Dihydroxyacetone ketose

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

Figure 9.2 Dihydroxyacetone and enantiomers of erythrulose. The triose, dihydroxyacetone, is the parent compound of the ketoses because it has the lowest number of carbon atoms but it does not contain an asymmetric centre. Therefore the d and l series of the ketoses are built up from the two enantiomers of the tetrose, erythrulose, which has one asymmetric carbon atom ( ).

Reaction of ribose 5-phosphate 116 with dihydroxyacetone phosphate, catalyzed by fructose 1,6-diphosphate aldolase from rabbit muscle (RAMA) affords the ketose diphosphate 117. Dihydroxyacetone phosphate was formed in situ from fructose 1,6-diphosphate by action of RAMA and triose phosphate isome-rase (TPI). The diphosphate 117 was dephosphorylated enzymatically using acid phosphatase, and the ketose 118 was reduced directly into the a-C-manno-side 119 by treatment with bistrimethylsilyltrifluoroacetamide, trimethylsilyl-triflate and triethylsilane (Scheme 28) [45]. 

The simplest carbohydrates, sometimes referred to as monosaccharides, or sugars, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses). They can be derived from polyalcohols (polyols) by oxidation of one carbinol group to a carbonyl group. For example, the simple three-carbon triol, glycerol, can be converted either to the aldotriose, glyceraldehyde, or to the ketotriose, dihydroxyacetone, by loss of two hydrogens (fig. 12.1). 

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). 

While the lyases that transfer a pyruvate unit form only a single stereogenic center, the group of dihydroxyacetone-phosphate-(DHAP, 41)-dependent aldolases create two new asymmetric centers, one each at the termini of the new C-C bond. A particular advantage for synthetic endeavors is the fact that Nature has evolved a full set of four stereochemically-complementary aldolases [189] (Scheme 6) for the retro-aldol cleavage of diastereoisomeric ketose 1-phosphates— D-fructose 1,6-bisphosphate (42 FruA), D-tagatose 1,6-bisphosphate (43 TagA), L-fuculose 1-phosphate (44 FucA), and L-rhamnulose 1-phosphate (45) aldolase (RhuA). In the direction of synthesis this formally allows the... 

Know the meaning of aldose, ketose, triose, tetrose, pentose, hexose, glyceraldehyde, dihydroxyacetone. 

Glyceraldehyde (an aldose) C HgOj Dihydroxyacetone (a ketose) CaHgOg... 

An enzymatic aldol condensation of dihydroxyacetone phosphate and D,L-glyceraldehyde has reportedly48 given a mixture of L-sorbose l-phos-phate along with D-fructose 1-phosphate. The ketoses may be set free with phosphatase. 

Structurally, the parent compound of the simple ketoses is dihydroxyacetone, a structural isomer of glyceraldehyde. 

Although dihydroxyacetone does not possess a chiral carbon atom, the simple ketoses are related to it structurally by the introduction of hydroxylated chiral carbon atoms between the keto group and one of the hydroxymethyl groups. Thus there are two ketotetroses, four ketopentoses, and eight ketohexoses. 

Some ketoses are not related structurally to dihydroxyacetone. They are named by considering the configurations of all the chiral carbon atoms as a unit, ignoring the carbonyl group. 

If the name of a ketose contains no number, it is assumed the ketose is related to dihydroxyacetone and the keto group is at position 2. 

Glyceraldehyde is an aldehyde sugar or aldose and dihydroxyacetone is a keto-sugar or ketose. That ending -ose just refers to a sugar. These two molecules combine to form the six-carbon sugar,... 

The family of D-ketoses, shown in Figure 27.5, is formed from dihydroxyacetone by adding a new carbon (bonded to H and OH) between C2 and C3. Having a carbonyl group at C2 decreases the number of stereogenic centers in these monosaccharides, so that there are only four D-ketohexoses. The most common naturally occurring ketose is D-fructose. 

Dihydroxyacetone is the simplest ketose. The stereochemical relation between d-ketoses containing as many as six carbon atoms are shown in Figure 11.3. Note that ketoses have one fewer asymmetric center than do aldoses with the same number of carbons. d-Fructose is the most abundant ketohexose. 

Much is known about the catalytic mechanism of triose phosphate isomerase. TIM catalyzes the transfer of a hydrogen atom from carbon 1 to carbon 2 in converting dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, an intramolecular oxidation-reduction. This isomerization of a ketose into an aldose proceeds through an enediol intermediate (Figure 16.6). 

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