Pathway with triose-phosphate

Stage 3 Regeneration of Ribulose 1,5-Bisphosphate from Triose Phosphates The first reaction in the assimilation of C02 into triose phosphates consumes ribulose 1,5-bisphosphate and, for continuous flow of C02 into carbohydrate, ribulose 1,5-bisphosphate must be constantly regenerated. This is accomplished in a series of reactions (Fig. 20-10) that, together with stages 1 and 2, constitute the cyclic pathway shown in Figure 20-4. The product of... 

An alternative pathway for synthesis of quinoli-nate from aspartate and a triose phosphate exists in bacteria and in plants and provides the major route of nicotinic acid synthesis in nature. In E. coli the reaction is catalyzed by two enzymes, one an FAD-containing L-aspartate oxidase which oxidizes aspartate to a-iminoaspartate.228 The latter condenses with dihydroxyacetone-P to form quinolinate (Eq. 25-13).229 There are at least two other pathways for synthesis of quinolinic acid as well as five or more salvage pathways for resynthesis of degraded pyridine nucleotide coenzymes.224/230/231... 

Alternatively, it is possible to write a sequence of reactions, including the action of phosphofructokinase and aldolase on seven-carbon intermediates, in which the carbon of ribulose-5-phosphate is converted mainly to glyceralde-hyde-3-phosphate. Such a pathway, with the triose phosphate entering the glycolytic sequence, amounts to a bypass, or shunt, around the first reactions of glycolysis, and the name hexose monophosphate shunt is sometimes used. Any amount of ribose-5-phosphate or erythrose-4-phosphate that may be needed for biosynthetic sequences can also be ob-... 

Suppose that you are seeking bacterial mutants with altered triose phosphate isomerase (TPI). The organism of interest is known to use the glycolytic pathway with the production of lactate. 

Glucose (Glc) is taken up and phosphorylated into glucose-6-phosphate (Glc6P), with consumption of ATP. Isomerization and phosphorylation afford fructose-l,6-bisphosphate (Frul,6P2), which is cleaved into two triose molecules D-glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone monophosphate (DHAP). These are equilibrated by triose phosphate isomerase as only GA3P is metabolized further, except approximately 5 mol% of DHAP that leaks out of the pathway via reduction to glycerol, which is excreted as a side-product. 

However, (26) should bind strongly to class II aldolases on account of its chelating ability and this has been found to be the case with rabbit muscle fructose diphosphate aldolase where (26) functions as a competitive inhibitor. Triose phosphate isomerase is also strongly inhibited by (26) this may be due to its similarity to the cw-eriediolate (28), which is an intermediate in the reaction pathway of this enzyme. Acyldihydroxyacetone phosphates are important intermediates in the biosynthesis of glycerolipids and the acyl... 

The details of the process and the oxidation-reduction balance can be pictured as in Eq. 17-25. Pyruvate is cleaved by the pyruvate formate-lyase reaction (Eq. 15-37) to acetyl-CoA and formic acid. Half of the acetyl-GoA is cleaved to acetate via acetyl-P with generation of ATP, while the other half is reduced in two steps to ethanol using the two molecules of NADH produced in the initial oxidation of triose phosphate (Eq. 17-25). The overall energy yield is three molecules of ATP per glucose. The "efficiency" is thus (3 x 34.5) H- 225 = 46%. Some of the glucose is converted to D-lactic and to succinic acids (pathway/. Fig. 17-9) hence the name mixed acid fermentation. Table 17-1 gives typical yields of the mixed acid fermentation of... 

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