Acids Research 14, 1615-28. 

C4 pathway Under normal atmospheric conditions, rubisco adds C02 to ribulose 1,5-bisphos-... 

Details of the C4 pathway are shown in Fig. 6. The steps involved are as follows ... 

Answer In maize, C02 is fixed by the C4 pathway elucidated by Hatch and Slack. Phospho-enolpyruvate is rapidly carboxylated to oxaloacetate, some of which undergoes transamination to aspartate but most of which is reduced to malate in the mesophyll cells. Only after subsequent decarboxylation of labeled malate does 14C02 enter the Calvin cycle for conversion to glucose. The rate of entry into the cycle is limited by the rate of the rubisco-catalyzed reaction. 

Answer Malic enzyme catalyzes oxidative decarboxylation of a hydroxycarboxylic acid in the C4 pathway ... 

The Hatch-Slack C4 pathway, alluded to briefly in the song, miglit be dealt with separately in ihe next edition, if there is a demand for it). 

In the C4 pathway, CO2 in the form of HCO3- reacts with phosphoenol-pyruvate (PEP) via the enzyme PEP carboxylase located in the cytosol of the mesophyll cells (Fig. 8-15b).8 The initial product is oxaloacetate, which is rapidly converted to malate and aspartate. For all chloroplasts in photo-respiring (C3) plants, and for the chloroplasts in the bundle sheath cells of C4... 

Although the RPP cycle is the fundamental carboxylating mechanism, a number of plants have evolved adaptations in which CO2 is first fixed by a supplementary pathway and then released in the cells in which the RPP cycle operates. One of these supplementary pathways, the C4 pathway, involves special leaf anatomy and a division of biochemical labor between cell types. Plants endowed with this path-... 

In the following sections, we will first describe the RPP cycle, the C4 pathway and CAM. We will then discuss what is known of the regulation of these pathways and the way in which the activity of the RPP cycle is coordinated with the utilization of photosynthate. 

Unlike the RPP cycle in which carboxylation and carbon reduction are restricted to the chloroplast, the C4 pathway involves the interaction of two cell types and several different compartments within these cells. C4 plants are characterized by a radial leaf anatomy (Kranz anatomy) in which one cell type, the mesophyll cells, surrounds the other type, bundle sheath cells. This arrangement of the cell types and the division of labor between them is central to the functioning of the C4 pathway. Carbon dioxide is first captured in the outer tissues (mesophyll) and then transported to the inner tissues (bundle sheath) where CO2 and reducing... 

The initial step in the C4 pathway is the carboxylation of phosphoeno/pyruvate (PEP) in the cytoplasm of the mesophyll cells. The reaction is catalysed by PEP carboxylase (Eqn. 4). 

Subsequent metabolism of oxaloacetate (OAA) varies according to species. Three main types of C4 pathway are recognized, of which the most extensively studied is that shown by plants such as Zea mays (corn) (Fig. 2). In these plants (here called type-1 C4 plants) OAA is reduced to malate via NADP-malate dehydrogenase in mesophyll chloroplasts. Malate is then transported to bundle sheath chloroplasts and oxidatively decarboxylated by NADP-malic enzyme to produce pyruvate, CO2 and NADPH. Pyruvate is recycled to the mesophyll cells while the CO2 and NADPH are used in the RPP cycle in the bundle sheath chloroplast. The original C3 carbon acceptor (PEP) is regenerated from pyruvate in the mesophyll chloroplast by the activity of pyruvate, Pj dikinase [8] (Eq. 5). 

Two other types of C4 pathways are recognized. In type-2 plants, Atriplex spongiosa) and type-3 Panicum maximum) plants, malate is replaced by aspartate as the major C4 acid transported to the bundle sheath cells. After transport, aspartate is converted to OAA by transamination. In type-2 plants, OAA is reduced to malate, which in turn is decarboxylated by NAD-malic enzyme in the bundle sheath cell mitochondria to give NADH, CO2 and pyruvate. In type-3 plants, OAA is decarboxylated in the cytosol by PEP carboxykinase in the presence of ATP, yielding PEP, CO2 and ADP. The return of carbon to the mesophyll cells for regeneration of the CO2 acceptor occurs as pyruvate (or alanine to maintain nitrogen balance) in type-2 and as PEP (or again perhaps as alanine) in type-3. These variations in the C4 pathway are summarized in Table I (see also Ref. 14). 

Although in each type of C4 pathway there is an initial carboxylation catalyzed by PEP carboxylase, the plant s ability to produce a net increase in fixed carbon depends on subsequent release of CO2 and refixation by the RPP cycle. In this sense, the RPP cycle is still the fundamental carboxylating mechanism of these plants. It should be noted that C3 plants also contain a cytosolic PEP carboxylase which is capable of fixing CO2. However, C3 plants lack the biochemical and structural specialization as well as the division of labor between cell types that make possible the classical C4 type of photosynthesis. 

Evolution of chloroplasts Section 19,1.2 Evolutionary origins of photosynthesis Section 19,6 Evolution of the C4 pathway Section 20.2.3... 

The C4 Pathway of Tropical Plants Accelerates Photosynthesis by Concentrating Carbon Dioxide... 

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