Regenerating substrate

Some bacteria use a "dicarboxylic acid cycle" to oxidize glyoxylate OHC-COO to C02. The regenerating substrate for this cycle is acetyl-CoA. It is synthesized from glyoxylate by a complex pathway that begins with conversion of two molecules of glyoxylate to tartronic semialdehyde ... 

The primary substrate of the citric acid cycle is acetyl-CoA. Despite many references in the biochemical literature to substrates "entering" the cycle as oxaloacetate (or as one of the immediate precursors succinate, fumarate, or malate), these compounds are not consumed by the cycle but are completely regenerated hence the term regenerating substrate, which can be applied to any of these four substances. A prerequisite for the operation of a catalytic cycle is that a regenerating substrate be readily available and that its concentration... 

The word amphibolic is often applied to those metabolic sequences that are part of a catabolic cycle and at the same time are involved in a biosynthetic (anabolic) pathway. Another term, anaplerotic, is sometimes used to describe pathways for the synthesis of regenerating substrates. This word, which was suggested by H. L. Komberg, comes from a Greek root meaning "filling up."80... 

Figure 17-6 The dicarboxylic acid cycle for oxidation of glyoxylate to carbon dioxide. The pathway for synthesis of the regenerating substrate Carbohydrate synthesis...

In bacteria, which lack formate dehydrogenase, formaldehyde can be oxidized to C02 to provide energy beginning with the reactions of Eq. 17-51. The resulting fructose 6-P is isomerized to glucose 6-P, which is then dehydrogenated via Eq. 17-12 to form C02 and the regenerating substrate ribulose 5-phosphate. 

A number of pseudomonads and other bacteria convert C compounds to acetate via tetrahydrofolic acid-bound intermediates and C02 using the serine pathway179 192 193 shown in Fig. 17-15. This is a cyclic process for converting one molecule of formaldehyde (bound to tetrahydrofolate) plus one of C02 into acetate. The regenerating substrate is glyoxylate. Before condensation with the "active formaldehyde" of meth-... 

A key enzyme in the glyoxylate cycle is isocitrate lyase, which cleaves isocitrate (Eq. 13-40) to succinate and glyoxylate. The latter is condensed with a second acetyl group by the action of malate synthase (Eq. 13-38). The L-malate formed in this reaction is dehydrogenated to the regenerating substrate oxalo-... 

The NADP-dependent TBADH was used for the laboratory-scale preparation of several chiral aliphatic and cyclic hydroxy compounds by reduction of the corresponding ketones. For the regeneration of NADPH, this reduction reaction can be coupled with the TBADH catalyzed oxidation of isopropanol. For the reduction of some ketones it was observed that the reaction rate was increased in the presence of the regenerating substrate isopropanol, for instance in the presence of 0.2 v/v isopropanol, the reduction rate of butanone or pentanone was increased 3-4-fold [57], In some cases, the enantiomeric excess of the reduction reaction is not very high, especially when small molecules are converted, but also for compounds such as acetophenone [138]. 

Fig. 12 Oxidation of cyclohexanol catalyzed by HLADH with acetaldehyde as a coenzyme regenerating substrate...

The S5mthetic pathway for the regenerating substrate of e dicarboxylic acid cycle is quite complex. Two molecules of glyoxylate imdergo a condensation with decarboxylation by glyoxylate carboligase ... 

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