Aldehydes, glyceraldehyde-3-phosphate

Alanine residues, glyceraldehyde-3-phos-phate dehydrogenase, 11,12 Alcohols, catalase smd, 388, 398, 401 Aldehydes, glyceraldehyde-3-phosphate dehydrogenase smd, 39 Algae, sulfate assimilation by, 279, 280 Alkali... 

One of the steps in the pathway is an aldol condensation between the ketone dihydroxyacetone phosphate and the aldehyde glyceraldehyde-3-phosphate. 

Scheme 9.6. A cartoon representation of the enzyme (named glyceraldehyde-3-phosphate dehydrogenase, EC 1.2.1.12) catalyzed oxidation of the aldehyde glyceraldehyde-3-phosphate to the carboxylic acid-phosphoric acid mixed anhydride, 1,3-bisphosphoglycerate (which can be hydrolyzed to the corresponding carboxyhc acid and phosphoric acid). Start in a clockwise direction from the npper left.

FIGURE 19.18 A mechanism for the glycer-aldehyde-3-phosphate dehydrogenase reaction. Reaction of an enzyme snlfliydryl with the carbonyl carbon of glyceraldehyde-3-P forms a thiohemiacetal, which loses a hydride to NAD to become a thloester. Phosphorolysls of this thloester releases 1,3-blsphosphoglycerate. 

The idea that enzymes whose normal substrate is orthophosphate can use arsenate in its place, and that the esters of arsenate formed are rapidly hydrolyzed, was given by Braunstein (16) in 1931, to explain the effects of arsenate on glycolysis. This idea was formulated more precisely for glyceraldehyde-phosphate dehydrogenase in 1939. This enzyme, E—SH, normally oxidizes its aldehyde substrate with phosphate uptake to give an acyl phosphate, as follows ... 

The reaction is effectively an aldol condensation between the enol of the keto-sugar phosphate and the electrophilic aldehyde of glyceraldehyde phosphate and the enzyme is named appropriately aldolase. The product is the keto-hexose fructose-1,6-diphosphate. 

Aldolase then catalyses the reversible cleavage of the six-carbon molecule into two three-carbon molecules, triose phosphates. Yeast aldolase is inactivated by cysteine and may be reactivated by Zn +, Fe + or Co + ions. The triose phosphates are a mixture of dihydroxyacetone phosphate and D-glyceraldehyde-3-phosphate. Only the latter undergoes further change in the EMP pathway, but an equilibrium between the two is maintained by enzymic conversion of some of the dihydroxyacetone phosphate into glycer-aldehyde-3-phosphate, catalysed by the enzyme triose-phosphate isomerase. 

Glyceraldehyde phosphate dehydrogenase probably holds the distinction of being the classic thiol enzyme in the minds of most biochemists . The thiol is believed to be involved in the initial attachment of the aldehyde substrate as a thiohemiacetal. The em me-bound thiohemiacetal is then oxidized by NAD generating an enzyme-bound thioester. In more sophisticated proposals for this mechanism the nicotinamide cofactor interacts with the active centre thiol as a charge transfer type of complex. This facilitates the reaction of the thiol with the carbonyl of the substrate. The thiol addition and the electron transfer to nicotinamide occur... 

Two pathways for PLP biosynthesis de novo are known in plant and microorganisms. The first was extensively studied in Escherichia coli. This pathway is articulated in two branches which join in a ring closure reaction catalysed by PNP synthase. One branch started from pyruvate and glycer-aldehyde 3-phosphate and the other from 4-phosphohydroxy-L-threonine (derived from erythrose 4-phosphate). The second route is when PLP is formed from glutamine, either ribose 5-phosphate or ribulose 5-phosphate and either dihydroxyacetone phosphate or glyceraldehyde 3-phosphate by action of the PLP synthase complex (Roje 2007). This pathway was discovered in fungi and it has become clear that it is much more widely distributed than the first pathway. It exists in Archaea, most eubacteria and plants. 

Because dihydroxyacetone phosphate and glyceraldehyde 3-phosphate enolize to give a common intermediate, they exist in equilibrium. The enzyme triose phosphate isomerase efficiently catalyzes the isomerization. Although the enediol intermediate is chiral, the enzyme forms only the i enantiomer of glyceraldehyde 3 phosphate. In aqueous solution, an acid-catalyzed reaction would yield a racemic mixture of aldehyde 3-phosphate. 

Transaldolase. While aldolase produces only trioses from hexoses (and the reverse) the analogous enzyme transaldolase transfers the dihydroxyacetone residue onto other aldoses. The enzyme is highly specific Transaldolase splits only fructose and sedoheptulose (reverse of aldol condensation) and transfers the C3 residue (a dihydroxyacetone, which remains bound to the e-amino group of a lysine residue on the enzyme surface) to a corresponding aldehyde, i.e. to glyceraldehyde phosphate or erythrose 4-phosphate, or possibly to ribose 5-phosphate ... 

Dehydrogenation of Glyceraldehyde Phosphate. Next, the aldehyde group of the triose is dehydrogenated to yield the carboxylic acid. This reaction could proceed in the simplest possible way, since it is strongly exergonic ... 

Ferredoxin rednctases in Pyrococcus furiosus inclnding aldehyde ferredoxin reductase, glyceraldehyde-3-phosphate ferredoxin oxidorednctase, and formaldehyde ferredoxin reductase (Roy et al. 1999). 

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. 

There is only one compound which carries an aldehyde group, so glyceraldehyde-3-P must be compound (viii) and acetone you may already know as a ketone, so compound (ii) is dihydroxyacetone phosphate, DHAP. 

A basic group removes a proton from the P-carbon of the iminium and forms the enamine. This enamine then reacts as a nucleophile towards the aldehyde group of glyceraldehyde 3-phosphate in a simple addition reaction, and the proton necessary for neutralizing the charge is obtained from an appropriately placed amino acid residue. Finally, the iminium ion loses a proton and hydrolysis releases the product from the enzyme. 

It is conceptually easier to consider initially the aldol reaction rather than the reverse aldol reaction. This involves generating an enolate anion from the dihydroxyacetone phosphate by removing a proton from the position a to the ketone group. This enolate anion then behaves as a nucleophile towards the aldehyde group of glyceraldehyde 3-phosphate, and an addition reaction occurs, which is completed by abstraction of a proton, typically from solvent. In the reverse reaction, the leaving group would be the enolate anion of dihydroxyacetone phosphate. 

In this step, the aldehyde group of glyceraldehyde 3-phosphate appears to be oxidized to an acid, which becomes phosphorylated, and hydrogen is passed to NAD+, which becomes reduced to NADH. We shall see shortly that the fate of this NADH is quite significant. 

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