CO2 fixation

Figure 4.8 The active site in all a/p barrels is in a pocket formed by the loop regions that connect the carboxy ends of the p strands with the adjacent a helices, as shown schematically in (a), where only two such loops are shown, (b) A view from the top of the barrel of the active site of the enzyme RuBisCo (ribulose bisphosphate carboxylase), which is involved in CO2 fixation in plants. A substrate analog (red) binds across the barrel with the two phosphate groups, PI and P2, on opposite sides of the pocket. A number of charged side chains (blue) from different loops as welt as a Mg ion (yellow) form the substrate-binding site and provide catalytic groups. The structure of this 500 kD enzyme was determined to 2.4 A resolution in the laboratory of Carl Branden, in Uppsala, Sweden. (Adapted from an original drawing provided by Bo Furugren.)...

Purple sulfur bacteria fix carbon dioxide using the Calvin-Benson cycle, but green sulfur bacteria use a completely different pathway, the reverse tricarboxylic acid cycle. Other photosynthetic bacteria use still different pathways for CO2 fixation (Perry and Staley, 1997). 

A new pathway of CO2 fixation proposed (to become the Wood-Ljungdahl... 

Fig. 10. Milestones in the study of CODH/ACS. Important discoveries beginning with the postulate of a new CO2 fixation pathway.

In this chapter we review recent developments, concentrating mostly on the effects of water stress because of its great importance, but trying where possible to identify general problems. As noted above, accumulation of ABA in leaves is common under stress, and we review its effects on gas exchange. If the sole direct effect of ABA were to reduce stomatal aperture, then assimilation rate would also decrease because of a lower intercellular partial pressure of C02,pi. Some experiments have suggested that pi is little affected by ABA and that the capacity for CO2 fixation has actually decreased. The situation has become complicated with the observation (Terashima et al., 1988) that application of ABA can cause non-uniform... 

Osmond, C.B., Smith, S.D., Ben, G.-Y. Sharkey, T.D. (1987). Stem photosynthesis in a desert ephemeral, Eriogonum inflatum characterization of leaf and stem CO2 fixation and HjO vapour exchange under controlled conditions. Oecologia, 72, 542-49. 

An intriguing stress-induced alteration in gene expression occurs in a succulent plant, Mesembryanthemum crystallinum, which switches its primary photosynthetic CO2 fixation pathway from C3 type to CAM (Crassulacean acid metabolism) type upon salt or drought stress (Winter, 1974 Chapter 8). Ostrem et al. (1987) have shown that the pathway switching involves an increase in the level of mRNA encoding phosphoenol-pyruvate carboxylase, a key enzyme in CAM photosynthesis. 

The development of CO2 fixation reactions in supercritical CO2 attracts increasing attention due to its gas-like low viscosities and high diffusivities and its liquid-like solubilizing power. Matsuda et al. attempted to carry out the con-... 

Photoelectrochemical experiments on the pyrite/H2S system, as well as theoretical considerations, led Tributsch et al. (2003) to the conclusion that CO2 fixation at pyrite probably could not have led to the syntheses proposed by Wachtershauser. The reaction mechanism involved in such reactions is likely to be much more complex than had previously been assumed. The Berlin group supports the objection of Schoonen et al. (1999) that, apart from other points, the electron transfer from pyrrhotine to CO2 is hindered by an activation energy which is too high. There is, thus, no lack of different opinions on the model of chemoautotrophic biogenesis hopefully future studies will shed more light on the situation ... 

Equations 1 to 3 show some of fixation reactions of carbon dioxide. Equations la and lb present coupling reactions of CO2 with diene, triene, and alkyne affording lactone and similar molecules [2], in a process catalyzed by low valent transition metal compounds such as nickel(O) and palladium(O) complexes. Another interesting CO2 fixation reaction is copolymerization of CO2 and epoxide yielding polycarbonate (equation 2). This reaction is catalyzed by aluminum porphyrin and zinc diphenoxide [3],... 

In spite of the importance of CO2 fixation reactions, relatively few theoretical works have been carried out on transition-metal CO2 complexes... 

Ivey DM. 1986. Generation of energy during CO2 fixation in acetogenic bacteria [dissertation]. Athens University of Georgia. 

Ragsdale SW. 1991. Enzymology of acetyl-CoA pathway of CO2 fixation. Crit Rev Biochem Mol Biol 26 261-300. 

The exploratory studies, as conducted, did not distinguish between effects Imposed on the stromal-associated CO2 fixation (Calvin cycle) reactions or on the light reactions associated with the thylakoids. Consequently, studies were conducted on light-induced electron transport and ATP synthesis associated with isolated spinach thylakold membranes. 

The most common and simplest procedure is to place a few microliters of the test solution over a small puncture wound on a detached leaf. The puncture wound enhances the access of the toxin to the leaf tissue. The leaf is then placed in a petri dish containing a filter paper saturated with water. The top cover of the plate is sealed with parafilm, and the plate is incubated under controlled light and temperature conditions. Toxin activity is usually indicated by chlorotic, necrotic, or colored spots on the leaf. Other methods for bioassay involving CO2 fixation, or effects on organelles, whole plants, protoplasts, tissue cultures, or plant parts are outlined (, 7). 

Phenolic compounds naturally occurring in plants have induced many physiological responses that duplicate those reported for ozone and/or peroxyacetylnitrate (PAN). Chlorogenic acid is a competitive inhibitor of lAA-oxidase (35) and plant growth is adversely affected by increased concentrations of auxins (36). Concentrations of chlorogenic acid are increased in tobacco tissue exposed to ozone ( ) Phenols inhibit ATP synthesis (37), oxidative phosphorylation ( ) and SH enzyme activity (27) they increase respiration (38), reduce CO2 fixation (22), modify both membrane permeability (40) and oxidation rate of reduced NADH... 

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