Pentoses ring structures

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

The cyclic forms adopted by the hexoses and pentoses can be depicted as symmetrical ring structures called Haworth projection formulae, which give a better representation of the spatial arrangement of the functional groups with respect to one another. The nomenclature is based on the simplest organic compounds exhibiting a similar five- or six-membered ring... 

Figure 9.7 Six- and five-membered cyclic ethers. The stable ring structures which are adopted by hexoses and pentoses are five- or six-membered and contain an oxygen atom. They are named as derivatives of furan or pyran, which are the simplest organic compounds with similar ring structures, e.g. glucofuranose or glucopyranose for five-or six-membered ring structures of glucose respectively.

Monomers and polymers of carbohydrates, (a) The most common carbohydrates are the simple six-carbon (hexose) and five-carbon (pentose) sugars. In aqueous solution, these sugar monomers form ring structures, (b) Polysaccharides are usually composed of hexose monosaccharides covalently linked together by glycosidic bonds to form long straight-chain or branched-chain structures. 

Kalckar117118119 has shown that the enzymatic phosphorolysis of inosine (hypoxanthine 9-D-ribofuranoside) may give rise to the formation of a pentose phosphate, isolable as its barium salt. The phosphate was found to be non-reducing although easily hydrolyzed by either acid or alkali to equimolar quantities of phosphate and pentose. In view of these properties and the fact that it could be used for the enzymatic synthesis of purine ribosides, Kalckar has tentatively assigned to it the D-ribose 1-phosphate structure its ring structure and configuration at carbon 1 remain undetermined. 

Pentose Rings (Figure 9.10, Figure 9.11, Table 9.2, Structure) Hexose Rings (Structure, Figure 9.12, Figure 9,13)... 

Both the pyrimidines and the purines are built up from small precursor molecules which are readily available in the metabolic pool (page 185). The free bases are not synthesized as such but, while being assembled, the partially constructed ring structure reacts with a special phosphorylated pentose known as PRPP (5-phosphoribosyl-l-pyrophosphate) and forms a ribonucleotide. The deoxyribonucleotides, with the exception of TMP which is formed by methylation of deoxyuridylate, are formed by reduction of the corresponding ribonucleoside diphosphate. The conversion is precisely controlled by allosteric effects which ensure that all four deoxyribonucleotides are available in amounts appropriate for nucleic acid synthesis. 

There are several differences in the chemical structures of DNA and RNA. First is the nature of the pentose ring in these macromolecules, i.e., ribofuranose for RNA and 2 -deoxyribofuranose for DNA (Fig. lA). Because of the presence of deoxyribose in DNA, the monomeric unit is called a deoxyribonucleotide or simply a deoxynucleotide, while the RNA monomer unit is called a ribonucleotide. 

Although the chemical formulas indicate that ribose is a pentose and fructose is a hexose, the ring portions of the structures are the same size. Proceeding clockwise around the rings from the oxygen atom, we see that the structures differ at the first two positions. In the first position, ribose has a carbon atom bonded to —H and —OH, while)S-fructose has a carbon bonded to —OH and — CH2 OH. In the second position the molecules have the same two bonds but in different orientations. The OH group points up in y6-fructose and down in ribose. The two molecules have the same structures at the other positions. 

---