142

chapter 9

Simple Carbohydrates

acid,

is a derivative of a ketonanose. It occurs in the

oligosaccharide side chains of some glycoproteins and in

gangliosides, both of which are constituents of cell mem-

branes (Chapter 10).

Sugar Phosphates

Sugar phosphates, which are phosphoric acid esters of

monosaccharides, occur as intermediates in carbohydrate

metabolism. Two of these compounds, namely, ribose

phosphate and deoxyribose phosphate, are constituents

of nucleotides and nucleic acids. Glucose can be phos-

phorylated either at the

Ce

primary hydroxyl group to

yield glucose

6

-phosphate or at the Cj anomeric hydroxyl

group to yield glucose 1-phosphate (Figure 9-16). In

glucose

1

-phosphate, the phosphate group can exist in

either the

a-

or /1-position. These two forms are not

interconverted in solution because the substitution of the

anomeric hydroxyl group by any group prevents the ring

opening responsible for the equilibration of anomers. The

reducing property is also lost.

Another class of sugar phosphates consists of nucle-

oside diphosphate sugars, in which a monosaccharide

is attached through the anomeric hydroxyl group to a

nucleoside diphosphate. A nucleoside contains D-ribose,

an aldopentose, attached to a purine or a pyrimidine base,

as in uridine diphosphate glucose (Figure 9-16). Such

compounds are important in the synthesis of polysaccha-

rides, the interconversion of sugars, and the synthesis of

glycosides.

Deoxy Sugars

In deoxy sugars, one or more hydroxyl groups of the

pyranose or furanose ring is substituted by hydrogen. A

well-known example is

2

-deoxyribose, which is a com-

ponent of deoxyribonucleotides, the repeating units of de-

oxyribonucleic acids (DNA). Another example is L-fucose

d - R ib o s e -5 - p h o s p h a te

( D -rib o fu ran o sy l-5 -p h o sp h ate)

2-D eoxy- o -rib o se -5 -p h o sp h a te

(2 -d eo x y -D -rib o fu ran o sy l-5 -p h o sp h ate)

D -G iucose-1 -p h o sp h a te

(a-D -g iu co p y ran o sy l p h o sp h ate )

C H 2OPC>3

H O '

,OH

)H O H

o -G lu c o se -6 -p h o sp h a te

(6 -p h o sp h o - D -g lucopyranose)

U ridine d ip h o sp h a te g lu c o se

F I G U R E 9 - 1 6

Sugar phosphates and a nucleoside diphosphate sugar. When the anomeric position of a sugar is not substituted, the

configuration at this position is not specified because the sugar can exist in either the

a -

or /

1

-anomeric form.