Chapter 11 Biochem... Carbohydrates and Glycoproteins Notes
Chapter 11: Carbohydrates and Glycoproteins
Monosaccharides
Definition: Simplest carbohydrates; carbon-based molecules rich in hydroxyl groups.
Empirical Formula: $(CH2O)n$, with possible additional groups.
Described as polyhydroxy aldehydes and ketones.
Characteristics:
Aldehydes or ketones with two or more hydroxyl groups.
Typically range from three to seven carbons.
Exist in several isomeric forms; also known as simple sugars.
Monosaccharide Examples
Dihydroxyacetone (ketose)
D-Glyceraldehyde (aldose)
L-Glyceraldehyde (aldose)
Monosaccharide Nomenclature
Classifications based on carbon chain length:
3 Carbons: Triose
4 Carbons: Tetrose
5 Carbons: Pentose
6 Carbons: Hexose
7 Carbons: Heptose
Based on the most oxidized group:
Ketose: contains keto group
Aldose: contains aldehyde group
Isomers
Constitutional Isomers: Identical molecular formulas but different atom order.
Stereoisomers: Identical bonding order but differ in spatial arrangement.
Can have D or L configurations; may be enantiomers or diastereoisomers.
The number of isomers: $2^n$, where $n$ is the number of asymmetric carbons.
Common Monosaccharides Types
Epimers: Sugars differing at one asymmetric center.
Examples: D-Ribose, 2-Deoxy-D-Ribose, D-Glucose, D-Mannose, D-Galactose, D-Fructose.
Cyclic Forms of Monosaccharides
Reaction: Aldehydes with alcohols form hemiacetals; ketones with alcohols form hemiketals.
Pyranose and Furanose Formation
Pyranose: Similar to pyran; contains a six-membered ring (e.g., $eta-D$-Glucopyranose).
Furanose: Similar to furan; contains a five-membered ring (e.g., $eta-D$-Fructofuranose).
Anomers of Glucose
Form when cyclic hemiacetal forms, creating two structures:
$ ext{α-D-glucopyranose}$: hydroxyl on C-1 opposite to C-6.
$ ext{β-D-glucopyranose}$: hydroxyl on C-1 same side as C-6.
D-Fructose Structures
Can interchange among four distinct ring forms; pyranose form is dominant due to steric hindrance.
Reducing Sugars
Defined as sugars that react with oxidizing agents; monosaccharides that adopt linear forms.
Fehling's solution: Tests for sugars adopting open structures.
Glycation of Sugars
Glycation: Non-enzymatic addition of a carbohydrate to another molecule.
D-Glucose can glycate proteins, forming advanced glycation end products (AGEs) linked to diseases.
Monitoring Diabetes with A1C
Glycated hemoglobin (A1C) measures blood glucose levels over time.
Normal <6%, uncontrolled diabetes can reach almost 10%.
Glycosidic Linkages
O-glycosidic linkage: Between an anomeric carbon of a carbohydrate and an alcohol.
N-glycosidic linkage: Between an anomeric carbon of a carbohydrate and an amine.
Complex Carbohydrates
Formed via glycosidic bonds; oligosaccharides contain two or more monosaccharides.
Disaccharides: Two sugars linked by O-glycosidic linkages; examples include sucrose, lactose, and maltose.
Storage Forms of Glucose
Glycogen: Main storage form in animals.
Structure: branched polymer of glucose; linked by $ ext{α-1,4}$ and $ ext{α-1,6}$ linkages.
Starch: Nutritional reservoir in plants; two forms - amylose (unbranched) and amylopectin (branched).
Cellulose
Main structural polysaccharide in plants; composed of β-1,4 linkages providing structural support.
Chitin
Structural polysaccharide found in fungi and arthropods; processed into versatile products like chitosan.
Glycoproteins and Glycosylation
Glycoproteins: Carbohydrates covalently attached to proteins; have diverse functions including cell adhesion.
N-Linked and O-Linked glycosylation based on the attachment to Asn and Ser/Thr, respectively.
Erythropoietin (EPO)
A glycoprotein that stimulates red blood cell production.
Contains carbohydrate components essential for its function and stability.
Mucins and Mucus Function
Mucins serve protective and adhesive roles in epithelial tissues and have significant impacts on health conditions.
Proteoglycans in Cartilage
Proteoglycans provide structural integrity, hydration, and cushioning of cartilage.
Blood Groups and Glycosylation
Blood types are determined by specific glycosylation patterns on red blood cell surfaces, with implications for transfusions.
I-Cell Disease
A lysosomal storage disease caused by defective glycosylation, leading to the accumulation of undigested materials.
