Lecture 2 Carbohydrate Biochemistry: Monosaccharides and Disaccharides

Question 1

Biological role of carbohydrates

Answer 1

• Carbohydrates serve as nutrients are a source and storage form of energy.
• Carbohydrates serve as components of cell membrane, extracellular matrix, and biomolecules such as nucleic acids, proteins, and lipids (e.g., phospholipids in membrane of all cells and blood group antigens in the membrane of red blood cells).

Question 2

chemical name for sugar unit

Answer 2

Saccharide

Question 3

What are simple sugars

Answer 3

monosaccharides

Question 4

General formula of monosaccharides

Answer 4

Question 5

How long are biologically relevant monosaccharides?

Answer 5

usually min 3C and max 6C

Question 6

Chemical structure of carbohydrates

Answer 6

In general, the structure of carbohydrates is based on a linear polymer:

• polyhydroxy aldehyde = HOCH2–(CHOH)x–CHO = aldose
• polyhydroxy ketone = HOCH2–(CHOH)x–(CO)–(CHOH)y–CH2OH = ketose

Question 7

tautomers

Answer 7

different compounds with the same atomic composition

• can sometimes be interconnected via isomerase enzymes

Question 8

Chiral vs achiral

Answer 8

• Chiral → no symmetry plane, cannot be superimposed
• Achiral → internal plane of symmetry, can be superimposed

Question 9

Isomers

Answer 9

molecules that have the same chemical formula

Question 10

Classes of isomers

Answer 10

Question 11

Structural (constitutional) isomers

Answer 11

have the same molecular formula but a different bonding arrangement among the atoms.

Question 12

Stereoisomers

Answer 12

have identical molecular formulas and arrangements of atoms. They differ from each other only in the spatial orientation of groups in the molecule.

• For organic chemistry, there are several types of stereoisomers including enantiomers and diasterioisomers (diastomer).

Question 13

What kind of isomer is Glyceraldehyde?

Answer 13

Glyceraldehyde has two isomers called D- and L-Glyceraldehyde. The D/L labeling says that the compound’s stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde—the dextrorotatory isomer of glyceraldehyde is named the D-isomer.

Question 14

Types of optical rotation

Answer 14

Dextrorotation is the rotation of plane-polarized light to the right side, whereas levorotation is the rotation of plane-polarized light to the left side.

Question 15

What must be in place for 2 structures to be identical?

Answer 15

• The same molecular formula
• Carbon connectivity
• Be superimposable
• Whether or not two structures are mirror images of each other will depend on the spatial distribution of specific substituents (constituents)

Question 16

Stereoisomers

Answer 16

Structures with the same molecular formula and carbon connectivity but differing in the spatial orientations of their atoms.

• look for point in structure (carbon atom) with 3 or 4 different substituents.

Question 17

Enantiomers

Answer 17

Stereoisomers that are non-superimposable mirror images of each other

Question 18

Diastereoisomers

Answer 18

Stereoisomers that are not mirror images of each and so they are not enantiomers.

Question 19

Stereocenter vs chiral center

Answer 19

• Stereocenter is a point in a molecule which can give rise to stereoisomers.
  
  • Atom in a molecule (e.g., a carbon atom) such that interchanging any two of its substituents leads to a stereoisomer
• Chiral center is a carbon atom to which four different atoms or groups of atoms are bonded.
  
  • Stereocenter in an enantiomer = atom (usually a carbon atom) with substituents that are not superimposable on their mirror image

Question 20

What do reactions of carbonyl groups in ketones and aldehydes and alcohol form?

Answer 20

hemiacetal or hemiketal

• Nucleophilic addition.
• An alcohol is added to aldehyde or a ketone

Question 21

formation of hemiacetal

Answer 21

alcohol with aldehyde carbonyl

Question 22

formation of hemiketal

Answer 22

Reaction of alcohol with ketone carbonyl

Question 23

Cyclization of glucose

Answer 23

• This type of reaction converts aldoses (e.g., glucose) from the open chain form to a cyclic hemiacetal which predominates in solution
• Occurs spontaneous without enzyme.
• In the cyclic form, the carbonyl group forms a covalent bond with the oxygen of a hydroxyl group along the sugar chain.
  
  • C1 with C5

Question 24

What is the general ring structure of glucose?

Answer 24

5C ring

Question 25

pyranose vs. furanose

Answer 25

• Pyranose → Sugar with a pyran-like ring structure (5 carbons + 1 oxygen form a 6- membered ring).
  
  • Ether linked at C1-C5
• Furanose → Sugar with a furan-like ring structure (4 carbons + 1 oxygen form a 5- membered ring).
  
  • Ether linked at C2-C5

Question 26

𝝰- vs. β- anomers

Answer 26

𝝰- OH on C1 points down (glucose)

β- OH on C1 points up (glucose)

Question 27

mutarotation

Answer 27

Cyclization is reversible

Question 28

Anomeric carbon

Answer 28

Anomeric carbon is the carbon derived from the carbonyl carbon compound (the ketone or aldehyde functional group) of the open-chain form of the carbohydrate molecule.

• Carbon 1 (in aldoses) and carbon 2 (in ketoses) are ‘anomeric’ carbons.
• In D-glucose the 𝝰-anomer has the OH pointing down, while the ß-anomer has the OH pointing up

Question 29

Glucose as reducing agent

Answer 29

• The aldose carbonyl (carbon 1) can be oxidized to a carboxyl group by mild oxidizing agents such as Fe3+ or Cu2+ ions. Sugars capable of reducing Fe3+ or Cu2+ ions are “reducing sugars”.
• Carbon 1 can only be oxidized in the linear form.
  
  • Has to have free aldehyde