CO2-fixing enzymes

Fmax at light saturation and at the optimal temperature for photosynthesis varies with plant species but is usually from 2 to 10 mol m-3 s-1. We can also estimate Vmax from measurements of the maximum rates of CO2 fixation by isolated chloroplasts. These maximum rates—which are sustained for short periods and are for optimal conditions—can be 100 mmol of CO2 fixed (kg chlorophyll)-1 s-1 [360 pmol (mg chlorophyll)-1 hour-1 in another common unit], which is approximately 3 mol m-3 s-1 (1 kg chlorophyll is contained in about 0.035 m3 of chloroplasts in vivo). In vitro, the key enzyme for CO2 fixation, ribulose-l,5-bisphosphate carboxylase/oxygenase, can have rates equivalent to 200 mmol (kg chlorophyll)-1 s-1. The estimates of Vmax using isolated chloroplasts or enzymes usually are somewhat lower than its values determined for a leaf Measurements using leaves generally indicate that KqOz is 5 to 20 mmol m-3. For instance, Kcch can be 9 mmol m-3 at 25°C with a Q10 of 1.8 (Woodrow and Berry, 1988 Q10 is defined in Chapter 3, Section 3.3B). 

Accordingly, of 799 genera in the Poaceae family of grasses, 407 possess the C4 assimilation mechanism (Watson Dallwitz, 1992). In C4 plants photorespiration is inhibited by a higher concentration of CO2 around the Rubisco carbon fixing enzyme, which also facilitates carbon assimilation with less water use (Downes, 1969). Therefore, higher temperatures and limited water availability allow C4 plants to outcompete C3 types, which is the observed pattern in the field. 

RuBP carboxylase is the central C02-fixing enzyme in all plants thus, it is the most abundant enzyme in the biosphere (71, 72). The enzyme requires a divalent metal ion but has no other cofactors. CO2 is the substrate, rather than HCOs". 

C4 Cycle A cycle in some plants that minimizes the wasteful effects of photorespiration by using an enzyme other than rubisco to perform the initial fixation of CO2. This enzyme is found in mesophyll cells, where it fixes CO2 into a four-carbon compound (hence C4). This fixed carbon is shuttled into sheltered bundle-sheath cells, where it is released as CO2 and enters the Calvin cycle. 

C4 plants incorporate CO2 by the carboxylation of phosphoenolpyruvate (PEP) via the enzyme PEP carboxylase to make the molecule oxaloacetate which has 4 carbon atoms (hence C4). The carboxylation product is transported from the outer layer of mesophyll cells to the inner layer of bundle sheath cells, which are able to concentrate CO2, so that most of the CO2 is fixed with relatively little carbon fractionation. 

While sulfide is toxic to cytochrome (c) oxidase the most important enzyme of the respiratoiy chain, at concentrations in the tens of pM, thiosulfate is not Thus, the animal has produced a soluble and excretable detoxification product and protected animal respiration. This thiosulfate still contains large amounts of energy and can serve as a substrate for bacterial chemoautotrophic metabolism. The bacterial endosymbionts can utilize the thiosulfate along with sulfide to fix CO2 (5Q) and supply the host s nutritional needs. 

While this efficiency is impressive, it also is rarely achieved. The difficulty is in the protein that carries out the first step of photosynthesis. Molecular oxygen, O2, competes with CO2 for the active site of ribulose bisphosphate carboxylase, leading to an oxidation and loss of the ribulose bisphosphate acceptor. This competition is apparently intrinsic to the enzyme, because attempts to increase the discrimination for CO2 by genetic engineering have resulted in a less-active enzyme, which fixes CO2 very poorly. 

During the light period, when COi is being fixed in the chloroplasts by the RPP pathway, it is likely that the mechanisms discussed above for C3 photosynthesis are also functional in CAM plants [19]. Additional control mechanisms are expected to provide for the efficient functioning of the diurnal cycle of CO2 fixation. The functioning of CAM cannot, however, be interpreted solely in terms of enzymolo-gy, but rather will involve cellular compartmentation of enzymes and metabolites together with intracellular transport processes [17]. 

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