1,3-Glyceraldehyde diphosphate

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate... 

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

Figure 17-2. The pathway of glycolysis. ( ,—P, HOPOj " .inhibition.) At asterisk Carbon atoms 1-3 of fructose bisphosphateform dihydroxyacetone phosphate, whereas carbons 4-6 form glyceraldehyde 3-phosphate. The term "bis-," as in bisphosphate, indicates that the phosphate groups are separated, whereas diphosphate, as in adenosine diphosphate, indicates that they are joined.

The isoprenoid side chains of quinones are biosynthesized mainly by the mevalonic acid pathway from acetyl-CoA. Another pathway to biosynthesizing isoprenoids is the so-called non-mevalonate ronte by which isopentenyldiphosphate (IPP) is formed from glyceraldehyde 3-phosphate and pyrnvate. The key molecule is the famesyl-diphosphate (FPP) that accepts other IPP molecules to form polyprenyl diphosphates. 

D-glyceraldehyde-3-phosphate, pyruvate (G3P) l-deoxy-D-xylulose-5-phosphate (DXP) 2C-methyl-D-erythritol-4-phosphate (MEP) 4-diphosph-2C-methyl-D-erythritol (CDP-ME) 4-diphosphocytidyl-2C-methyl-D-erythritol-2-phophate (CDP-MEP)... 

The hexose phosphate, fructose-1,6-diphosphate, is split by aldolase into two triose phosphates glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Aldolase consists of four 40-kDa subunits. Three tissue-specific forms exist in human tissues aldolase A (ubiquitous and very active in the muscle), aldolase B (liver, kidney, and small intestine), and aldolase C (specific to the brain). These three isozymes have nearly the same molecular size but differ in substrate specificity,... 

Aldolase 4.1.2.3 C Fructose-1,6-diphosphate NADH Glyceraldehyde-3- phosphate Glycerophosphate dehydrogenase... 

The basis of the action of iodoacetate on muscle contraction was uncovered by Dickens and Rapkine (ca. 1933). They found iodoacetate alkylated SH groups on proteins, especially those in glyceraldehyde 3-phosphate dehydrogenase (G-3-PDH). When the enzyme was inhibited precursors accumulated—hexose mono- and diphosphates—as in Lundsgaard s experiments. 

The natural substrate for the dehydrogenase, glyceraldehyde-3-phosphate (G-3-P), had been synthesized earlier by Hermann Fischer, Emil Fischer s son, and Baer in 1932. In 1934 Meyerhof and Lohmann synthesized hexose diphosphate, establishing it to be fructose 1,6 bisphosphate (F-l, 6 bis P). With F-1,6 bisP as substrate and hydrazine to trap the aldehydic and ketonic products of the reaction, G-3-P was identified in the mixture of G-3-P and dihydroxyacetone phosphate which resulted. Triose phosphate isomerase was then isolated and the importance of phosphorylated 3C derivatives established. 

Both the aldol and reverse aldol reactions are encountered in carbohydrate metabolic pathways in biochemistry (see Chapter 15). In fact, one reversible transformation can be utilized in either carbohydrate biosynthesis or carbohydrate degradation, according to a cell s particular requirement. o-Fructose 1,6-diphosphate is produced during carbohydrate biosynthesis by an aldol reaction between dihydroxyacetone phosphate, which acts as the enolate anion nucleophile, and o-glyceraldehyde 3-phosphate, which acts as the carbonyl electrophile these two starting materials are also interconvertible through keto-enol tautomerism, as seen earlier (see Section 10.1). The biosynthetic reaction may be simplihed mechanistically as a standard mixed aldol reaction, where the nature of the substrates and their mode of coupling are dictated by the enzyme. The enzyme is actually called aldolase. 

In Box 10.4 we saw that an aldol-like reaction could be used to rationalize the biochemical conversion of dihydroxyacetone phosphate (nucleophile) and glyceraldehyde 3-phosphate (electrophile) into fructose 1,6-diphosphate by the enzyme aldolase during carbohydrate biosynthesis. The reverse reaction, used in the glycolytic pathway for carbohydrate metabolism, was formulated as a reverse aldol reaction. 

Aldolase catalyses both aldol and reverse aldol reactions according to an organism s needs. In glycolysis, the substrate fmctose 1,6-diphosphate is cleaved by a reverse aldol reaction to provide one molecule of glyceraldehyde 3-phosphate and one molecule of dihydroxyacetone phosphate. In carbohydrate synthesis, these two compounds can be coupled in an aldol reaction to produce fmctose 1,6-diphosphate. 

Glyceraldehyde phosphate and dihydroxyacetone phosphate react in the presence of aldolase to yield fructose diphosphate as the only product under physiolocial conditions. 

By the reaction of D-glyceraldehyde and 1,3-dihydroxypropane (both as monophosphate ester), D-fructose as the 1,6-diphosphate ester is formed. The process is readily reversible and is catalyzed by an enzyme known as aldolase. 

