Table of Contents
Fundamentals of Biochemistry
Biochem 380 - Fall 2006
Lecture 023
Outline
- Announcements
- Questions from previous lecture
- Section 8.4: Derivatives of Monosaccharides
- Section 8.5: Disaccharides and Other Glycosides
Announcements
- Lecture 22 notes are online
- Reminder: homework problems for Chapter 8: 1, 5, 8, 10, 11, 12
Questions
- Any questions on the material from the previous lecture?
Section 8.4: Derivatives of Monosaccharides
- So far, we have learned about the 'standard' monosaccharides that can vary in chain length and
configuration of their hydroxyl groups
- These basic sugars can be modified in a number of ways to facilitate their use in energy metabolism,
nucleic acid synthesis and protein modification. We'll talk about the following kinds of sugar derivatives:
- Sugar phosphates
- Deoxy sugars
- Amino sugars
- Sugar alcohols
- Sugar acids
Sugar Phosphates
- The addition of a phosphate group is something we've already seen in proteins. Phosphorylation of
sugars is used during glycolysis to 'activate' them in the process of being oxidized for energy production
- The energy released in hydrolysis of phosphate groups is also used for transferring sugar units
from one substrate to another, as we have discussed in section 7.3 with the cosubstrate UDP-glucose.
An example of UDP-glucose transfer occurs in glycogen synthesis (Chapter 12)
Deoxy Sugars
- Another important modification of monosaccharides is the removal of a hydroxyl group to produce
a deoxy sugar. An example is the removal of the 2' OH from ribose to give β-2-deoxy-D-ribose,
which is a component of DNA
- The missing oxygen changes the structural properties of the furanose ring, causing it to favor a 2'-endo conformation.
This in turn affects the larger helical structure of DNA, enabling the regular and stable 'B-form' helix that is used for
informational storage
- Fucose is another deoxy sugar, having an unusual L-configuration. Fucose is added to a number of different
glycoproteins and glycolipids as a structural modifier. One example that we'll discuss next time is its use in
the ABO blood groups
Amino Sugars
- Amino sugars are monosaccharides where one or more of the hydroxyl groups have been replaced with amino groups.
One simple amino sugar is α-D-glucosamine (GlcN), which has an amino group on the C2 carbon.
The amino group is often further modified by acylation, as seen in N-acetyl-α-D-galactosamine
(GalNAc)
- Amino sugars are used as components in glycosaminoglycans, which we'll talk about next time. These
are composed of repeating disaccharide units, often further modified with anionic groups, to increase viscosity
and serve as lubricants
Sugar Alcohols
- In sugar alcohols, the carbonyl group has been reduced to a hydroxyl, producing a polyhydroxyl molecule
- We've already seen one example of this with D-Ribitol, a component in Riboflavin, FMN and FAD
- Glycerol and myo-inositol are components used in lipid synthesis that we'll see in the next chapter
(Chapter 9)
- In general, when a sugar is converted to a sugar alcohol, the suffix of the name is changed from an -ose
to an -itol
Sugar Acids
- The aldehyde carbon of a sugar can be oxidized to a carboxylate group to produce an
aldonic acid, a type of sugar acid. An example is D-gluconate, which is a derivative of glucose
- If the acid group in gluconate reacts with the terminal alcohol, a ring structure is produced, containing an intramolecular ester.
The resulting molecule is a lactone
- If the primary alcohol of glucose is reduced instead, the result is an alduronic acid,
D-glucuronate. This is a precursor to ascorbate (vitamin C)
Some Abbreviations of Monosaccharides
Section 8.5: Disaccharides and Other Glycosides
- In the previous section, a variety of monosaccharide derivatives have been shown, with modifications
to various parts of a sugar. A bond involving the anomeric carbon has special significance,
because it is the primary means of linking a sugar to other compounds
- When the anomeric carbon is condensed with an alcohol, thiol or amine, the result is a glycosidic bond.
Compounds that have glycosidic bonds are glycosides
- We'll look at a number of common glycosides, including compounds of two sugars (disaccharides)
- In addition to the basic type of sugar in a linkage, the anomeric form of the monosaccharide
(α or β) will contribute to the properties of the resulting glycoside
Maltose
- Maltose is a disaccharide of 2 glucose monomers, with a glycosidic bond from the α anomer of
one glucose to the C4 carbon of the second one. The formal name of the resulting disaccharide is
α-D-glucopyranosyl-(1→4)-β-D-glucopyranose
- The anomeric carbon of the first sugar is fixed in the α configuration, but the
the terminal sugar can convert between the α, β and open-chain configurations
- Maltose is released by the hydrolysis of starch, a homopolymer of glucose residues. Starch
is an energy storage molecule found in plants such as corn and other grains. After being cleaved from starch,
maltose disaccharide can be cleaved into two glucose monomers by the enzyme maltase
Cellobiose
- Cellobiose is another disaccharide of two glucose monomers, but in this case it has
a glycosidic bond from the β anomer of the first glucose. This minor difference has large consequences
on the structure of the resulting polymer of cellobiose, cellulose
- The linkage produces straight polysaccharide chains that pack tightly together in fibers and
sheets in plant structures, giving high tensile strength
- Unlike starch and other glucose polymers with α linkage, the β linkage of cellulose
makes it indigestible to animals such as humans. Herbivores such as cows rely upon bacteria to provide the
enzymes to break down and digest cellulose
Lactose
- Lactose is another common disaccharide, found in milk
- In this case, it is composed of a monomer of galactose joined to a glucose in a β linkage.
Galactose is an epimer of glucose, and has a single hydroxyl group in a different configuration
that distinguishes the lactose disaccharide from cellobiose
Sucrose
- Sucrose is a disaccharide composed of glucose and fructose
- Note that the anomeric ends of both the glucose and the fructose are joined together
- Consequently, the anomeric carbons are not free to reconvert back to aldehyde groups
- For this reason, sucrose is an example of a non-reducing sugar
Reducing and Nonreducing Sugars
- A sugar such as glucose contains a free aldehyde group in the open form which can be oxidized
in the presence of a suitable agent such as cupric ion (Cu2+):
- A sugar that reacts in this way is called a reducing sugar because it is able to reduce the Cu ion
- Sugars that are modified in some way so that the aldehyde or ketone group is not available for
oxidation are called nonreducing sugars
Nucleosides and Other Glycosides
- Additional examples of glycosides are seen with guanosine, a component of nucleic acids,
and linkages between galactose and glycerol to produce a β-galactoside, an abundant type of
glycoside
Questions
- Questions about the material covered today?
Next Lecture: Sections 8.6 - 8.7