Unit 2 - Carbohydrates

Bioch 310: Structures of carbohydrates, aldoses, ketoses, optical features and stereochemistry, hemiacetals and hemiketals.

Function of carbohydrates

Serve as nutrients and are a source and storage form of energy.

Where to find carbohydrates

In the cell membrane, extracellular matrix, biomolecules like nucleic acids, proteins and lipids

Isomerase

Enzyme responsible for the isomerization from an aldehyde (aldose) to a ketone (ketose)

Monosaccharides

Simple sugars with the general chemical formula (CH2O)n where 3 ≤ n ≤ 9

Note: sugars with less than 3 carbons are just referred to as aldoses and ketoses

Aldose

sugar molecule with an aldehyde group

Ketose

Sugar molecule with a ketone group

What are the only 2 trioses?

1. Glyceraldehyde

2. Dihydroxyacetone

*Both are present in glycolysis

Triose

3 carbon sugar

Hexose

6 carbon sugar

Isomers

Same formula but different structure with distinct chemical properties

What molecule is used as a reference to describe all other isomers

Glyceraldheyde

What does D- conformation mean

(Dextarotation) the -OH is to the right and the structure rotates plane polarized light to the right

What does L- conformation mean

(Levorotation) The -OH is to the left and the structure rotates plane polarized light to the left

All biological relevant sugars are what type of sugars?

D-sugars

All biologically relevant amino acids are what type of amino acids?

L-amino acids

Stereoisomers

Have the same chemical formula and carbon connectivity but differ in the spatial orientation of their atoms.

Spatial orientation of certain chemical groups can affect the optical properties of the molecule

Isomers

Always have the same chemical formula

Constitutional isomers

same molecular formula but a different bonding arrangement among the atoms

Stereoisomers

identical molecular formulas and arrangements of atoms but differ in the spatial orientation of groups in the molecule.

Two types: 1. enantiomers 2. diastereoisomers

What defines D- and L-

the carbon stereogenic centre furthest away from the carbonyl group

Epimers

Stereoisomers that differ only in the configuration at one chiral centre

Controlled by epimerases

Aldotriose

Glycerladehyde

What defines D and L

The chiral carbon furthest away from the carbonyl group

Ketotriose

Dihydroxyacetone

Aldotetroses

Erythrose, Threose

Ketotetrose

Erythrulose

Aldehyde + alcohol =

Hemiacetal formation via nucleophilic addition

Ketone + alcohol =

Hemiketal formation via nucleophilic addition

Cyclization of glucose

Converts aldoses (like glucose) from the open chain form to a cyclic hemiacetal form which predominates in aq environments

**Spontaneous process (no enzymes present)

α

Hydroxyl group on anomeric carbon pointing down

β

Hydroxyl group on anomeric carbon pointing up

Pyranose

Sugar with a pyran-like ring structure (5 carbons and 1 oxygen form a 6-membered ring)

Furanose

Sugar with a furan-like ring structure (4 carbons and 1 oxygen form a 5-membered ring)

Mutarotation

The reverse process of cyclization

Anomeric carbon

Carbon derived from the carbonyl carbon of the open-chain form of the carbohydrate molecule

In aldoses: carbon 1 is anomeric

In ketoses: carbon 2 is anomeric

Reducing sugars

sugars capable of reducing Fe3+ or Cu 2+ ions

eg. D-glucose

Monosaccharide

Simple sugar (eg glucose)D

Disaccharide

Chain of two monosaccharide units

Oligosaccharides

Chain of a few (<20) monosaccharide units

Polysaccharides

Chain of 20 or more monosaccharide units (eg. glycogen, cellulose)A

Acetal

Functional group formed by the nucleophilic addition of an alcohol to aldehyde or ketone (contains two -OR groups)

Critical difference between acetal and hemiacetal

Hemiacetal contains one -OH and one -OR group, whilst acetal contains two -OR groups

Cellulose

• Most abundant polymer in the world

• linear polymer of β(1→4)-linked glucose units

  • digested by cellulase

Starch

• Branched polymer of α(1→4) and α(1→6) linked glucose units

• Made of amylose and amylopectin

  • Fuel in plants

Amylose

• Linear polymer of α(1→4) linked glucose units

• Digested by amylases (salivary, pancreatic)

  • does not fold into helix-like structure, uses much space

Amylopectin

• Branched polymer of α(1→4) and α(1→6) linked glucose units

• folds into helix-like structure

• compact storage

  • digested by amylases (salivary and pancreatic)

Glycogen

• Highly branched polymer of α(1→4) and α(1→6) glucose units

• Fuel in animals

• Stored in the liver and skeletal muscle