Glucose interconvertibility

Although the mutarotation of glucose interconverts only anomeric pyranose forms, some aldohexoses form a four-component mixture of anomeric pyranoses and anomeric furanoses. Fructose also forms a four-component mixture of anomeric furanoses and pyranoses. 

When this isomeiization reaction is catalyzed by alkah, it is termed the Lobry de Bmyn-Alberda van Ekenstein reaction. By it, D-glucose, D-mannose, and D-fmctose can be interconverted. The isomerizations involve a common intermediate, the 1,2-enediol. In the Glu—Man—Fm interconversions... 

Glycogen synthase also exists in two distinct forms which can be interconverted by the action of specific enzymes active, dephosphorylated glycogen synthase I (glucose-6-P-independent) and less active phosphorylated glycogen synthase D (glucose-6-P-dependent). The nature of phosphorylation is more complex with glycogen synthase. As many as nine serine residues on the enzyme appear to be subject to phosphorylation, each site s phosphorylation having some effect on enzyme activity. 

This enzyme interconverts ribulose-5-P and ribose-5-P via an enediol intermediate (Figure 23.30). The reaction (and mechanism) is quite similar to the phosphoglucoisomerase reaction of glycolysis, which interconverts glucose-6-P and fructose-6-P. The ribose-5-P produced in this reaction is utilized in the biosynthesis of coenzymes (including N/ DH, N/ DPH, F/ D, and Big), nucleotides, and nucleic acids (DNA and RNA). The net reaction for the first four steps of the pentose phosphate pathway is... 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a and P structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1... 

The guilty party is the triose phosphate isomerase (TIM) reaction that interconverts DHAP and G3P. To be converted to pyruvate, the DHAP first has to be converted to G3P. TIM just moves the carbonyl group between the two carbons that don t have phosphate attached. TIM doesn t touch the phosphate. So, if the DHAP is labeled at the carbon that has the phosphate attached, the G3P that comes from DHAP will be labeled at the carbon with the phosphate attached. The carbon with the phosphate attached in the G3P that was produced directly by the aldolase reaction came from C-6 of glucose, but the carbon with the phosphate attached in the G3P that was produced from DHAP came from C-l of glucose. After TIM does it stuff, the carbon of G3P that has the phosphate will be... 

This is interconverted to form glyceraldehyde 3-phosphate and both combine, via the enzyme aldolase, to produce fructose bisphosphate, en route to form glucose or glycogen. 

In the production of the sweetener high fructose com symp (HFCS), glucose isomerase is used to convert glucose into fmctose (equilibrium mixture appr. 50 50) since fmctose is nearly three times as sweet as glucose. The enzyme belongs to the subclass intramolecular oxidoreductases interconverting aldoses and ketoses (EC 5.3.1). About 8 million tonnes pr. year of glucose is treated in this way. For further details, see Chapter 4. 

An alternative method is to use electrochemical mediators that are at a higher concentration that O2 and can therefore be shuttled back and forth between the protein and the electrode faster than the enzyme is reduced, so that the arrival of the glucose is always rate-limiting. A typical chemical that works in this way is ferrocene, which is an iron cation between two cyclopentadienyl anions, as shown in Figure 6.47. It exists in neutral and - -1 oxidation state that are readily interconvertible at metal or carbon electrodes. 

The glucose 6-phosphate thus formed can enter glycolysis or another pathway such as the pentose phosphate pathway, described in Section 14.5. Phosphoglucomutase employs essentially the same mechanism as phos-phoglycerate mutase (p. 531). The general name mu-tase is given to enzymes that catalyze the transfer of a functional group from one position to another in the same molecule. Mutases are a subclass of isomerases, enzymes that interconvert stereoisomers or structural or positional isomers (see Table 6-3). 

Phosphoglucomutase interconverts glucose 1-phosphate and glucose 6-phosphate. Glucose... 

In spite of their easy interconversion in solution, a and p forms of carbohydrates are capable of independent existence, and many have been isolated in pure form as crystalline solids. When crystallized from ethanol, D-glucose yields a-D-glucopyranose, mp 146°C, [a]D +112.2°. Crystallization from a water-ethanol mixture produces p-D-glucopyranose, mp 148-155°C, [a]D +18.7°. In the solid state the two forms do not interconvert and are stable indefinitely. Their structures have been unambiguously confirmed by X-ray crystallography. 

The rearrangement is of interest because the corresponding enzymatic interconversion of aldoses and ketoses is an important part of the biosynthetic, photosynthetic, and metabolic pathways, as we shall see in Section 20-9. Although the biochemical rearrangement also may proceed by way of enediol intermediates, it is highly stereospecific and yields only one of two possible stereoisomeric aldoses. For example, glucose, but not mannose, can be enzymatically interconverted with fructose as the 6-phosphate ester derivative ... 

Also, it should be pointed out in this connection that the reaction mechanism (see Fig. 4 and Section III,D,5,a) postulated on the basis of kinetic observations may be an oversimplification of the actual situation. For example, the possibility exists that separate phosphoryl-enzyme intermediates may be formed with various phosphate substrates (10). If such intermediates were interconvertible with a single phosphoryl-enzyme complex which was common to reactions involving all such substrates, and if the rates of these interconversions were very rapid relative to the other steps in the catalytic process, the system would be kinetically indistinguishable from that depicted in Fig. 4. Further work relating to the mechanism of reaction, and the experimental assessment of the possible involvement of this multifunctional catalyst in intra- and extracellular glucose transport, would appear to be in order. 

Phosphoglucomutase Interconverts Glucose-1-phosphate and Glucose-6-phosphate... 

Hemiacetal formation was first observed in optical studies on D-glucose. The optical rotation ([ ) of a freshly dissolved sample of D-glucose changes with time because it possesses two stereoisomeric hemiacetals (anomers) that are interconvertible in solution (fig. 12.5). One of these anomers, a-D-glucose, has [a]D=113 the other, a-L-glucose, has [a]D= 19. The optical rotation of a freshly prepared solution of either of these compounds eventually approaches an intermediate value that depends on the equilibrium between the two anomers. 

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