Carbohydrates Flashcards
Carbohydrates
Carbo(sugar) - hydrate (water)
Named because many have the formula (CH2O)n.
Produced from CO2 and H2O via photosynthesis in plants.
Photosynthetic bacteria are able to bind carbon dioxide to water to form carbohydrates that are stored in plants in the form of cellulose or lignin etc
Size ranges from small (e.g glyceraldehyde, Mw = 90 Da) to huge (e.g amylopectin, Mw = 200, 000, 000 Da)
Fulfil a wide range of functions, including:
- Energy source and energy storage
- structural component of cell walls and exoskeletons
- informational molecules in cell-cell signaling
Can be covalently linked with proteins to form glycoproteins and proteoglycans.
Carbohydrate
- Chemistry of sugars, carbonyl group, chiral centres
- open-chain and ring forms of monosaccharides
- structures and names of monosaccharides
- structures and properties of disaccharides
- biological function of polysaccharides
Aldoses and ketoses
Two classes of sugars, based on carbonyl group
An aldose contains an aldehyde group (at the end of the chain)
A ketose contains a ketone group (somewhere in the middle of the chain)
Aldoses and ketoses can have between 3-6 carbon sugars
Chiral centres and enantiomers
Enantiomers
- Steroisomers that are (nonsuperimposable) mirror images
In sugars that contain many chiral centers, only the one that is most distant from distant from the carbonyl carbon is designated as D (right) or L (left).
Enantiomer = mirror image
D and L isomers of a sugar are enantiomers
-for example, L and D glucose have the same water solubility
Most hexoses in living organisms are D stereoisomers because thats what enzymes have evolved to fit in their active sites
So D stereoisomers are favoured because evolution has favoured the enzymes (proteins) that use D stereoisomers
Some simple sugars occur in the L-form, such as L-arabinose.
When you shine polarised light on a molecule (that has a chiral carbon), it rotates the plane polarised light to either left or right
Left = levo
Right = dextro (D)
The more R groups the carbons are attached to in a sugar molecule, the more enantiomers there are
Epimers
Epimers are two sugars that differ only in the configuration around one carbon atom.
Two sugars that look identical but only one carbon has been rotated
Hemiacetals and Hemiketals
What differentiates a sugar chain structure from a ring structure
Hemi means half
So hemiacetal - half of acetyl group. Hemiketal- half of ketal group
Acetal: aldehyde. Ketal: ketone
Aldehyde and ketone carbons are electrophilic: the carbon in the carbon-oxygen double bond, is slightly electropositive because most of the electrons spend most of their time in the double bond or the lone pairs in the oxygen. This is aldehydes and ketones want electrons (electrophillic)
Alcohol oxygen atom is nucleophile- the oxygen atom in an OH group of an alcohol is quite electron rich so they are nucleophilic; ready to donate electrons
The carbonyl group can attack down the sugar which is what forms the ring. The nucleophilic site of an alcohol attacks the electrophilic site of the aldehyde, the ring forms.
When aldehyde are attacked by alcohols, hemiacetals form
When ketones are attacked by alcohols, hemiketals form
Cyclization of monosaccharides
Pentose and hexoses readily undergo intramolecular cyclization.
The former carbonyl carbon becomes a new chiral center, called the anomeric carbon (=hemiacetal carbon).
The former carbonyl oxygen becomes a hydroxyl group; the position of this group determines if the anomer is alpha or beta.
If the hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety the configuration is alpha.
If the hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety, the configuration is beta.
The hemiacetal (or hemiketal) is still reactive- it can link to another monosaccharide
Chemical reaction
Aldehyde + (excess) alcohol -> hemiacetal
Ketone + (excess) alcohol -> hemiketal
Both hemiketal and hemiacetal have single bonds around the carbon; because the alcohol attacks the carbonyl group and breaks the double bonds and forms a single bond by adding.
The glycosidic bond
Two sugar molecules can be joined via a glycosidic bond between an anomeric carbon and a hydroxyl carbon
The glycosidic bond (an acetal) between monomers is less reactive than the the hemiacetal at the second monomer.
- second monomer, with the hemiacetal, is reducing. (Reducing meaning that the end is still reactive and can carry on making chains).