REVIEW GUIDE
————————————-
Formula of a Monosaccharide: The empirical formula for monosaccharides is $(CH2O)n$, where $n$ is typically between 3 to 7. They consist of carbon backbone with hydroxyl groups and may include additional functional groups.
Structure of Glucose:
Flat Form: The flat structure of glucose, also known as its Fischer projection, shows the carbon atoms arranged in a linear chain with the aldehyde group at one end.
Ring Form: Glucose can cyclize into a ring form through a reaction between the carbonyl group and a hydroxyl group, leading to the formation of either $α-D$-glucopyranose or $β-D$-glucopyranose, depending on the orientation of the hydroxyl group on the anomeric carbon (C-1).
Types of Isomers:
Constitutional Isomers: These have identical molecular formulas but differ in the connectivity of their atoms.
Stereoisomers: These have the same bonding sequence but differ in their spatial arrangement (e.g., D and L configurations;
Enantiomers: A type of stereoisomer that are mirror images of each other.
Diastereoisomers: Stereoisomers that are not mirror images of each other.
Boat vs. Chair Formation of Glucose: The chair conformation is more stable than the boat conformation due to less steric hindrance between bulky groups, allowing for more favorable interactions between atoms.
Importance of D-Glucose: D-Glucose is essential for most organisms as it serves as a primary source of energy for cellular processes through cellular respiration, and is a crucial building block for other carbohydrates.
Reducing Sugar: Reducing sugars are those that can donate electrons to oxidizing agents, hence they can reduce substances such as Fehling's or Benedict's solutions. A test for reducing sugars involves heating a sugar solution with Fehling's solution, leading to the formation of a brick-red precipitate if reducing sugars are present.
Glycation Importance in Diabetes: Glycation refers to the non-enzymatic addition of sugars to proteins, leading to the formation of advanced glycation end products (AGEs). Increased levels of AGEs in the body are linked to diabetic complications, as they can contribute to insulin resistance and vascular damage.
Glycosidic Linkage:
O-Glycosidic Linkage: This type of bond forms between the anomeric carbon of a monosaccharide and the hydroxyl group of an alcohol.
N-Glycosidic Linkage: This bond forms between the anomeric carbon of a monosaccharide and the nitrogen of an amine.
Phosphorylation of Sugars: The addition of phosphate groups to sugars is relevant for cellular metabolism as it helps in the regulation of sugar metabolism, promotes activation of sugars for further enzymatic reactions, and facilitates the retention of sugars within cells.
Disaccharides: There are reducing and non-reducing disaccharides:
Reducing: Lactose (glucose + galactose), Maltose (glucose + glucose)
Non-reducing: Sucrose (glucose + fructose). The reducing end refers to the capability of one of the monosaccharide units to undergo oxidation/reduction.
Glycogen, Starch, and Chitin:
Glycogen: Animal storage form of glucose, branched structure with $α-1,4$ and $α-1,6$ linkages.
Starch: Plant storage form of glucose; consists of amylose (unbranched) and amylopectin (branched).
Chitin: A structural polysaccharide found in the cell walls of fungi and exoskeletons of arthropods, composed of N-acetylglucosamine linked by $β-1,4$ linkages.
Insoluble vs. Soluble Fiber:
Insoluble Fiber: Provides bulk, aiding in digestive health and preventing constipation.
Soluble Fiber: Can lower blood cholesterol and glucose levels, contributing to heart health.
Glycoproteins:
Classes:
Glycoproteins: Have carbohydrate moieties influencing cell recognition and signaling.
Proteoglycans: Composed mainly of carbohydrate; provide structural support.
Mucins: Help in lubrication and protection of epithelial surfaces.
N-linkage vs. O-linkage:
N-linkage: Involves attachment of carbohydrates to nitrogen on asparagine.
O-linkage: Involves attachment of carbohydrates to oxygen on serine or threonine.
Post-translational Modification: Primarily occurs in the rough endoplasmic reticulum (RER) and Golgi apparatus, which are responsible for folding and processing proteins after protein synthesis, facilitating their functional maturation.
Blood Group Antigens and Glycosylation: Various blood types arise from specific glycosylation patterns on red blood cells, modified by glycosyltransferases. Enzyme expression determines which sugars are added and thus influences a person’s blood type.
I-Cell Disease: This lysosomal storage disease is caused by defective glycosylation, preventing lysosomal enzymes from reaching their intended location. Normally, lysosomal proteins have a mannose 6-phosphate tag that directs them to lysosomes, and this process is disrupted in I-Cell disease.
Lectins: Focus on C-type lectins, which are important for cell-cell recognition and selectin function in the immune response.
Influenza Virus: The influenza virus exploits glycoproteins on the host cell surface to attach and enter the cells. Viral particles exit by budding from the host cell membrane, where they also acquire a lipid bilayer that includes host glycoproteins essential for infectivity.