Fructose, anomers

Fructose is sweeter than sucrose at low temperatures ( 5° C) at higher temperatures, the reverse is true. At 40°C, they have equal sweetness, the result of a temperature-induced shift in the percentages of a- and p-fructose anomers. The taste of sucrose is syneigistic with high intensity sweeteners (eg, sucralose and aspartame) and can be enhanced or prolonged by substances like glycerol monostearate, lecithin, and maltol (19). 

D-rlbose anomers 3, D-xylose 4, D-mannose 5, D-fruc-tose 6, a-D-galactose 7, a-D-glucose 8, D-fructose anomer( ) 9, 8-D-glucose. 

Figure 25.5 Pyranose and furanose forms of fructose in aqueous solution. The two pyra-nose anomers result from addition of the C6 -OH group to the C2 carbonyl the two furanose anomers result from addition of the C5 -OH group to the C2 carbonyl.

Problem 25.12 Figure 25.5 shows only the jS-pyranose and /3-furanose anomers of D-fructose. Draw the cr-pyranose and n-furanose anomers. 

Step 3 of Figure 29.7 Phosphorylation Fructose 6-phosphate is converted in step 3 to fructose 1,6-bisphosphate (FBP) by a phosphofmctokinase-catalyzed reaction with ATP (recall that the prefix bis- means two). The mechanism is similar to that in step 1, with Mg2+ ion again required as cofactor. Interestingly, the product of step 2 is the tv anomer of fructose 6-phosphate, but it is the (3 anomer that is phos-phorylated in step 3, implying that the two anomers equilibrate rapidly through the open-chain form. The result of step 3 is a molecule ready to be split into the two three-carbon intermediates that will ultimately become two molecules of pyruvate. 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

Figure 9.6 Anomeric forms of D-glucose and D-fructose. The alpha and beta anomers are named with reference to the configuration of the glycosidic hydroxy] group associated with the oxygen bridge.

In sucrose, fructose is present as the P anomer. Now, one of these sugars has acted as an alcohol to make a bond to the other sugar. We can look at this in two ways. Either frnctose acts as an alcohol to react with the hemiacetal glucose to form an acetal, or alternatively, glucose is the alcohol that reacts with the hemiketal fructose to form a ketal. In sucrose, the pyranose ring is an acetal, whilst the fnranose ring is a ketal. This all seems rather... 

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

Mutarotation changes and rates of change for D-fructose show that / -D-fructofuranose is a plentiful component at equilibrium in water and that both of the a-anomers are relatively unimportant (2). The recently-described 13C NMR spectrum of D-fructose (14) confirms and amplifies this early polarimetric evidence, giving an equilibrium composition of... 

Figure 20-2 Structure and configuration of the o-ketoses from C3 to C6. As with the aldoses (Figure 20-1), the cyclic form is predominantly an oxacyclohexane (pyranose) ring in the free sugar, but the oxacyclopen-tane (furanose) form is shown for fructose because it occurs widely in this form as the disaccharide, sucrose. Only the a anomers are shown (see Section 20-2B).

In the presence of dilute base, D-glucose rearranges to a mixture containing the anomers of D-glucose (—64%), D-fructose (—31%), and D-mannose (3%). This interconversion undoubtedly involves enolization of the hexoses by way... 

A five-membered cyclic sugar ring is called a furanose. Fructose prefers a fu-ranose ring system, and is formally named fructofuranose. Like glucose, fructose can cyclize and can form either an alpha anomer or a beta anomer. Notice that,... 

---