Dihydroxyacetone phosphate biosynthesis

Dihydroxyacetone phosphate (82) is a substrate for a-glycero-phosphate dehydrogenase, aldolase, and triose phosphate isomerase, and its O-alkyl ethers are intermediates in the biosynthesis of phospholipids. In neutral aqueous solution at 20 °C, dihydroxyacetone phosphate exists as an equilibrium mixture of the keto (82), gem-d o (83), and enol (84) forms, as shown by n.m.r. spectroscopy. The proportion of (82) to (83)... 

Agranofl, B. W., Hajra, A. K. The acyl dihydroxyacetone phosphate pathway for glycerolipid biosynthesis in mouse liver and Ehrlich ascites tumor cells. Proc. Natl. Acad. Sci. U.S. 68, 411-415 (1971). 

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. 

D-bifunctional protein deficiency [5], 2-methyl acyl-CoA racemase (AMACR) deficiency [3] and sterol carrier protein (SCP-x) deficiency [6], the disorders of etherphospholipid biosynthesis (dihydroxyacetone phosphate acyltransferase and alkyl- dihydroxyacetone phosphate synthase deficiency) [2], the disorders of phytanic acid alpha-oxidation (Refsum disease) [15], and the disorders of glyoxylate detoxification with hyperoxaluria type 1 as caused by alanine glyoxylate aminotransferase deficiency as a sole representative. 

Write the sequence of steps and the net reaction for the biosynthesis of phosphatidylcholine by the salvage pathway from oleate, palmitate, dihydroxyacetone phosphate, and choline. Starting from these precursors, what is the cost (in number of ATPs) of the synthesis of phosphatidylcholine by the salvage pathway ... 

In the first phase of phospholipid synthesis from glyc-erol-3-phosphate to phosphatidic acid, the pathways in E. coli and eukaryotes are very similar (see fig. 19.2). The major difference is that one additional pathway exists for generation of phosphatidic acid from dihydroxyacetone phosphate, an intermediate in glycolysis. Once phosphatidic acid is made, it is rapidly converted to diacylglycerol or CDP-diacylglycerol (see fig. 19.2) both of which are intermediates for the biosynthesis of eukaryotic phospholipids. 

Fatty acids released by lipoprotein lipase are taken up by the tissue where this enzyme is located, where they may be oxidized (see later) or stored in the form of triglycerides, such as adipose tissue. Triglyceride biosynthetic enzymes are located in the endoplasmic reticulum. Triglyceride biosynthesis is summarized in Figure 19.4. It is seen that dihydroxyacetone phosphate (see Chapter 18) is a key intermediate. It can combine with an acyl residue carried by acyl coenzyme A... 

Figure 27 Four possible pathways for ABA biosynthesis. Open and closed circles show the 13C label from [1-13C]-d-glucose in the mevaloic acid pathway and the MEP pathway, respectively. DAP, dihydroxyacetone phosphate DXP, 1-deoxy-xylulose-5-phosphate FDP, farnesyl diphosphate GAP, glyceraldehyde-3-phosphate GGDP, geranylgeranyl diphosphate HMG-CoA, 3-hydroxy-3-methylglutaryl CoA IDP, isopentenyl diphosphate MEP, 2-C-methyl-D-erythritol-4-phosphate.

Bacteria and plants use aspartate (54) and dihydroxyacetone phosphate (50) as precursors for the biosynthesis of nicotinamide via quinolinic acid (53, Fig. 6B) (33). 

Stable isotope-labelled intermediates playing an important role in the study of the mevalonate as well as the deoxyxylulose phosphate pathway of isoprenoid biosynthesis have been prepared. C- and " C-labelled 4-diphosphacytidyl-2C-methyl-D-erythrytol (52) and 2C-methyl-D-erythrytol-4-phosphate (53) have been obtained in milimol quantity and in high yield by sequences of one-pot reactions using C-labelled pyruvate, or dihydroxyacetone phosphate... 

Peroxisomes contain dihydroxyacetone phosphate acyl-transferase and alkyldihydroxyacetone phosphate synthase, which are involved in synthesis of the plasmalogens (Chapter 19). Peroxisomes may also participate in the biosynthesis of bile acids. The conversion of trihydrox-ycholestanoic acid to cholic acid (Chapter 19) has been localized to peroxisomes. 

