Table of Contents
Fundamentals of Biochemistry
Biochem 380 - Fall 2006
Lecture 022
Outline
- Announcements
- Questions from previous lecture
- Section 8.1: Monosaccharides
- Section 8.2: Cyclization of Aldoses and Ketoses
- Section 8.3: Conformations of Monosaccharides
Announcements
- Lecture 21 notes are online
- Answer key and scores for midterm 1 are online
- Homework problems for Chapter 8:
Questions
- Any questions on the material from the previous lecture?
Chapter 8: Carbohydrates
- Carbohydrates are the second major class of biomolecules that we will cover. The term
refers to 'hydrate of carbon', with structural formula (CH2O)n
- The basic building blocks of carbohydrates are the monosaccharides.
These are linked together in longer chains to form oligosaccharides (2 - 20 residues)
and polysaccharides (> 20 residues)
- Carbohydrates find many uses as structural elements and energy storage molecules
- Although they are not constructed with the precise primary sequence of proteins and nucleic acids,
carbohydrates are also used for informational purposes, by being joined to other biomolecules such as proteins
and lipids to confer greater identity and structural diversity
Section 8.1: Monosaccharides
- Monosaccharides are the basic units of carbohydrates, and are classified into two types, depending
on the position of the carbonyl group
- The first type is an aldose, which is an aldehyde with two or more hydroxyl groups. The
second type is a ketose, which is a ketone with two or more hydroxyl groups
- Shown above are the two simplest monosaccharides: glyceraldehyde
and dihydroxyacetone. These are both trioses, which are three-carbon sugars
Stereochemistry of Monosaccharides
- Like the amino acids, most monosaccharides are chiral, having atoms with asymmetric centers.
Shown above are the two stereoisomers of glyceraldehyde
- Glyceraldehyde is actually the basis for the L and D nomenclature, where a solution of D-glyceraldehyde
rotates polarized light to the right (dextrorotatory) and L-glyceraldehyde rotates light to the left
(levorotatory)
- This rotational direction only applies to glyceraldehyde specifically, as the direction of rotation
of light through an arbitrary chiral molecule can vary
- When the molecule is oriented with the C1 aldehyde at the top, pointing away from the viewer, this
defines a convention where the C2 hydroxyl group will be on the left for L-glyceraldehyde,
and on the right for D-glyceraldehyde
Fischer Projections
- Because monosaccharides can have multiple chiral centers, there are some
conventions used for drawing their structures. For linear chains, the stereochemistry is often represented
as a Fischer Projection:
- In a Fischer projection the carbon chain is oriented in the vertical direction, with a conformation
that projects the carbon bonds onto a flat plane, and with all horizontal bonds projecting out, in front of the plane
Stereochemistry of Longer Monosaccharides
- For longer monosaccharides, the assignment of the L and D configuration is determined by
the configuration of the chiral carbon farthest away from the C1 carbonyl
- For example, in glucose, a 6-carbon sugar, the C5 carbon is used. If the C5 hydroxyl group
is on the left, the molecule is L-glucose. If the hydroxyl group is on the right, it is D-glucose.
In contrast to the L-amino acids, D-sugars are more commonly synthesized in living cells
- In a carbon chain with 2 possible configurations for each chiral center, there are a
total of 2n stereoisomers for a compound with n chiral carbons
Some D-Aldose Isomers
Some D-Ketose Isomers
Section 8.2: Cyclization of Aldoses and Ketoses
- In solution, monosaccharides can undergo a reaction between the carbonyl carbon and a
hydroxyl group to form a cyclic or ring structure
- In the case of an aldose, the result is a cyclic hemiacetal, and for a ketose the result
is a cyclic hemiketal
- In each case, a new chiral center is created, shown above as C*
Pyranose and Furanose Ring Formation
- Stable ring structures typically have either 5 or 6 members in the ring. When a monosaccharide
has a 6-membered ring, it is called a pyranose, because of its similarity to the hydrocarbon pyran
- A monosaccharide with a 5-membered is called a furanose, similar to furan
- Shown above are Haworth projections of some of the pyranose and furanose forms of the 5-carbon sugar D-ribose
Formation of Glucopyranose
- The stereochemistry of a monosaccharide ring can be shown in a Haworth projection. The hydroxyl groups to the right
in a Fischer projection point down in a Haworth projection. Likewise, the hydroxyl groups to the left in
a Fischer projection point up.
- In the cyclization of a monosaccharide, two different anomers can be formed at the carbonyl carbon,
also called the anomeric carbon. For α-D-sugars, the anomeric hydroxyl will be below the ring
and for β-D-sugars, the anomeric hydroxyl group will be above the ring
Equilibrium of Ring Structures
- In solution, ring closing and opening occurs rapidly for monosaccharides, and they exist in
an equilibrium between different forms
- For example, D-ribose exists as a mixture of 21.5% α-D-ribopyranose, 58.5% β-D-ribopyranose,
6.5% α-D-ribofuranose and 13.5% β-D-ribofuranose
- A small percentage of the sugar also exists in the open-chain form
Section 8.3: Conformations of Monosaccharides
- The actual 3D conformations of monosaccharides are not actually planar. Because of the
tetrahedral bonding and steric interactions between the atoms in the ring, cyclic monosaccharides
adopt a number of bent or twisted conformations with atoms above and below the plane of the ring
- For ribofuranose, shown above, if the C3 carbon is above the ring it is in the C3'-endo conformation,
and if the C2 carbon is above the ring, the conformation is C2'-endo
Pyranose Conformations
- Pyranoses also have non-planar conformations. They can adopt a greater variety of distinct conformations,
with two general types: chair and boat
- The substituents of the carbons in the ring can also have two different orientations:
axial and equatorial, in which they are oriented perpendicular to or parallel to the
plane of the ring, respectively
Questions
- Questions about the material covered today?
Next Lecture: Sections 8.4 - 8.5