Glyceraldehyde, carbon atom reactions

This curious phenomenon of inversion of groups about the asymmetric carbon atom, first studied by Walden (1893, 1985) is called Walden Inversion. In a number of other reactions, the inversion was so quantitative that the yield of the opitcal isomer was 100% while in others the product was a mixture of the (+) and (-) forms in unequal amounts signifying that the inversion was partial. The above conversion has been shown to occur in two steps. The step in which the actual inversion occurs constitutes a Walden inversion. Change in the sign of rotation does not necessarily mean that an inversion of configuration has occurred as is clear from the oxidation of D(+) glyceraldehyde to D(-) glyceric acid. 

It should be noted that the aldehydic carbon atom of the triose enediol released from the reducing end originates from C-3 of the hexose, whereas the carbonyl group of the D-glyceraldehyde formed from the nonreducing end originates from C-4. Thus, in the absence of isomerization, hexoses labeled at either C-l or C-6 would mainly yield lactic-3-14C acid. However, extensive isomerization does occur, and lactic-2-14C acid and lactic-I-14C acid are also found in considerable proportion. The presence of lactic-2-14C acid is explained by the reaction of the dicarbonyl compound 78 that undergoes a benzilic... 

Transaldolase catalyzes a two-step conversion. The first step, an aldol cleavage of the bond between C-3 and C-4 of a ketose, is essentially identical to the reaction catalyzed by aldolase. However, the dihydroxyacetone that is produced in the transaldolase reaction from carbons 1, 2, and 3 is not released. Rather, it is held at the catalytic site while the glyceraldehyde-3-phosphate produced diffuses away and is replaced by erythrose-4-phosphate. An aldol condensation then generates the second product of the reaction, a ketose that contains the first three carbon atoms of the original ketose attached to C-1 of the acceptor aldose (fig. 12.32). 

Transketolase is one of several enzymes that catalyze reactions of intermediates with a negative charge on what was initially a carbonyl carbon atom. All such enzymes require thiamine pyrophosphate (TPP) as a cofactor (chapter 10). The transketolase reaction is initiated by addition of the thiamine pyrophosphate anion to the carbonyl of a ketose phosphate, for example xylulose-5-phosphate (fig. 12.33). The adduct next undergoes an aldol-like cleavage. Carbons 1 and 2 are retained on the enzyme in the form of the glycol-aldehyde derivative of TPP. This intermediate condenses with the carbonyl of another aldolase. If the reactants are xylulose-5-phosphate and ribose-5-phosphate, the products are glyceraldehyde-3-phosphate and the seven-carbon ketose, sedoheptulose-7-phosphate (see fig. 12.33). 

The carbon that lost a proton is now a nucleophilic center and can therefore react with formaldehyde. As mentioned above, formaldehyde has an electrophilic center on carbon. Therefore, the carbon of formaldehyde can react with the nucleophilic carbon on the enolate of glycolaldehyde to form a new compound containing three carbon atoms, a three-carbon carbohydrate called glyceraldehyde. The overall reaction sequence is often called an aldol addition reaction, here of formaldehyde and glycolaldehyde. 

The requirement for NADPH far exceeds an equal requirement for ribose 5-phosphate (necessary for the production of nucleic acids and nucleotides), and so the second phase of the pentose phosphate pathway converts the C5 sugar, by a series of reversible reactions, into the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. This interconversion is shown in Fig. 11-27. Not only does the second phase of the pathway conserve all the carbon atoms of the C5 sugar, but it produces erythrose 4-phosphate (C4), xylulose 5-phosphate (C5), and sedoheptulose 7-phosphate (C7), which are available to other metabolic processes. 

Nonoxidative phase of the pentose phosphate pathway. Numbers in parentheses show the distribution of carbon atoms between the various branches of each reaction. TK = transketolase GAP = glyceraldehyde 3-phosphate DHAP = dihydroxyacetone phosphate P = phosphate. 

TPP is a coenzyme for transketolase, the enzyme that catalyzes the conversion of a ke-topentose (xylulose-5-phosphate) and an aldopentose (ribose-5-phosphate) into an al-dotriose (glyceraldehyde-3-phosphate) and a ketoheptose (sedoheptulose-7-phosphate). Notice that the total number of carbon atoms in the reactants and products does not change (5+5 = 3+ 7). Propose a mechanism for this reaction. 

One molecule of glyceraldehyde-3-phosphate has already been produced by the aldolase reaction we now have a second molecule of glyceraldehyde-3-phosphate, produced by the triosephosphate isomerase reaction. The original molecule of glucose, which contains six carbon atoms, has now been converted to two molecules of glyceraldehyde-3-phosphate, each of which contains three carbon atoms. 

Two enzymes, transhetolase and transaldolase, are responsible for the reshuffling of the carbon atoms of sugars such as ribose-5-phosphate and xylulose-5-phosphate in the remainder of the pathway, which consists of three reactions. Transketolase transfers a two-carbon unit. Transaldolase transfers a three-carbon unit. Transketolase catalyzes the first and third reactions in the rearrangement process, and transaldolase catalyzes the second reaction. The results of these transfers are summarized in Table 18.2. In the first of these reactions, a two-carbon unit from xylulose-5-phosphate (five carbons) is transferred to ribose-5-phosphate (five carbons) to give sedoheptulose-7-phosphate (seven carbons) and glyceraldehyde-3-phosphate (three carbons), as shown in Figure 18.15, bottom, red numeral 1. 

The discovery that -imines obtained from a variety of isopropylidene protected sugar open-chain aldehydes and ketenes, or ketene equivalents afford cz5-substituted 8-lactam adducts usually with a high asymmetric induction and definite relative geometry depending on the absolute configuration of the stereogenic center next to the imine carbon atom (Scheme 1) [21-24], prompted many laboratories to exploit this reaction in a number of syntheses. Imines derived from both easily available enantiomeric forms of 2,3-0-isopropylidene-glyceraldehyde are particularly attractive. 

The NADPH and ATP generated are used within the chloroplast to fix carbon dioxide (CO2) and reduce it to sugar. This sequence of reactions is called the Calvin-Benson-Bassham cycle and this cycle results in the production of the phosphorylated 3-carbon sugar glyceraldehyde-3-phosphate, also called triose. This triose sugar is exported out of the chloroplast and in this course we will refer to it as CH2O which is the chemical composition of sugar when normalized to 1 carbon atom. 

A number of synthesis of amino-sugars from chiral non-carbohydrate starting materials have been reported. A reaction sequence used previously to synthesize 2-amino-2-deoxy-D-ribose from 2,3-0-isopropylidene-D-glyceraldehyde (V0I.I6, p.92-3) has been improved K) achieve better stereoselectivity in the initial aldol condensation used to extend the chain by two carbon atoms. The synthesis of a 6,6,6-trifluoro-L-daunosamine derivative from 2,3-0-cyclohexylidene-D-glyceraldehyde is covered in Chapter 8, and of 2,5-dideoxy-2,5-imino-pentonic acids from tartaric acid in Chapter 16. 

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