An mCyN unit is also present in the core of the polyketide antibiotic asukamycin from Streptomyces nodosas subsp. asukaensis [96]. This has been shown to arise not from a variant of the shikimate pathway, but from the condensation of a C4 unit from the TCA cycle, closely related to succinate, with a C3 unit, possibly dihydroxyacetone phosphate, from the triose pool. Related studies concerning 3-amino-4-hydroxybenzoic acid biosynthesis in Streptomyces murayamaensis mutants MC2 and MC3 support this hypothesis [97]. 

Figure 5. Biosynthetic pathways for diacyl, plasmalogen and alkyl-ether molecular subclasses of phospholipids. Monoacyl dihydroxyacetone phosphate is the key branch-point intermediate whose utilization determines the phospholipid subclass distribution of newly synthesized phospholipids. Reduction of monoacyl dihydroxyacetone phosphate leads to the biosynthesis of diacyl phospholipids. Fatty alcohol exchange, catalyzed by alkyl dihydroxyacetone phosphate synthase, is the first committed step in the biosynthesis of alkyl-ether and plasmalogen subclasses of phospholipids.

A series of early biochemical studies have identified the primary precursors of methanofuran as phosphoe-nolpyruvate, dihydroxyacetone phosphate, tyrosine, glutamate, 2-ketoglutarate, acetate, and CO2 as illustrated in Figure 2. Isotope incorporation studies with H- and C-labeled precursors have shown that the fiiran moiety of methanofuran is formed from phosphoenolpyruvate and dihydroxyacetone phosphate. Following dehydration, the next step in the biosynthesis of the fiiran would be a reduction of the carboxylic acid to an aldehyde. The final step in the biosynthesis of the fiiran moiety of methanofuran is expected to be a transamination reaction to form the 2-aminomethyI subunit. 

Fig. 4. Ether phospholipid synthesis from dihydroxyacetone-phosphate. (A) Dihydroxyacetone-P acyl transferase (DHAPAT). The first step of ether phospholipid synthesis is catalyzed by peroxisomal DHAPAT. This enzyme is a required component of complex ether lipid biosynthesis and its role cannot be assumed by a cytosolic enzyme that also forms acyldihydroxyacetone-P. (B) Ether bond formation by alkyl-DHAP synthase. The reaction that forms the 0-alkyl bond is catalyzed by alkyl-DHAP synthase and is thought to proceed via a ping-pong mechanism. Upon binding of acyl-DHAP to the enzyme alkyl-DHAP synthase, the pro-f hydrogen at carbon atom 1 is exchanged by enolization of the ketone, followed by release of the acyl moiety to form an activated enzyme-DHAP complex. The carbon atom at the 1-position of DHAP in the enzyme complex is thought to carry a positive charge that may be stabilized by an essential sulfhydryl group of the enzyme thus, the incoming alkox-ide ion reacts with carbon atom 1 to form the ether bond of alkyl-DHAP. It has been proposed that a nucleophilic cofactor at the active site covalently binds the DHAP portion of the substrate.

In contrast, sterols, which are derived from a C30 isoprenoid carbon skeleton, are produced in the cytosol (Lichtenthaler 1999). In higher plants, the carbon feedstocks required to support biosynthesis outside the chloroplast are exported mainly as the C3 carbohydrate derivative, dihydroxyacetone phosphate (Schleucher et al. 1998). As a result of these factors, plastidic fatty acids and cytosolic sterols, both derived from acetate, can have different isotopic starting points. Moreover, even when starting points and downstream processes are closely similar, the separation of pathways between compartments can mean that divisions of carbon flows at branch points differ significantly and, therefore, that final isotopic compositions differ sharply. 

Figure 19.2 schematically depicts the primary pathways of prokaryotic and eukaryotic glycerophospholipid biosynthesis. Note that the center pathway shown in purple occurs in both prokaryotic and eukaryotic cells. Phosphatidic acid, the branch point between the synthesis of fats and other glycerophospholipids, can be made via three different pathways in eukaryotes-from glycerol-3-phosphate (Figure 19.3), from diacylglycerol (Figure 19.2), and from dihydroxyacetone phosphate (DHAP) (see here). 

Because o-glucose has twice as many carbons as pymvate, it should not be surprising that one of the steps in the biosynthesis of o-glucose is an aldol addition. An enzyme called aldolase catalyzes an aldol addition between dihydroxyacetone phosphate and... 

Early in fermentation when yeast is growing, removal of pyruvate for biosynthesis might be expected to lead to a build up of NADH and thus to a halt in catabolism. To avoid this, cells reduce dihydroxyacetone phosphate to glycerol phosphate. This, in turn, is dephosphorylated to produce glycerol which is excreted. 

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