Triose

We shall consider this condensation again when we consider the hexose sugars. 

A tetrose, known as erythrose, has been obtained by the oxidation of the tetra-hydroxy alcohol, erythritol, or hutan-tetrol. 

It has also been prepared by the aldol condensation of glycolic aldehyde, 

Erythrose yields an osazone with phenyl hydrazine and reduces Fehling s solution but is not fermentable with yeast zymase. When 22 

Pentose sugars may be obtained by the oxidation of the normal penta-hydroxy alcohol. The usual method of preparing them, however, is by decreasing the carbon content of a hexose sugar by means of the oxime reaction (p. 330). 

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R. C. Wade, M. E. Davis, B. A. Luty, J. D. Madura, and J. A. McCammon. Gating of the active site of triose phosphate isomerase Brownian dynamics simulations of flexible peptide loops in the enzyme. Biophys. J., 64 9-15, 1993. 

Godzik A, J Skolnick and A Kolinski 1992. Simulations of the Folding Pathway of Triose Phosj Lsomerase-type a// Barrel Proteins. Proceedings of the National Academy of Sciences USA 89 1 2633. 

Further steps m glycolysis use the d glyceraldehyde 3 phosphate formed m the aldolase catalyzed cleavage reaction as a substrate Its coproduct dihydroxyacetone phosphate is not wasted however The enzyme triose phosphate isomerase converts dihydroxyacetone phosphate to d glyceraldehyde 3 phosphate which enters the glycol ysis pathway for further transformations... 

Silver carbonate, alone or on CeHte, has been used as a catalyst for the oxidation of methyl esters of D-fmctose (63), ethylene (64), propylene (65), trioses (66), and a-diols (67). The mechanism of the catalysis of alcohol oxidation by silver carbonate on CeHte has been studied (68). 

Figure 2.20 Motifs that are adjacent in the amino acid sequence are also usually adjacent in the three-dimensional structure. Triose-phosphate isomerase is built up from four P-a-p-a motifs that are consecutive both in the amino acid sequence (a) and in the three-dimensional structure (b).

The enzyme triose phosphate isomerase eatalyzes the intereonversion of D-glyeeraldehyde 3-phosphate and dihydroxyaeetone phosphate. 

FIGURE 6.28 Examples of protein domains with different numbers of layers of backbone strnctnre. (a) Cytochrome c with two layers of a-helix. (b) Domain 2 of phosphoglycerate kinase, composed of a /3-sheet layer between two layers of helix, three layers overall, (c) An nnnsnal five-layer strnctnre, domain 2 of glycogen phosphorylase, a /S-sheet layer sandwiched between four layers of a-helix. (d) The concentric layers of /S-sheet (inside) and a-helix (outside) in triose phosphate isomerase. Hydrophobic residnes are bnried between these concentric layers in the same manner as in the planar layers of the other proteins. The hydrophobic layers are shaded yellow. (Jane Richarelson)... 

Another important parallel /3-array is the eight-stranded parallel j8-barrel, exemplified in the structures of triose phosphate isomerase and pyruvate kinase (Figure 6.30). Each /3-strand in the barrel is flanked by an antiparallel a-helix. The a-helices thus form a larger cylinder of parallel helices concentric with the /3-barrel. Both cylinders thus formed have a right-handed twist. Another parallel /3-structure consists of an internal twisted wall of parallel or mixed /3-sheet protected on both sides by helices or other substructures. This structure is called the doubly wound parallel j8-sbeet because the structure can be... 

FIGURE 6.30 Parallel /3-array proteins—the eight-stranded /3-barrels of triose phosphate iso-merase (a, side view, and b, top view) and (c) pyruvate kinase. (Jane Richardson)... 

Triose phosphate isomerase with substrate analog 2-phosphoglycerate shown in red. 

The triose phosphate isomerase reaction completes the first phase of glycolysis, each glucose that passes through being converted to two molecules of glyceraldehyde-3-phosphate. Although the last two steps of the pathway are... 

FIGURE 19.15 A reaction mechanism for triose phosphate isomerase. 

Dihydroxyacetone phosphate is of course an intermediate in glycolysis. D-Gly-ceraldehyde can be phosphorylated by triose kinase in the presence of ATP to form D-glyceraldehyde-3-phosphate, another glycolytic intermediate. 

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. 

Knowles, J., and Albery, W., 1977. Perfection in enzyme catalysis The energetics of triose phosphate isomerase. Accounts of Chemical Research 10 105-111. 

The hydroxylated compound thus formed would be hydroxymalonaldehyde —i.e. tartronic dialdehyde (8). This compound has never been obtained or studied, but its enol form, which is the so-called triose reductone (11) (14), is well known, and it is generally agreed that, in solution, the equilibrium... 

On the basis of experiments with myoinositol (50), it has been suggested that triose reductone should be oxidized by periodate to yield two molar equivalents of formic acid and one molar equivalent of carbon dioxide. However, it has been reported by two groups (1,29) that crystalline triose reductone is oxidized by two moles of periodic acid to give formic acid and glyoxylic acid, free iodine being liberated during the... 

The appearance of free iodine during the periodate oxidation of compounds having an active hydrogen atom (27) or an ene-diol structure (1,39) has frequently been observed, and this implies that further reduction of iodate, formed from periodate during the main reaction, takes place. It has, in fact, been shown that, in acid solution, iodate is fairly readily reduced by such compounds as triose reductone (27), dihydfoxy-fumaric (39), and tartronic (32) acids. 

We examined the reaction of triose reductone with both periodate and iodate (55,56), and found that, whereas iodine was invariably set free from both sodium periodate and sodium iodate if the concentration of the reductone were greater than 10 3M, no iodine was liberated at lower concentrations (e.g. 6 x 10 4M) of substrate, even in the presence of relatively large amounts of the oxidants. 

The reaction of iodate with triose reductone is not only a function of the concentration of the reagents, it is also dependent on the pH of the solution. In solutions of triose reductone more dilute than 10"3M, iodine is set free from iodate, if the pH of the solution is lower than about 3 (55). Dihydroxyfumaric and L-ascorbic acids (26), which also have free ene-diol structures, behave similarly. 

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