Carbon cycle enzymes

The latent ATPase activity of CF undergoes light activation in intact chloroplasts (Mills, Hind 1979). The activation depends on both AyH and thiol-reduction of CF. The latter is probably mediated by thioredoxin which undergoes photoreduction via ferredoxin-thioredoxin reductase, thus resembling the activation of a few carbon cycle enzymes (Buchanan 1980 ... 

Kozarich JW (1988) Enzyme chemistry and evolution in the )S-ketoadipate pathway. In Microbial Metabolism and the Carbon Cycle (Eds SR Hagedorn, RS Hanson, and DA Kunz), pp. 283-302. Harwood Academic Publishers, Chur, Switzerland. 

The importance of zinc may extend even further as the processes that control carbon uptake by marine phytoplankton are important in the carbon cycle, and the zinc enzyme carbonic anhydrase could be a limiting factor. The concentration of zinc available to some marine phytoplankton has... 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

Racker and Schroeder (85) questioned the importance of the alkaline FDPase in photosynthesis because of its lack of activity at neutral pH, its apparent cytoplasmic localization, and the presence of a second enzyme or enzymes which appeared to be associated with the chloroplasts and which hydrolyzed both FDP and SDP. Later work, however, has clearly established the function of this enzyme in the photosynthetic carbon cycle. Smillie has shown that the alkaline FDPase is associated with photosynthetic tissues in higher plants and Euglena (101, 102). The enzyme was also shown to be localized in the chloroplasts and to be absent in nonphotosynthetic tissue or bleached algae. It was the only FDPase detected in the photosynthetic bacterium Chromatium grown under autotrophic conditions (102). Preiss et al. (103) have pointed... 

Tetrahydrofolate cofactors participate in one-carbon transfer reactions. As described above in the section on vitamin B12, one of these essential reactions produces the dTMP needed for DNA synthesis. In this reaction, the enzyme thymidylate synthase catalyzes the transfer of the one-carbon unit of N 5,N 10-methylenetetrahydrofolate to deoxyuridine monophosphate (dUMP) to form dTMP (Figure 33-2, reaction 2). Unlike all of the other enzymatic reactions that utilize folate cofactors, in this reaction the cofactor is oxidized to dihydrofolate, and for each mole of dTMP produced, one mole of tetrahydrofolate is consumed. In rapidly proliferating tissues, considerable amounts of tetrahydrofolate can be consumed in this reaction, and continued DNA synthesis requires continued regeneration of tetrahydrofolate by reduction of dihydrofolate, catalyzed by the enzyme dihydrofolate reductase. The tetrahydrofolate thus produced can then reform the cofactor N 5,N 10-methylenetetrahydrofolate by the action of serine transhydroxy- methylase and thus allow for the continued synthesis of dTMP. The combined catalytic activities of dTMP synthase, dihydrofolate reductase, and serine transhydroxymethylase are often referred to as the dTMP synthesis cycle. Enzymes in the dTMP cycle are the targets of two anticancer drugs methotrexate inhibits dihydrofolate reductase, and a metabolite of 5-fluorouracil inhibits thymidylate synthase (see Chapter 55 Cancer Chemotherapy). 

This step leaves two cleavage products. The first, derived from the two carbons at the carboxyl end of the fatty acid, is acetyl-CoA, which can be further metabolized in the TCA cycle. The second cleavage product is a shorter fatty acyl-CoA. Thus, for example, the initial step of digesting a fatty acid with 16 carbons is an acyl-CoA molecule where the acyl group has 14 carbons and a molecule of acetyl-CoA. The P-oxidation scheme may be used to accommodate unsaturated fatty acids also. The reactions occur as described previously for the saturated portions of the molecule. Where a trans carbon-carbon double bond occurs between the %- and p-carbons of the acyl-CoA, the accommodation is fairly simple reaction 1 isn t needed. Where the double bonds are in the cis configuration, or are between the P and y carbons, isomerase enzymes change the location of the double bonds to make recognizable substrates for P-oxidation. 

Evans, Buchanan, and Arnon (41a) have recently found that the ferredoxin-dependent pyruvate and a-ketoglutarate synthesizing reactions function in a new carbon cycle for the photosynthetic fixation of C02. The new cycle was named the reductive carboxylic acid cycle, and apart from pyruvate and a-ketoglutarate synthases, it includes certain of the enzymes associated with Krebs citric acid cycle, operating in the synthetic direction. Photoreduced ferredoxin and ATP, formed by photo-... 

The interaction between the carbonate cycle and the organic carbon cycle takes place under a variety of circumstances. At one end of the spectrum, carbonate chemistry may be under direct enzymic control (see Chapter 2.2). It may take place within cells, within organisms, or within micro-environments in close contact with living tissues (e.g., molluscan mantle). At the other extreme, where products of metabolic activities modify the overall chemistry of the environment, carbonate dissolution or precipitation may be influenced indirectly. The closer the contact between the organism and the substrate, the more specific are the biogenic dissolution and crystallization patterns that remain as traces of biological activity in sediments. 

McLean, P., Reid, E., and Gurney, M. W., Effect of azo-dye carcinogenesis on enzymes concerned with urea synthesis in the rat. Biochem. J. 91, 464-473 (1964). McLean, P., and Rossi, F., Changes in the activities of urea-cycle enzymes after the administration of carbon tetrachloride. Biochem. J. 91, 261-270 (1964). McMurray, W. C., Mohyuddin, F., Bayer, S. M., and Rathbun, J. C., Citrul-linuria a disorder of amino acid metabolism associated with mental retardation. Int. Copenhagen Congr. Sci. Study Meat. Retard., Proc. 3rd 1, 117 (1964). McMurray, W. C., Rathbun, J. C., Mohyuddin, F., and Koegler, S. J., Citrul-linuria. Pediatrics 32, 347-357 (1963). 

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