Lecture 16: Carbohydrate Structure and Transport

Question 1

Carbohydrates

Answer 1

• Energy producing macronutrient found in diet
• Recommended dietary intake: 130 g/day
• Each g of glucose produces 4 kcal of energy

Question 2

Simplest Monosaccharide

Answer 2

-Triose(three carbons)

Question 3

Carbohydrate Numbering

Answer 3

• Alpha carbon is most oxidized
  
  • eg. carboxyl group

Question 4

Carbohydrate Configuration

Answer 4

• 2^x
  
  • x = chirals
  • L or D assigned to asymmetric chiral carbon furthest away to the carbonyl

Question 5

Diversity of Sugar Structures

Answer 5

• Sugars all have chemical formula (CH2O)n
• Sugars can often be found in ring structures
  
  • lowest energy structure

Question 6

Carbohydrate Cyclization

Answer 6

• Aldehydes, ketones, and hydroxyl groups are very reactive
• Aldehyde/ketone carbonyl undergo nucleophilic attack by hydroxyl groups
• Intramolecular reactions for a 5 member furan ring or 6 member pyran ring
• Alpha carbohydrate: OH on bottom
• Beta carbohydrate: OH on top

Question 7

Anomeric Carbons are Chiral

Answer 7

• Intramolecular cyclization is reversible in solution through linear chain
• Chirality of cyclic structures can be detected by mutarotation(change in optical rotation of polarized light), following interconversion between alpha and beta anomer
• Reactivity of aldehydes also means sugars can react with other molecules but only in linear form

Question 8

Carbohydrate Modification

Answer 8

• Sugars can be phosphorylated, methylated, or N-containing functional groups can be added
• Hydroxyls can be removed
• Increases complexity of carbohydrate structure

Question 9

Carbohydrate Terminology

Answer 9

• Isomers: Same molecular formula, different structure
• Constitutional Isomers: different order of functional group binding
• Stereoisomers: Same formula/order
  
  • Enantiomers: Non-superimposable mirror images
  • Diastereomers: Not mirror images
    - Epimer: differ at one asymmetric carbon
    - Anomer: Differ at a newly formed asymmetric C in ring structure

Question 10

Glycosidic Bond

Answer 10

• Glycosidic bond formation is a condensation reaction where H2O is lost
• Cleaving glycosidic bonds requires H2O(hydrolysis)
• Monosaccharides are joined by glycosidic linkages to form disaccharides and more complex structures

Question 11

Other types of glycosidic bonds

Answer 11

• Intermolecular glycosidic bonds are formed between a hydroxyl or amine, and a reactive anomeric carbon with another molecule
• Glycoproteins may be O linked(Ser/Thr) or N linked(Asn)

Question 12

Complex Carbohydrates

Answer 12

• Amylose: alpha - 1,4
• Lactose: beta- 1,4
• Amylopectin: alpha - 1,6
• Sucrose: alpha - 1,2

Question 13

Polysaccharides

Answer 13

• Large sugar polymers(thousands of monosaccharides), linked in linear or branched fashion
• Important for energy storage, cellular structure and recognition
• Homeopolymer: same monosaccharides
  
  • eg. Glycogen and starch(homoglycans)
• Heteropolymer: Different monosaccharides
  - eg. sugars found in glycoproteins(heteroglycans)

Question 14

Starch

Answer 14

Amylose: Unbranched glucose units. alpha(1,4) linkages
- Amylopectin: Linear glucose chains joined by alpha(1,4) linkages. alpha(1,6) linkages at branch points once every 30 glucose units

Question 15

Cellulose

Answer 15

• Most abundant organic compound, serving structural role in plants
• Unbranched chains of glucose joined by beta(1,4) linkages with many H bonds
• Cellobiose is a disaccharide of glucose
• Humans don’t produce cellulase(cannot break down cellulose)

Question 16

Glycogen

Answer 16

• Storage form of long, branched chains of glucose
• Contains branch point every 8-12 glucose units
• Contains dimer of glycogenin at the centre to bind branches together
• Glucose units are added and removed from non-reducing ends
• Found in liver and muscle

Question 17

Cleavage to Monosaccharides

Answer 17

• Cells can only transport and use monosaccharides for fuel
• Disaccharides/Polysaccharides must be broken down to monosaccharides by enzymes to use
• Lactase, maltase, and sucrase help cleave associated disaccharides

Question 18

Protein Recognition of Sugars

Answer 18

• Amino acid side chains in the active sites of enzymes confer specificity through non-covalent interactions

Question 19

Glucose Transport

Answer 19

• Moving glucose around body is critical
• Glucose is a very polar molecule and cannot diffuse through the lipid bilayer
• Transporters help move glucose from one area to another based on concentration gradient
• Different cells use different glucose transporters
  
  • Sodium-Glucose Co-transporter
    - Secondary active transport
    - Glucose Transporters
    - Facilitated Diffusion
    
    • Fructose Transporters
      
      • Facilitated Diffusion

Question 20

Glucose Transporter

Answer 20

• GLUT1: Glucose, Galactose, RBC, Blood Brain Barrier
• GLUT2: Glucose, Galactose, Fructose, Small Intestine, Liver, Pancreas
• GLUT3: Glucose, Galactose, Neurons
• GLUT4(Insulin Dependent): Glucose, Muscle, Adipose
• GLUT5: Fructose, Small Intestine