Dihydroxyacetone phosphate formation

Aldolase 4.1.2.3 Fructose-1,6-diphosphate Dihydroxyacetone phosphate Formation of a hydrazone... 

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). 

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

Figure 10.18 Enzymatic in situ generation of dihydroxyacetone phosphate from fructose 1,6-bisphosphate (b), with extension to an in vitro artificial metabolism for its preparation from inexpensive sugars alongthe glycolysis cascade (a), and utilization for subsequent stereoselective carbon-carbon bond formation using an aldolase with distinct stereoselectivity (c).

Figure 10.20 Substrate analogs of dihydroxyacetone phosphate accessible by the CPO oxidation method, and spontaneous, reversible formation of arsenate or vanadate analogs of dihydroxyacetone phosphate/n s/tu for enzymatic aldol additions.

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

The reverse aldol reaction results in the formation of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Dihydroxyacetone phosphate... 

Treatment with sodium borohydride of the enzyme-substrate complex of aldolase A and dihydroxyacetone phosphate leads to formation of a covalent linkage between the protein and substrate. This and other evidence suggested a Schiff base intermediate (Eq. 13-36). When 14C-containing substrate was used, the borohydride reduction (Eq. 3-34) labeled a lysine side chain in the active site. The radioactive label was followed through the sequence determination and was found on Lys 229 in the chain of 363 amino acids.186/188 188b Tire enzyme is another (a / P)8-barrel protein and the side chain of Lys 229 projects into the interior of the barrel which opens at the C-terminal ends of the strands. The conjugate base form of another lysine,... 

Aldolases such as fructose-1,6-bisphosphate aldolase (FBP-aldolase), a crucial enzyme in glycolysis, catalyze the formation of carbon-carbon bonds, a critical process for the synthesis of complex biological molecules. FBP-aldolase catalyzes the reversible condensation of dihydroxyacetone phosphate (DHAP) and glyceralde-hyde-3-phosphate (G3P) to form fructose-1,6-bisphosphate. There are two classes of aldolases the first, such as the mammalian FBP-aldolase, uses an active-site lysine to form a Schiff base, whereas the second class features an active-site zinc ion to perform the same reaction. Acetoacetate decarboxylase, an example of the second class, catalyzes the decarboxylation of /3-keto acids. A lysine residue is required for good activity of the enzyme the -amine of lysine activates the substrate carbonyl group by forming a Schiff base. 

Fig. 10.1 Cellular formation and metabolism of methylglyoxal (MG). AGEs advanced glycoxida-tion endproducts DHAP dihydroxyacetone phosphate G3P glyceraldehyde 3-phosphate F-6P fructose 6-phosphate F-1,6P2 fmctose 1,6-bisphosphate Gly-I II glyoxalase I II SSAO semicarbazide-sensitive amine oxidase AMO amine oxidase.

Answer Problem 1 outlines the steps in glycolysis involving fructose 1,6-bisphosphate, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate. Keep in mind that the aldolase reaction is readily reversible and the triose phosphate isomerase reaction catalyzes extremely rapid interconversion of its substrates. Thus, the label at C-l of glyceraldehyde 3-phosphate would equilibrate with C-l of dihydroxyacetone phosphate (AG ° = 7.5 kJ/mol). Because the aldolase reaction has AG ° = -23.8 kJ/mol in the direction of hexose formation, fructose 1,6-bisphosphate would be readily formed, and labeled in C-3 and C-4 (see Fig. 14-6). 

In glycolysis, the conversion of dihydroxyacetone phosphate to glyceraldehyde 3-phosphate, catalyzed by triosephosphate isomerase, is reversible. Given that at equilibrium the reaction strongly favors the formation of dihydroxyacetone phosphate, how does glycolysis proceed ... 

Figure 5 Formation of the pyridoxine ring in vitamin B5. (A) deoxyxylulose phosphate-dependent pathway (B) deoxyxylulose phosphate-independent pathway. 43, 1-deoxy-D-xylulose 5-phosphate 47, 3-amlno-1-hydroxyacetone 1-phosphate 48, pyridoxine 5 -phosphate 49, ribulose 5-phosphate 50, dihydroxyacetone phosphate 39, pyridoxal 5 -phosphate.

Fig. 10.9 Formation of Phosphatidic Acid from Glycerol or Dihydroxyacetone Phosphate and its Conversion to Triacylglycerol or Phospholipids.

An aldolase-based strategy conveniently provided access to thioketoses with sulfur in the ring. For example, RAMA-catalyzed carbon-carbon bond formation of a 3-thioglycerinaldehyde with dihydroxyacetone phosphate gave 6-thio-D-fructose 36 [32] (Figure 9.8), also available by enzymatic isomerization of 6-thio-D-glucose or 6-thio-L-sorbose with glucose isomerase (EC 5.3.1.5 vide infra) [32,41]. 

The enzymatic aldol reaction represents a useful method for the synthesis of various sugars and sugar-like structures. More than 20 different aldolases have been isolated (see Table 13.1 for examples) and several of these have been cloned and overexpressed. They catalyze the stereospecific aldol condensation of an aldehyde with a ketone donor. Two types of aldolases are known. Type I aldolases, found primarily in animals and higher plants, do not require any cofactor. The x-ray structure of rabbit muscle aldolase (RAMA) indicates that Lys-229 is responsible for Schiff-base formation with dihydroxyacetone phosphate (DHAP) (Scheme 13.7a). Type II aldolases, found primarily in micro-organisms, use Zn as a cofactor, which acts as a Lewis acid enhancing the electrophilicity of the ketone (Scheme 13.7b). In both cases, the aldolases accept a variety of natural (Table 13.1) and non-natural acceptor substrates (Scheme 13.8). 

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