- anomeric carbon involved in the glycosidic linkage is nonreducing.
Nonreducing Because the glycosidic bond it forms with another sugar makes it much less reactive
For example, the disaccharide formed upon condensation of two glucose molecules via 1->4 bond is called maltose. This is a reducing sugar (still has a free hemiacetal).
Glycosidic bonds are unreactive; needs an enzyme to cut it apart again
However, in a disaccharide, one end of the chain is still left with a hemiacetal. So it is a reducing sugar.
Nonreducing disaccharides
Two sugar molecules can also be joined via a glycosidic bond between two anomeric carbons
The product has two acetal groups and no hemiacetal.
There are no reducing ends, this is a nonreducing sugar
Examples: trehalose, lactose, sucrose
No remaining hemiacetals so no reducing sugars
Therefore these are non-reducing sugars
Polysaccharides
Natural carbohydrates are usually found as polymers
These polysaccharides can be
-homopolysaccharides (made up of same types of sugars)
-heteropolysaccharides (made up of different sugars)
-linear (sugars that are stitched into long chains)
-branched (branch points that come off the chains)
Polysaccharides do not have a defined molecular weight.
- This is in contrast to proteins and DNA, since no template is used to make polysaccharides
Polysaccharides do not hold information that codes for the cell generation to generation. Due to this, there is enormous difference in their molecular weight compared to proteins and DNA.
Sugar code- some proteins and lipids are decorated with sugars in a specific order put together by enzymes. Enzymes put them in a defined metabolic order and these sugar code play an important role. They can play a part in information signalling pathway for a cell.
Glycogen
Glycogen is a branched homopolysaccharide of glucose.
- glucose monomers form (alpha 1-> 4) linked chains.
- branch-points with (alpha 1-> 6) linkers every 8-12 residues
- molecular weight reaches several millions
- functions as the main storage polysaccharide in animals
The enzymes that activate the glycogen can get the many free ends available in the glycogen and that rapidly releases free glucose into the blood.
Starch
Starch is a mixture of two homopolysaccharides of glucose
Amylose is an unbranched polymer of (alpha 1-> 4) linked residues. Specific Enzymes aid t synthesis this glycosidic link in plants.
On one end of an amylose, it is non-reducing and other it is reducing.
Amylopectin is branched like glycogen but the branch-points with (alpha 1 -> 6) linked occur every 24-30 residues.
Molecular weight of amylopectin is up to 200 million!
Starch is the main storage polysaccharide in plants
Cellulose
Cellulose is a branched homopolysaccharide of glucose
The enzymes that carry out the synthesis of sugars to form cellulose are different
- glucose monomers form (beta 1->4)
-hydrogen bonds form between adjacent monomers
These H bonds line themselves between the monomers as the beta 1->4 bonds are made between each monomer. The way these H bonds are formed are very similar to the way H bonds formed between bases of two strands in DNA. These make the structure very stable and these H bonds sort of point towards each other between each adjacent monomer.
-additional H-bonds between chains
-structure is now tough and water-insoluble
-most abundant polysaccharide in nature; photosynthesis produces a lot of cellulose
-cotton is nearly pure fibrous cellulose
Cellulose metabolism
The fibrous structure and water-insolubility make cellulose a difficult substrate to act on.
Fungi, bacteria and protozoa secrete cellulases, which allow them to use wood as source of glucose
Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyse (beta 1-> 4) linkages
Ruminants and termites live symbiotically with microorganisms that produce cellulases
Ruminants and termites have microbes in their gut that breaks down cellulose to release sugar that is absorbed into the bloodstream on the animal
Cellulases hold promise in the fermentation of biomass into biofuels
Cellulases break down cellulose to sugars that can be fermented to form alcohol or biofuel
Both fungi and the gribble, release cellulases that break down cellulose releasing glucose that they use for their own metabolism.
Chitin
Chitin is a linear homopolysaccharide of N-acetylglucosamine. This sugar is formed by different groups attaching itself onto the glucose
N-acetylglucosamine monomers form (beta 1-> 4) linked chains.
-hard, insouble, cannot be digested by vertebrates
-structure is tough but flexible, and water-insouble
-found in cell walls in mushrooms, and in exoskeletons of insects, spiders, crabs, lobsters, and other arthropods
