The Metabolic Pathway of CAM

The Metabolic Pathway of CAM Table 3.2. Proiiucts of dark and light CO2 fixation by Opuntia (data of Ting and Dugger, 1968) ... 

There is overwhelming evidence that high night temperatures inhibit malic acid accumulation in CAM plants, whereas low night temperatures have the opposite effect (see reviews of Wolf, 1960 Ranson and Thomas, 1960 see Chap. 4.3.1). Because malic acid is the end product of the metabolic pathway of dark CO2 fixation, it is reasonable to predict that CO2 exchange during the night would also be affected by temperature. 

An important problem which remains to be elucidated is the evolution of CAM. Is it truly an example of convergent evolution The occurrence of CAM in the gymnosperm Welwitschia and in ferns suggests a positive answer to that question. If so, are the specific metabolic details of CAM in each family similar or are there distinctive differences which are representative of the taxonomic categories In this context, did these plants with the CAM photosynthetic pathway actually all develop in the tropics and then move into arid environments of the subtropics and deserts of the world ... 

Finally, it should be made clear that the Cactaceae type of CAM shift is quite different from the shift reported in the Aizoaceae, Portulacaceae, and certain Crassulaceae. In the latter, the response to photoperiod and water stress appear to be a real shift in metabolic pathway. Not only do acids begin to fluctuate concomitant with gas exchange at night, but enzymes change quantitatively. Hence, the evidence suggests a reversible shift from C3 to CAM. 

These observations on Sedum acre suggest that CAM does not contribute considerably to net CO2 uptake of this plant. Carbon gain should be highest in the spring with cool days and water stress situations being rare. During these periods, CO2 is harvested via the C3 pathway of carbon metabolism. CAM in Sedum acre seems to be restricted to an auxilary role as a mechanism which preserves respiratory CO2 by nocturnal CO2 refixation rather than contributing substantially to CO2 uptake from the atmosphere. This may explain why, in spite of a CAM capability, S. acre has Ci3 values of a C3 plant. 

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. 

In their paper in 1987, Hibbs et al. (1987a) proposed that the characteristic pattern of metabolic dysfunction inflicted by CAMs is due to iron loss from aconitase and other iron-sulfur-containing enzymes resulting from nitrite or oxygenated nitrogen intermediates in the pathway of nitrite and nitrate synthesis. Much data have since been published to support this proposal, although as described below the chemical details of this process are still not clear. 

Figure 8-15. Carboxylase reactions and locations for the three photosynthetic pathways (a) C3, (b) C4, and (c) Crassulacean acid metabolism (CAM). The reactions for C3 and C4 plants occur during the daytime. The indicated decarboxylations of C4 acids occur in the cytosol of bundle sheath cells for C4 plants and the cytosol of mesophyll cells for CAM plants.

Fig. 3. Carbon flow during Crassulacean acid metabolism (CAM). The simplifled pathway shown is that occurring in malic enzyme type plants. The location of the decarboxylation reaction is believed to be the mitochondria (NAD-malic enzyme type) or the cytosol [16] or chloroplast (NADP-malic enzyme type) [15]. Abbreviations G6P, glucose 6-phosphate F6P, fructose 6-phosphate F16P, fructose 1,6-bisphosphate GAP, glyceraldehyde 3-phosphate PEP, phosphoeno/pyruvate PYR, pyruvate.

Jahren et al, 2001). Today most C4 plants are tropical grasses, and most CAM plants are submerged aquatic plants and desert succulents. Most other kinds of plants use the C3 photosynthetic pathway. There is the potential to recognize these various metabolic pathways from the isotopic composition of organic carbon in paleosols and in fossil plants, and in the fossils of animals which ate the plants (Ceding et al, 1997 MacFadden et ah, 1999 Krull and Retallack, 2000). 

CAM plants use the same chemistry but package it differently. Specifically, they lack the Kranz anatomy that is the defining characteristic of the C4 plants. Kranz is the German word for wreath and refers to the appearance—in a cross-sectioned leaf—of the cells which sheath the vascular bundles in C4 plants. CAM stands for Crassulacean Acid Metabolism. There is no such thing as crassulacean acid. The name instead refers to the initial discovery of this pathway of carbon fixation, in which oxaloacetic, malic, and pyruvic acids play key roles, in plants from the family Crassulaceae. cam plants open their stomata, take in CO2, and produce malate at night. Temperatures and, consequently, water losses are lower. During the day, the stomata are closed and the malate is processed as in the bundle-sheath cells of C4 plants. Diffusive losses of CO2 are, however, greater than those in C4 plants. 

An endogenous amine, cysteamine (CAM) is a cystine-depleting compound with antioxidative and anti-inflammatoiy properties it is used for treatment of cystinosis - a metabolic disorder caused by deficiency of the lysosomal cystine carrier. CAM is widely distributed in organisms and considered to be a key regulator of essential metabolic pathways [89],... 

---