Sunday, August 21, 2011

CARBOHYDRATES

What are carbohydrates?

- Sugars/saccharides

- Contains mainly C H and O

- General formula (CH2O)n where n> or = 3

Biological role of carbohydrates

- Energy metabolism

- Biological structural material

- Molecular recognition among cells

- As water trapping molecules

Types of carbohydrates

- Monosaccharides

· Basic sub unit of all carbs

· Cannot be hydrolyzed to small units

· Produced from CO2 and H2O via photosynthesis

· General formula (CH2O)n

- Disaccharides

· Formed by condensation of 2 units of mono

· Joined by covalent GLYCOSIDIC bonds

· Sucrose,Maltose,Lactose

- Oligosaccharides

· Short chain carbs

· 3-12 units of monos

· Joined by glycosidic bonds

· Most are joined with PROTEINS

· Blood group antigens

- Polysaccharides

· Long chained carbs

· Large number of mono units

· Form macromolecules/biopolymers

· 2 types

(a) Homopolysaccharides – Composed of similar mono units

Eg:- Starch,Cellulose,Chitin,Glycogen

(b) Heteropolysaccharides – Composed of different sub units

Eg:- Glycoproteins

Classification of monosaccharides

- Number of C

· 3-triose-Glyceraldehyde

· 4-tetrose-Erythrose

· 5-pentose-Ribose,Arabiose,Xylose

· 6-hexose-Glucose,Fructose,Galactose

· 7-heptose-Sedoheptose

· 8-octose-monooctulose

· 9-nonose-nuraminic acid


- Active group

· Aldose – Sugars with aldehyde group – Glyceraldehyde,Ribose,Glucose,Galactose

· Ketose – Sugars with ketone group – Fructose,Ribulose


Structural characteristics of monosaccharides

1) formation of ring structure

- Occur in monos with 5 or more C

- Two possible configurations

· Open/strait chain structure

· The closed ring structure


-
Configurations are inter changeable in SOLUTIONS

- Under physiological conditions they exist in ring form because it is more stable

- Two types of ring structures are possible

· 6 membered ring – pyranoses

· 5 membered ring – furanoses

- This depends on the reaction of keto/aldehyde group with OH

- The distribution of the forms depends on;

· The structure of the sugar

· pH

· Temperature

· Solvent

Haworth projection is a common way of representing cyclic structure of monosaccharides with a simple 3d perspective.

The groups below the plane of the ring in Haworth projections are equivalent to those on the right-hand side of a Fischer projection.


- The pyranose forms of the sugars are more stable than the furanose forms in solution.

2.) Formation of steroisomers

Sterioisomers have same molecular formula but different spatial arrangement

- 2 possible sterioisomers

- Atoms are arranged such that they are mirror images of each other

- They are not superimposable

- Such are called ENANTIOMERS

- Molecules capable of exsisting in non super imposable mirror images are CHIRAL MOLECULES

- 2 types of enantiomers are found in biological systems

· D form

· L form

- Only one form is dominant

· L – amino acids

· D – sugars

- A C atom with 4 different groups bonded to it is a chiral/asymmetric carbon

- * The chiral C furthest from the carbonyl group determines the configuration L or D of the sugar*

- OH group on the left is L-glucose and on the right D-glucose

3.) Asymetric of carbon

- Ring formation has resulted in an asymmetric center at the first C atom

- Resulting in formation of 2 configurations

- Depending on the orientation of the OH group

· Alpha

· Beta

- These have different properties

- But they are not mirror images of each other

- They are called ANOMERS

- Carbon 1 is referred to as the ANOMERIC CARBON ATOM

-** Monosacs undergo interconversion between Alpha and Beta forms through the open chain linear form as an intermediate state

This is called MUTAROTATION **

2 comments:

  1. nice one. I've never seen anybody using this strategy to study for exams before

    ReplyDelete
  2. Thanks!I don't know really, even this strategy is not doing any good to my exams. :-/ I was never able to girawafy anything and now I know I'll never be able to either. sigh!

    ReplyDelete