Triose conversion

Triose phosphate isomerase catalyzes the conversion of dihy-droxyacetone-P to glyceraldehyde-3-P. The standard free energy change, AG°, for this reaction is +7.6 kj/mol. However, the observed free energy change (AG) for this reaction in erythrocytes is +2.4 kj/mol. 

These catalysts facilitate the interconversion of isomeric compounds and include racemases, optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose phosphate isomerase. 

FIGURE 20-11 Transketolase-catalyzed reactions of the Calvin cycle, (a) General reaction catalyzed by transketolase the transfer of a two-carbon group, carried temporarily on enzyme-bound TPP, from a ketose donor to an aldose acceptor, (b) Conversion of a hexose and a triose to a four-carbon and a five-carbon sugar (step of Fig. 20-10). (c) Conversion of seven-carbon and three-carbon sugars to two pentoses (step of Fig. 20-10). 

One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other live triose phosphate molecules (15 carbons) are rearranged in steps to (S) of Figure 20-10 to form three molecules of ribulose 1,5-bisphosphate (15 carbons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic C02 assimilation, six NADPH and nine ATP are required. 

Conversion of Triose Phosphates to Sucrose and Starch Is Tightly Regulated... 

Energy investment phase (continued) Conversion of fructose 6-phosphate to triose phosphates. 

The 500-residue subunits of pyruvate kinase consist of four domains,891 the largest of which contains an 8-stranded barrel similar to that present in triose phosphate isomerase (Fig. 2-28). Although these two enzymes catalyze different types of reactions, a common feature is an enolic intermediate. One could imagine that pyruvate kinase protonates its substrate phosphoenolpyruvate (PEP) synchronously with the phospho group transfer (Eq. 12-42). However, the enzyme catalyzes the rapid conversion of the enolic form of pyruvate to the oxo form (Eq. 12-43) adding the proton sterospecifically to the si face. This and other evidence favors the enol as a true intermediate... 

Formation of pyruvate. The conversion of glucose to pyruvate requires ten enzymes (Fig. 17-7), and the sequence can be divided into four stages preparation for chain cleavage (reactions 1-3), cleavage and equilibration of triose phosphates (reactions 4 and 5), oxidative generation of ATP (reactions 6 and 7), and conversion of 3-phosphoglycerate to pyruvate (reactions 8-10). 

Variations of the alcoholic and homolactic fermentations. The course of a fermentation is often affected drastically by changes in conditions. Many variations can be visualized by reference to Fig. 17-9, which shows a number of available metabolic sequences. We have already discussed the conversion of glucose to triose phosphate and via reaction pathway a to pyruvate, via reaction c to lactate, and via reaction d to ethanol. 

Now let us consider the further conversion of PEP and of the triose phosphates to glucose 1-phosphate, the key intermediate in biosynthesis of other sugars and polysaccharides. The conversion of PEP to glucose 1-P represents a reversal of part of the glycolysis sequence. It is convenient to discuss this along with gluconeogenesis, the reversal of the complete glycolysis sequence from lactic acid. This is an essential part of the Cori cycle (Section F) in our own bodies, and the same process may be used to convert pyruvate derived from deamination of alanine or serine (Chapter 24) into carbohydrates. 

Unusual features of vitamin A as observed by some investigators include (1) decreases serum cholesterol in large-quantity administration (chicks) (2) dietary protein required to mobilize liver reserves of vitamin A (3) decreased quantities in tumors (4) coenzynre Q10 accumulates in A-deficient rat liver (5) Ubichromenol-50 accumulation in A-deficient rat liver (6) retinoic acid functions as vitamin A except for visual and reproductive functions (7) anti-infection properties and anti-allergic properties (8) decreases basal metabolism (9) detoxification of poisons in the liver aided by vitamin A and (10) vitamin A is involved in triose —> glucose conversions. 

The Conversion of Triose Phosphates to Phosphoglycerates Occurs in Two Steps The Three-Carbon Phosphorylated Acids Constitute a Third Metabolic Pool... 

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