Terpenes, biogenetically, arise from two simple five-carbon moieties. Isoprenyl-diphosphate (IPP) and dimethylallyldiphosphate (DMAPP) serve as universal precursors for the biosynthesis of terpenes. They are biosynthesised from three acetylcoenzyme A moieties through mevalonic acid (MVA) via the so-called mevalonate pathway. About 10 years ago, the existence of a second pathway leading to IPP and DMAPP was discovered involving l-deoxy-D-xylulose-5-phos-phate (DXP) and 2C-methyl-D-erythritol-4-phosphate (MEP). This so-called non-mevalonate or deoxyxylulose phosphate pathway starts off with the condensation of glyceraldehyde phosphate and pyruvate affording DXP. Through a series of reactions as shown in Fig. 4.1, IPP and DMAPP are formed, respectively [3,7, 42, 43]. 

The formation of ATP (or GTP) at the expense of the energy released by the oxidative decarboxylation of a-ketoglutarate is a substrate-level phosphorylation, like the synthesis of ATP in the glycolytic reactions catalyzed by glyceraldehyde 3-phosphate dehydrogenase and pyruvate kinase (see Fig. 14-2). The GTP formed by succinyl-CoA synthetase can donate its terminal phosphoryl group to ADP to form ATP, in a reversible reaction catalyzed by nucleoside diphosphate kinase (p. 505) ... 

Some bacteria that lack the usual aldolase produce ethanol and lactic acid in a 1 1 molar ratio via the "heterolactic fermentation." Glucose is converted to ribulose 5-phosphate via the pentose phosphate pathway enzymes. A thiamin diphosphate-dependent "phosphoketolase" cleaves xylulose 5-phosphate in the presence of inorganic phosphate to acetyl phosphate and glyceraldehyde 3-phosphate. 

Starting with the simple compounds acetyl-CoA, glyceraldehyde-3-phosphate, and pyruvate, which arise via the central pathways of metabolism, the key intermediate isopentenyl diphosphate is formed by two independent mutes. It is then converted by bacteria, fungi, plants, and animals into thousands of different naturally occurring products. These include high polymers, such as rubber, as well as vitamins, sterols, carotenoids, and over 30,000 different terpenes and related compounds. Many of the latter are found only in specific plants where they may function as defensive compounds or pheromones. 

The terpenes, carotenoids, steroids, and many other compounds arise in a direct way from the prenyl group of isopentenyl diphosphate (Fig. 22-1).16a Biosynthesis of this five-carbon branched unit from mevalonate has been discussed previously (Chapter 17, Fig. 17-19) and is briefly recapitulated in Fig. 22-1. Distinct isoenzymes of 3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase) in the liver produce HMG-CoA destined for formation of ketone bodies (Eq. 17-5) or mevalonate.7 8 A similar cytosolic enzyme is active in plants which, collectively, make more than 30,000 different isoprenoid compounds.910 However, many of these are formed by an alternative pathway that does not utilize mevalonate but starts with a thiamin diphosphate-dependent condensation of glyceraldehyde 3-phosphate with pyruvate (Figs. 22-1,22-2). 

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-... 

Figure 22-2 The glyceraldehyde 3-phosphate pyruvate alternative pathway of isoprenoid biosynthesis. The intermediate 1-deoxyxylulose 5-phosphate may enter terpenes, vitamin B6, and thiamin. Isopentenyl diphosphate is shown as the final product, but the intermediate steps are uncertain. See Lange et al 2 ...

Another assay for phosphoffuctokinase involves converting the fructose 1,6-diphosphate to dihydroxyacetone phosphate and glyceraldehyde 3-phosphate with aldolase, equilibrating the triosephosphates with triosephosphate isomerase, and then measuring the production of NADH on the oxidation of the glyceraldehyde phosphate by glyceraldehyde 3-phosphate dehydrogenase. 

The formation of deoxyribose, die pentose moiety of deoxyribonucleic acid, can occur directly from ribose while the latter is in the form of a nucleotide diphosphate. Deoxyribose-5-phosphate can also be formed by condensation of acetaldehyde and glyceraldehyde-3-phosphate. 

Photosynthesis. The formation of carbohydrates in green plants by the process of photosynthesis is described in ihc entry on Photosynthesis. The synthetic mechanism involves the addition of carbon dioxide to ribulose-1,5-diphosphate and the subsequent formation of two molecules of 3-phosphoglyccric acid which are reduced to glyceraldehyde-3-phosphate. The triose phosphates are utilized to again from ribulose-5-phosphates by enzymes of the pentose phosphate cycle Phosphorylation or ribulose-5-phosphate with ATP regenerates ribulose-1.5-diphosphate to accept another molecule of carbon dioxide. See also Phosphorylation (Photosynthetlc). 

At this stage the enzyme aldolase catalyzes the aldol cleavage of fructose 1,6-diphosphate. One product is glyceraldehyde 3-phosphate and the other is... 

For studies of the specificity of FDPases, and particularly with sedoheptulose 1,7-diphosphate, the assay for inorganic phosphate is most convenient. The disappearance of SDP can also be followed spectrophoto-metrically using aldolase, glyceraldehyde-3-phosphate dehydrogenase, and DPN 21). 

Consider the glyceraldehyde-3-phosphate dehydro-genase-phosphoglycerokinase enzymes of glycolysis and the succinate thiokinase of the TCA cycle. Compare the mechanisms of incorporation of inorganic phosphate into the respective nucleoside diphosphates. 

Deoxy-D-xylulose 5-phosphate is formed from the glycolytic pathway intermediates pyruvic acid and glyceraldehyde 3-phosphate with the loss of the pyruvate carboxyl (Figure 5.6). Thiamine diphosphate-mediated decarboxylation of pyruvate... 

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