Biochemistry I Spring Term
Lecture 23
Carbohydrates I
March 18, 2013
Assigned reading in Horton: 8.1-8.4. Nelson 5e: 7.1
Key Terms:






aldose
ketose
epimers
hemiacetal
anomeric carbon
Haworth Representation
Carbohydrates are:
1.
2.
3.
4.
The primary energy reservoir in biosphere.
Biosynthetic precursors to amino acids and nucleic acids.
Targeting of proteins for trafficking within the cell.
Structural and mechanical components.
 Cell walls in plants
 Cell walls in bacteria
CH2OH
O
CH2OH
Structural Hierarchy:
dihydroxyacetone
1. Monosaccharides: cannot be hydrolyzed to simpler sugars.
(no chiral center)
2. Oligosaccharides: 'a few' covalently linked
monosaccharides.
3. Polysaccharides: 'many' covalently linked
H
O
O
monosaccharides.
H
Monosaccharides: All carbons in monosaccharides are
H
OH
HO
H
'hydrated' -hence the name carbohydrate (general formula
CH2OH
CH2OH
(CH2O)N)
S(L) glyceraldehyde
D(R) glyceraldehyde
1. The simplest monosaccharides contain three carbons:
2. When the C=O group is at the 2nd position it's called an ketose.
3. When the C=O group is at the very end it's an aldose.
4. Note that the aldose, glyceraldehyde, has a chiral center and therefore exists in D and L forms, or mirror
images of each other. The D-form is the "root" compound for all other naturally occurring aldoses.
5. Additional hydrated carbons (HO-C-H) are added just below the aldehyde or ketone group. Therefore, the
chiral center of D-glyceraldehyde is preserved. The added carbon generates a new chiral center. The two
different molecules generated by the addition of another carbon are called epimers because the differ in only
one chiral center. For example, the addition of a CH2OH unit to D-glyceraldehyde gives the following two
epimers: erythrose and threose.
Aldoses
D-Glyceraldehyde
Erythrose
1
H
O
H
H
H
OH
OH
OH
CH2OH
H
O
H
H
H
H
OH
OH
OH
OH
CH2OH
O
H
H
OH
OH
CH2OH
HO
H
H
H
H
OH
OH
OH
CH2OH
H
OH
CH2OH
HO
H
Threose
HO
H
H
O
O
H
H
H
H
H
H
H
HO
H
H
O
OH
H
OH
OH
CH2OH
O
H
H
OH
OH
CH2OH
H
HO
HO
H
H
H
HO
H
O
H
OH
OH
OH
CH2OH
H
H
H
HO
H
O
H
OH
CH2OH
O
H
OH
H
OH
CH2OH
O
OH
OH
H
OH
CH2OH
H
HO
H
HO
H
HO
HO
H
O
H
OH
H
OH
CH2OH
H
HO
HO
HO
H
O
H
H
H
OH
CH2OH
O
H
H
OH
CH2OH
H
H
HO
HO
H
O
OH
H
H
OH
CH2OH
Biochemistry I Spring Term
Lecture 23
March 18, 2013
Ketoses:
CH2OH
O
CH2OH
HO
CH2OH
O
CH2OH
dihydroxyacetone
H
CH2OH
CH2OH
O
H
O
OH
HO
CH2OH
H
H
H
OH
OH
CH2OH
Important sugars to remember are:
Aldose
Ketose
Glyceraldehyde (C3) [Important in energy metabolism]
Fructose (C6)
[Important in
metabolism]
Ribose (C5) [Building block of DNA and RNA]
Glucose (C6) [Energy metabolism and structural
elements]
Ring formation:
+
H
In general, alcohols can attack the C=O group in sugars to form
hemiacetals. Since sugars have OH groups, they can form
hemiacetals by an intramolecular reaction, forming closed rings.
O aldehyde
O
R2
Only long (>C4) saccharides can form internal hemiacetals, giving
closed rings (Includes ribose, glucose, fructose). No atoms are lost or
gained in this reaction!
H
H
R2
O
R1
H
R1
H
Ring Formation in Glucose
alcohol
1. Six membered ring created by forming a bond between C1 and
O5 (most stable ring size).
2. This form is called pyranose, i.e. glucopyranose after the organic
compound, pyran.
3. The C1 carbon becomes chiral and is called the anomeric carbon
4. The new OH group (on C1) can exist in either the  or  form.
5. The  and  forms can readily inter-convert via the linear intermediate.
O
Hemiacetal
O
pyran
CH2OH
CH2OH
H
O
H
H
O
O
OH
H
HO
OH
H
OH
O
OH
OH
OH
OH
OH
HO
H
OH
OH
H
OH
OH
CH2OH
CH2OH
O
OH
CH2OH
H
OH
CH2OH
H
OH
H
O
O
H
OH
OH
OH
OH
O
H
OH
2
Biochemistry I Spring Term
Lecture 23
March 18, 2013
C5 Aldose: Ribose
1. Formation of a 5 membered ring can occur
by forming a bond between C1 and O4.
2. The cyclic form is called a
O
furanose (i.e. ribofuranose)
after the organic compound,
furan.
H
CH2OH
O
CH2OH
O
H
O
H
O
OH
H
OH
H
OH
H
O
OH
CH2OH
H
O
H
OH
OH
OH
CH2OH
O
O
CH2OH
OH
OH
H
OH
OH
OH
H
C6 Ketose: Fructose
H
Although this is a 6-carbon
sugar, because it is a ketose a
five membered ring is formed.
O
OH
1
CH2OH
C
C
This is also called a furanose (i.e.
fructo-furanose).
HO
3
H
4
C
OH C
2
4
2
CH2OH
O
HO
O
3
C
O
CH2OH
CH2OH
5
C
OH
HO
C
OH
C
H
OH
O
H
5
C
O
CH2OH
H
OH
CH2OH
CH2OH
O
HO
5
C
4
CH2OH
C
OH C
2
O
CH2OH
3
C
OH
OH
How to identify the anomeric carbon:
Presentation of structures: Fischer, Haworth, Reduced Haworth Representations.
H
O
H
OH
HO
H
H
OH
H
OH
CH2OH
CH2OH
H
H
HO
H
OH
H
OH
H
H
O
H
OH H
H
OH H
H
OH
H
OH H
O
H
CH2OH
OH
O
OH
H
OH H
OH
H
CH2OH
OH H
O
OH
OH
H
OH
CH2OH
OH
O
OH
CH2OH
glucose
OH
H
H
galactose
O
OH
HO
H
HO
H
H
OH
CH2OH
OH
O
OH
CH2OH
i) tip clockwise, ii) move –O- to back, iii) down = down, up = up.
3