Reductive tricarboxylic acid cycle

C. aurantiacus grows best as a photoheterotroph, but autotrophy has been successfully demonstrated in some strains [9j. Either hydrogen or sulfide can serve as electron donors for photoautotrophy. The pathway of CO2 fixation is not known but does not appear to be either the reductive pentose phosphate pathway or the reductive tricarboxylic acid cycle [10]. 

The reductive tricarboxylic acid cycle is basically the reverse of the oxidative tricarboxylic acid cycle that heterotrophs use to generate reducing equivalents (NADH, FADH2) that function as electron donors for energy generation. The 3-hydroxyproprionate cycle was found in the green nonsulfur bacterium Chloroflexus (Strauss and Fuchs, 1993) and more recently also in some autotrophic Archea (Mendez et al., 1999). In this pathway, 3-hydroxyproprionate is a key intermediate. 

ShibaH., Kawasumi T., Igarashi Y., andMinoda Y. (1985) The CO2 assimilation via the reductive tricarboxylic acid cycle in an obligately autotrophic aerobic hydrogen-oxidizing bacterium, Hydrogenobacter thermophilus. Arch. Microbiol. 141, 198-203. 

A second ATP-dependent citrate lyase (ACL) activity is responsible for the formation of acetyl-CoA 30 and oxaloacetate 35 from citrate 39 with concomitant hydrolysis of ATP to ADP and phosphate (Equation (22)). ° It is proposed to play a vital role in maintaining acetyl-CoA and oxaloacetate levels in most mammals, whereas in some bacteria it is an essential enzyme of the reductive tricarboxylic acid cycle (RTCA). 

Wachtcrshauser s prime candidate for a carbon-fixing process driven by pyrite formation is the reductive citrate cycle (RCC) mentioned above. Expressed simply, the RCC is the reversal of the normal Krebs cycle (tricarboxylic acid cycle TCA cycle), which is referred to as the turntable of metabolism because of its vital importance for metabolism in living cells. The Krebs cycle, in simplified form, can be summarized as follows ... 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

NADH oxidation via complex I takes place on the inside of the membrane—i. e., in the matrix space, where the tricarboxylic acid cycle and 3-oxidation (the most important sources of NADH) are also located. O2 reduction and ATP formation also take place in the matrix. 

All microbes, including chemoorganohetero-trophs, possess some ability to engage in reversible carboxylation (i.e., CO2-C assimilation into an organic compound) and decarboxylation reactions, some of which lead to the incorporation of a significant amount of CO2 (Wood, 1985). Here, we briefly consider the biochemical pathways that photo- and chemolithotrophic bacteria deploy in order to produce the majority of their biomass. The four major C02-fixing pathways are the Calvin cycle, the acetyl-CoA pathway, the reductive tricarboxylic acid (TCA) cycle, and the 3-hydroxypriopionate cycle. 

Aspartase deficiency in Y. pestis is another example of a substantial effect of a single base transversion (in this case a missense mutation) that nevertheless results in 99.99% reduction in enzyme activity [32], AspA activity catalyzes the deamination of L-aspartate to form fumarate, a component of the tricarboxylic acid cycle. Comparison of aspA in Y. pestis and closely related Y. pseudotuberculosis defines only a single base transversion (G.C-T.A) at a.a. position 363. This causes exchange of valine (GUG) in... 

The reduction in urinary elimination in untreated diabetic patients affects not only citric acid. Indeed, Osteux and Laturaze (07) have shown that the same holds true for the other acids of the tricarboxylic acid cycle treatment with insulin re-establishes a normal elimination of these acids. Treatment with antidiabetic sulfonamides also increases the urinary citrate in such patients (OlO). 

Figure 16. A schematic view of the flow of carbon within the tricarboxylic-acid cycle. The labels m, c, and b designate respectively carbon from the methyl and carboxyl positions of acetyl-CoA and from bicarbonate used to produce oxaloacetate from phosphoenolpymvate. Inputs and outputs of water, of redox cofactors (NAD, etc.), and of coenzymes (CoASH) have been omitted in order to focus on the carbon skeletons. The amination of a-ketoglutarate in order to produce glu is the principal means of importing N for use in the amino-acid pool. The process involves multiple steps, including a reduction (indicated by addition of [H]) and is represented here only schematically. The boldface T indicates transamination, an example of which is shown in equation 21. The circled P represents a phosphate group, POs. Pj represents inorganic phosphate, HP04 .

Fig. 7. Proposed pathway for assimilation of ammonia produced by Nj reduction in the bacteroids and synthesis of asparagine by enzymes located in the plant fraction of lupin nodules. The utilization of photosynthate to provide oxaloacetate, NADH and ATP is also indicated. Phosphoenolpyruvate, PEP tricarboxylic acid cycle, TCA. (Based on Scott et al., 1976, and reproduced with permission.)...

Mitochondria are surrounded by two lipoprotein membranes, together about 180 A thick. The inner membrane is folded into the cell as a series of invaginations known as cristae. About one-quarter of the protein part of the cristae consists of oxysomes (respiratory assemblies), i.e. ordered arrangements of riboflavine-protein, coenzyme Q, cytochromes b, c, c, a, and a (in that sequence) together with their specific proteins. Ferredoxins (Section 11.0) also play an important part. The tricarboxylic acid cycle ensures the reduction of the first two members of the above chain, and each member is oxidized by the member on its right (in the above list), and so on to the end of the chain at cytochrome a which is in equilibrium with atmospheric oxygen. 

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