Page 98 - The Manga Guide to Biochemistry

D-form and L-form

Monosaccharides, such as glucose, can exist as D-form or L-form isomers. All monosaccha-
rides that appear in this book are in D-form.

To tell the difference between the two forms, first find the asymmetric carbon. This
is the carbon for which the four bonded substances all differ—in glucose, it’s the fifth one.
When the OH connected to this asymmetric carbon is on the right side in the structural
formula of the open-chain form, this monosaccharide is the D-form. When the OH is on
the left side, it’s the L-form. Using this as a basis, the H and OH that are bonded to the car-
bons at the second through fifth positions of the D-form for glucose are all reversed in the
L-form.

The fact that the H and OH are reversed for all carbons from the second to the fifth
positions is important. If, for example, only the H and OH at the fourth position of glucose
are reversed, a different monosaccharide called galactose is formed (see page 62).

Note that most monosaccharides that exist in the natural world are known to be of the
D-form.

HO HO
C C

H C OH HO C H
HO C H H C OH

H C OH HO C H
H C OH D-form HO C H L-form

CH2OH CH2OH

D-glucose L-glucose

84 Chapter 2