Glyceraldehyde rearrangement

Dilute alkali may also cause degradation of reducing sugars and the enolisation may extend to the 2,3-dienol together with the production of such breakdown products as glyceraldehyde. Rearrangements can also take place with concentrated alkali with the formation of saccharinic acids. D-Glucose (CLI) yields saccharinic acid (CL) with dilute alkali and a mixture of iso- (CLII) and wcte-saccharinic acid (CLIII) with more concentrated alkali. 

When carbon rearrangements are balanced to account for net hexose synthesis, five of the glyceraldehyde-3-phosphate molecules are converted to dihy-droxyacetone phosphate (DHAP). Three of these DHAPs then condense with three glyceraldehyde-3-P via the aldolase reaction to yield 3 hexoses in the form... 

This reaction rearranges the carbonyl and hydroxyl groups on carbons 1 and 2. However, more than 80% of the enzymatic rate acceleration has been traced to enzyme-substrate interactions involving the phosphate group on carbon 3 of the substrate. This was determined by a careful comparison of the enzyme-catalyzed reactions with glyceraldehyde 3-phosphate and with glyceraldehyde (no phosphate group at position 3) as substrate. 

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. 

MECHANISM FIGURE 22-18 Tryptophan synthase reaction. This enzyme catalyzes a multistep reaction with several types of chemical rearrangements. An aldol cleavage produces indole and glyceraldehyde 3-phosphate this reaction does not require PLP. Dehydration of serine forms a PLP-aminoacrylate intermediate. In steps and this condenses with indole, and the product is hydrolyzed to release tryptophan. These PLP-facilitated transformations occur at the /3 carbon (C-3) of the amino acid, as opposed to the a-carbon reactions described in Figure 18-6. The /3 carbon of serine is attached to the indole ring system. Tryptophan Synthase Mechanism... 

The pathway also operates in some bacteria and apparently is the sole source of isoprenoid compounds for the unicellular alga Scenedesmus.28 The pathway is outlined in Fig. 22-2. Pyruvate is decarboxylated by a thiamin diphosphate-dependent enzyme,29 and the resulting enamine is condensed with D-glyceraldehyde 3-phosphate to form 1-deoxyxylulose 5-phosphate.28, i0 31a The latter undergoes an isomeroreductase rearrange-... 

Griffiths and Gutsche (23) recently studied the interconversion of deuterated mandelaldehyde dimer and 2-hydroxyacetophenone in pyridine to obtain information concerning the glyceraldehyde-dihydroxy-acetone rearrangement. Their results support an enolization mechanism requiring a base and an acid catalyst. They found a deuterium isotope effect of ca. 1.3 for the transformation of the aldehyde to the ketone. When they corrected this for the apparently differing amounts of the aldehyde form in equilibrium with the proteo dimer and the deuterio dimer, they obtained a value of 3.9. By the Swain-Schaad equation (26) ... 

How can the imine behave as the carbon donor in addition to the aldehyde carbonyl of glyceraldehyde 3-phosphate It is unlikely to do so directly, but it can rearrange to an enamine which, as we will explain in Section 17-4B, can act as a carbon nucleophile ... 

Glyceraldehyde is present in cells as its phosphate which is in equilibrium with dihydroxyacetone phosphate. This looks like a complicated rearrangement but it is actually very simple—the two compounds have a common enol through which they interconvert. 

Table 7b 2,3-Wittig Rearrangement of Noniacemic Attylic Propargylie Ethers Derived from (/ )-Glyceraldehyde Acetonide...

In the absence of Zn, the similar dependence of both the hydride-shift pathway and the enolisation pathway on [OD ] established that the hydrate diol anion was inert to both types of rearrangement, and that the hydride-shift mechanism probably went through the glyceraldehyde anion formed by deprotonation of 02. 

---