Carbohydrates 1 Flashcards
What are the most smallest monosaccharides composed of ?
Three carbons: note that glyceraldehyde and dihydroxyacetone are constitutional isomers of each other (same atoms but in different order)
Examples of six monosaccharides ?
Especially common: ribose and deoxyribose (in RNA and DNA), glucose, mannose, galactose and fructose
Examples of aldoses and ketoses ?
Aldoses = glucose Ketoses = fructose
What is Stereochemistry the study of ?
study of arrangement of atoms in 3D space (remember D and L amino acids!)
Stereoisomers are ?
Molecules that have the same bonds connecting the same atoms but different relative orientations of the bonds
What is meant by chiral?
Identical molecules can be made to superimpose by rotating and flipping, but different compounds cannot be made to superimpose
What is meant by asymmetric ?
Not the same as their mirror image: hand-like property
So what would asymmetric molecules be termed as ?
Chiral
Carbohydrates can have more than one ?
Chiral carbon: in general a molecule with n chiral centers can have 2n stereoisomers
All monosaccharides (except dihydroxyacetone) contain ?
One or more assymetric (chiral) carbons
Most hexoses in living organisms are ?
D stereoisomers
What are epimers
- Sugars that differ only in configuration around one of many chiral carbons. (Carbohydrates which vary in one position for the placement of the -OH group)
What is an enantiomer ?
A chiral compound and its mirror image
What is the stereochemical configuration of a sugar defined by ?
By the chiral carbon at the greatest distance from the carbonyl carbon
If hydroxyl (OH) on chiral carbon at the greatest distance from carbonyl carbon:
to the right – D-glyceraldehyde
to the left – L-glyceraldehyde
- Dexter = right in Latin
- Laevus = left in Latin
How are chiral compounds usually drawn and how are chiral carbohydrates usually represented ?
- Chiral compounds can be drawn using perspective formulas (wide end of wedge projects towards you, dashed line projects away)
- However, chiral carbohydrates are usually represented by Fischer projections
In a Fischer Projection what do the bonds mean?
Horizontal bonds are pointing toward you; vertical bonds are projecting away from you
What type of amino acid do proteins contain in nature ?
L amino acids
Enzymes evolved to fit ?
Mostly D stereoisomers
In aqueous solution many monosaccharides have ?
Cyclic structures
What is the chemical basis for ring formation ?
- It is the reaction of an aldehyde with an alcohol to form a hemiacetal
- Whereas a ketone can react with an alcohol to form a hemiketal
What would be the anomeric carbon ?
The hemiacetal or hemiketone carbon
What is Haworth Projections ?
Cyclic structures depicted by 3D drawing adhering to a set of rules
Hexoses commonly form two types of ring structure. What are these?
- Most common 5 membered ring is furanose
- Most common 6 membered ring is pyranose
What is Furan derived from ?
The furan ring: furan 4 carbon ring with 1 oxygen
What is Pyranose derived from ?
Pyran ring: pyran ring is 5 carbon ring with 1 oxygen
The open-chain form of glucose cyclizes when ? and forms ?
- The C-5 hydroxyl group attacks the oxygen atom of the C-1 aldehyde group to form an intramolecular hemiacetal
- Two anomeric forms, designated α and β, can result
The open-chain form of fructose cyclarises to a five-membered ring when? and forms ?
The C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the α anomer is shown.
Why is pyranose ring not planar ?
Due to geometry of saturated C atoms
What kind of conformations do cyclohexane rings have ?
“chair” and “boat” conformations
What ratio anomer does a solution of glucose contain ?
A solution of glucose contains one-third α anomer, two-thirds β anomer, and about 1% open chain
As the two anomeric forms are in equilibrium that passes through an open-chain form, the free open-chain form reacts with ?
Oxidising agents, therefore glucose is a reducing sugar
What can aldehyde reduce ?
Cu2+ to Cu+ (Fehling’s/Benedict’s test) or can reduce Ag+ to Ag0 (Tollens’ test)
What do modern detection techniques use?
Colorimetric and electrochemical tests
Phosphorylation is a common modification of carbohydrates. What does it make ?
It makes the sugars anionic and prevents them from leaving the cell
What does Phosphorylation facilitate?
The metabolism of sugars
Phosphorylated sugars are key intermediates ?
In energy generation and biosynthesis: the three sugars are all glycolytic intermediates
- G-6P
- DHAP
- GAP
Modified carbohydrates are often expressed on ?
Cell surfaces as parts of glycoproteins and glycolipids
A bond formed between the anomeric carbon atom and an oxygen atom of an alcohol is called ?
An O-glycosidic bond
A bond formed between the anomeric carbon atom and an amine is called ?
An N-glycosidic bond
