Oxido-reductive metabolism

In this scenario, the next key step was the discovery by early living organisms of alternative pathways to synthesize nucleoside triphosphates (stage 3 in Scheme 40), for instance by the introduction of an oxido-reduction metabolism. In the particular case of ATP, the increase in its concentration could have forced the chemical flux of the system to be reverted since the unstable adenylate anhydride was stabilized in the active site of the adenylate binding protein (E) allowing the activation of amino acids by ATP through the process that became predominant. 

Silveira, W. B., Passos, F. J. V., Mantovani, H. C., Passos, F. M. L. (2005). Ethanol production from cheese whey permeate by Kluyveromyces mandanus UFV-3 a flux analysis of oxido-reductive metabolism as a function of lactose concentration and oxygen levels. Enzyme and Microbial Technology, 36, 930—936. 

A) Thermodynamics forbids to cross the line ArGgrowth = 0- Experimental data obtained during oxido-reductive metabolism of S. cerevisiae (+ O) or K. marxianus ( ). (Redrawn from References [66], [67], [68], respectively, with permission of the author and publisher). 

Figure 23 Aerobic batch culture of S. cerevisiae on glucose. = oxido-reductive metabolism on glucose (D = glucose exhaustion = pyruvate consumption = consumption of ethanol and acetate (D = consumption of ethanol.

Metabolism of the major fuels, described above, generates hydrogen atoms or electrons. These are oxidised, not directly, but via a series of oxidations and reductions (oxido-reduction reactions or, alternatively, redox reactions) that... 

The literature of metabolism in proteinoids and proteinoid microspheres is reviewed and criticized from a biochemical and experimental point of view. Closely related literature is also reviewed in order to understand the function of proteinoids and proteinoid microspheres. Proteinoids or proteinoid microspheres have many activities. Esterolysis, decarboxylation, animation, deamination, and oxido-reduction are catabolic enzyme activities. The formation of ATP, peptides or oligonucleotides is synthetic enzyme activities. Additional activities are hormonal and inhibitory. Selective formation of peptides is an activity of nucleoproteinoid microspheres these are a model for ribosomes. Mechanisms of peptide and oligonucleotide syntheses from amino acids and nucleotide triphosphate by proteinoid microspheres are tentatively proposed as an integrative consequence of reviewing the literature. 

Summary of Coenzyme-linked Oxido-reductions in the Metabolism of Alditols by Yeasts... 

A study of the reduction of [24- C]3-oxo-5j8-cholanic acid in bile fistula rats given [l- Hjjethanol showed that all metabolites had a 3a-hydroxy group and all radioactive products (lithocholate, 3a,6/8-dihydroxy-5 -cholanate, chenodeoxycho-late and y8-muricholate) contained about 13 atom% excess deuterium in the 3/9 position. Thus, the 3)8-hydroxy-5/9-steroid dehydrogenase isoenzyme of alcohol dehydrogenase [172] has no function in the reductive metabolism of bile acids. Cholic acid was not radioactive but contained deuterium at the 3)8, 5)8 and other positions, probably because of the transfer of deuterium from ethanol via NADH to NADPH, which it utilized in the biosynthesis of cholesterol and bile acids and in oxido reduction of the 3-hydroxyl group of the latter [173]. 

Additionally it should be remembered that nicotine metabolites still retain a pyridyl moiety and this functional group can release nicotinamide from NADPH and generate an analogue of the coenzyme via a glycohydrolase. As these analogues may not be able to participate in the normal oxido/reduction reactions of intermediary metabolism certain pathways may be inhibited leading to accumulation of substrates e.g. glucose-6-phosphate and diminution of availability of products e.g. ribose, and thereby affect purine, pyrimidine and nucleic acid biosynthesis. 

The first step of xylose catabolism is its conversion to xylulose. In bacteria, it takes place by the direct isomerization catalysed by xylose isomerase. In PeniciUium chrysogenum, a sequence of enzymes in the initial steps of pentose metabolism was observed that differs from xylose isomerization in bacteria [91, 92]. These enzymes were common in yeast and filamentous fungi. In this oxido-reductive pathway, xylose is first reduced to the xyhtol in the presence of NAD(P)-linked xylose reductase, which is then reoxidized by NAD(P)-hnked dehydrogenase to give xylulose (Fig. 1). It has been assumed that this oxido-reductive pathway is common among fungi [93]. Both the enzymes involved, xylose reductase and xylitol dehydrogenase, were found to be inducible and relatively specific for the D-xylose and xyhtol in F. oxysporum, whereas D-xylose isomerase was not detected. 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

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