Carbohydrates & Glycoproteins Study Notes

Historically recognized as crucial for fueling and structural roles in cells.
- Initially thought to be peripheral to key cellular activities.
It is now understood that carbohydrates attached to proteins and lipids play a more significant role than previously believed.
- They are essential for cell survival and facilitate cell-to-cell interactions.
Secreted proteins are often extensively glycosylated (decorated with carbohydrates).

Carbohydrates consist of small individual units called monosaccharides, which can combine to form complex polymers known as polysaccharides.
They can be explained as polyhydroxy aldehydes or ketones.
Properties of carbohydrates:
- They are hydrophilic, meaning they interact well with water.
- The general formula for carbohydrates is commonly represented as $(CH2O)n$ where $n$ indicates the number of repeating units.

The simplest carbohydrates are the three-carbon sugars:
- Dihydroxyacetone: $CH_2OH|O=C|H|H|C'$
- Formula: $(CH2O)3$ or $C3H6O_3$.
- D-Glyceraldehyde (an aldose):
- Structure: $CH2OH|H-C-OH|HO|C-H|CH2OH$
- L-Glyceraldehyde (an aldose):
- Structure is similar to D-Glyceraldehyde but varies in spatial arrangement.
Common notation:
- Monosaccharides that contain a carbonyl group (≥C=O) are referred to as aldoses (with an aldehyde) or ketoses (with a ketone).
Suffix usage:
- Sugars are often named with the suffix “-ose”, while ketoses may be designated with the suffix “-ulose”.

Isomers: Compounds with the same molecular formula but different structural configurations.
- Constitutional Isomers: Differ in the order of atomic attachment.
- Stereoisomers: Atoms connected in the same order but differ in spatial arrangement.
Types of Isomers:
- Enantiomers: Non-superimposable mirror images of each other.
- Diastereoisomers: Not mirror images and have more than one stereocenter.
- Epimers: Differ at only one asymmetric carbon atom among a set of stereoisomers.
- Anomers: Isomers that differ at the new asymmetrical carbon created upon ring closure.

Glyceraldehyde and Dihydroxyacetone are examples of isomers with identical molecular formula $(C3H6O_3)$ but different structures.
The substances D-Glucose and D-Mannose are epimers, differing at one of their multiple asymmetric carbon atoms.
Anomers such as $eta$-D-Glucose vs. $eta$-D-Fructose differ at the configuration of the carbon formed by the ring closure related to their carbonyl group.

Carbohydrate molecules are not rigid and can exist in different forms, including linear and cyclic configurations.

When carbohydrates form rings:
- They provide a stable and resistant structure against oxidation.
Mechanisms involved in ring formation include:
- Hemiacetal Formation (in aldoses) or Hemiketal Formation (in ketoses).
Ring types:
- Pyran: 6-membered ring structure.
- Furan: 5-membered ring structure.

D-Fructose, a simple sugar, has a 6-carbon structure with a cyclic form called a hemiketal.
In this form, it retains six carbon atoms but is structured differently compared to its open-chain form.

Anomers are a specific type of stereoisomer that arises during the formation of a cyclic form from an open-chain form when a new asymmetric carbon atom is generated.
- Designations of anomers:
- $ ext{α}$ means the hydroxyl group on the carbon-1 is below the plane of the ring.
- $ ext{β}$ means the hydroxyl group on the carbon-1 is above the plane of the ring.

Glycoproteins are proteins that have carbohydrate groups covalently attached to them.
- Approximately 50% of the proteome consists of glycoproteins.
The attachment of carbohydrates occurs via:
- N-linkage: connecting through the amide nitrogen atom in the side chain of asparagine.
- O-linkage: connecting through the hydroxyl oxygen atom in the side chains of serine or threonine.

Structural representation for N-linked glycosidic linkage (Asn) and O-linked glycosidic linkage (Ser, Thr).