Carbohydrates Flashcards
monosaccharides
1 sugar unit
disaccharides
2 sugar units covalently linked
oligosaccharides
several sugar units
- most important are the ones attached to proteins or lipids outside of the cell
- important in cell- cell interactions in the nervous system
- brain
- coat blood cells and determine blood group
polysaccharides
long chains of 100s-1000s
- linked by glycosidic bonds
- a(1-4) or B(1-4)
- can be straight or branches
What are carbs involved in?
- Energy transport and storage (glycogen)
* glucose gets broken down to release energy - Structural
* bacterial cell walls, cellulose and peptidoglycan - Information
* signals on proteins and membranes
* blood group antigens- when transfusing you have to be careful because if they have type A you cant give type B or type AB because carbos on the surface of red blood cells wont match, causes immune response which isnt favourbale
What are the 2 major carb families?
- Aldoses- carbonyl is always on c1
* derive from D- glyceraldehyde - Ketoses- carbonyl is always on C2
* derive from dihydroxyacetone
BOTH have asymmetric chiral carbons
EXCEPT FOR dihydroxyacetone
glucose
Glc
glucosamine
GlcN
amino sugar
- sugar derivative
Fructose
Fru
Galactose
Gal
Galactosamine
GalN
- amino sugar
- sugar derivative
N-Acetylglucosamine
GlcNac
- sugar amide
- sugar derivative
Ribose
Rib
- deoxy- sugar
- sugar derivative
Why do ketohexoses have 1 less chiral centre
because of symmetry carbonyl group will go down into molecule will change overall structure because of position
How are most sugars found?
As D and most amino acids are in L
How do you identify if a molecule is D?
Look at the chiral carbon furthest from the carbonyl and if it has OH on the right its D
What are enantiomers?
non super imposable mirror images
- one is D and one is L
- if you were to mirror each of the groups are aligned but if you put them on top of each other the groups wouldn’t match
What are diastereomers?
non mirror image stereoisomers
Epimers
diastereomers that differ in position of only one carbon
What is the epimeric carbon?
the carbon that differs between molecules
How do carbs exist in solution?
cyclic form
- carbonyl groups react with alchols
Why do some carbs exist as cyclic and not others?
the main restriction is steric- small rings (4 or fewer atoms in the ring) are unstable so they don’t have the flex to form cyclic
* have carbonyl group thats an electrophile Oh thats a nucleophile but dont have the ability to form cyclic without straining themselves
A carbon thats 5 or 6 atoms in backbone are most stable when they have formed cyclic structure because they don’t have issues with steric strain and the reactions create an additional chiral centre
formation of cyclic forms is reversible- molecules can shift between linear and cyclic according to stability in solution
anomers
isomers that differ only at the hemiacetal or hemiketal C and the C is called the anomeric carbon
mutarotation
the alpha anomer and beta anomer can interconvert through trace amounts of the non-cyclic form
- measured by rotation of plane- polarized light
- pure alpha glucose rotates light at 112
- pure B rotates at 19
at equ the mixture rotates light +53- weighted average (beta is more prevalant)
Equitorial
group in plane with planar part of ring, less steric hinderance for bulky groups(best for bulky groups, want as many as them in this position as possible)
axial
group perpendicular to planar part of the ring
sugar derivatives
K
What can glucose form?
It can form 2 cyclic structures involving 6- membered ring
they are different because the new chiral centre can be alpha or beta
6 membered ring is preferred over the less stable 5 membered ring
* structure is called pyranose
B- anomer
OH on the C1 is on the same side of the ring as c6
a- anomer
OH on the c1 is on the opposite to C6
Describe cyclic ketoses
the rings are puckered
- different positions they take on for stability
- have groups in positions that will provide greatest stability and lowest delta G
Furanose
5 membered ring structures
*called this because of their resemblance to furan
maltose
- reducing sugar made from starch and amylase
- Glc (a1-4) Glc
isomaltose
- Glc (a1-6) Glc
- a reducing sugar made from hydrolysis of dextrans
- hydrolyzing a branched sugar
Cellobiose
- Glc (beta 1-4) Glc
- a reducing sugar produced by acid hydrolysis of cellulose
Are humans able to breakdown cellulose?
No we are not able to hydrolyze cellulose because we do not have the enzymes to break beta bonds
Lactose
Gal (beta 1-4) Glc
- reducing sugar (able to mutarotate, linearize, donate electrons)
- major sugar of milk
Sucrose
- Glc (alpha 1-B2) Fru
- non reducing sugar made by plants- table sugar
- formed by reacting the anomeric carbons so no hemiacetal or ketal function
- doesnt undergo mutarotation
What happens when sucrose is hydrolyzed?
When hydrolyzed with acid to Glc + Fru an effect called inversion of sucrose happens- 1:1
- sucrose rotates light by +66
- hydrolysis gives a 1:1 mixture of D-Glu (a=+52) and D Fru (a=-92) which rotates light with a=-39
- mixture is called “invert sugar” because of change of sign
Trehalose
- Glc (alpha 1-a1) Glc consists of 2 Glc units linked together through their anomeric carbon
- non reducing
- cant mutarotate
- used for energy storage in insects
Why are oligosaccharides on the outside of cells?
Want them to be on the outside so immune system can recognize and if it doesn’t then it can mount a response against blood group and cause rejection
What are the important roles of polysaccharides in biology?
- Storage of sugar units (compact (polymeraize or depolymearize easily) Units cane be easily added or removed by enzymes as needed
glycogen, starch - Structural
- tough molecules, resistant to degradation by enzymes, fold up into organized structures to maintain cell walls
* peptidoglycan, chitin- help resist osmotic rupture
Why don’t we store sugar as free glucose, why do we make glycogen?
Non polymerized glc= osmotically active
- if we pack a bunch of it into the cell water rushes in and it will burst and kill you
Starch
the storage form of D-Glc in plants
- 2 types: amylose and amylopectin
Amylose
- unbranched chains of a(1-4) linked glc units up to 4,000 per chain
- form a helix(NOT SAME AS SECONDARY STRUCTURE IN PROTEIN)
Amylopectin
- up to 200 amylose chains linked by a(1-6) branches
- cant form a helix
Why can amylopectin not form a helix?
- because to large and bulky to much rigidity
Glycogen
animal cell storage of Glc
- similar to amylopectin but more branched- 15-30 sugars/branch
- stored in liver and muscle only
Dextrans
- bacterial polysaccharides with a(1-6) links and some a(1-2) and A(1-4) links
- incomplete hydrolysis of starch and glycogen gives some disaccharide
- can be broken down by disaccharides
- most of it is maltose linked coming from the straight portions of the polysaccharide plus some isomaltose come from branches
Explain why glycogen is stored only in the liver and muscle
- because its not effective storage
- better to store energy as fats because you get 2X the amount of energy
- we store in the liver because its fast (easy to access in times of need)
In the intestine how do you absorb sugar
ALL sugar must be monosaccharides because we don’t have the means to bring in
Cellulose
- linear chains of 10-15,000 glucose units linked as in D-cellobiose
- strong rod like structure of parallel chains packed side by side (for strength and rigidity)
- fibrous structure is used for textiles(cotton, linen) industrial fibre and paper
cant be digested by humans but cows have an extra stomach (extra pocket inside tract) and secrete cellulase, an enzyme that cleaves B1-4 glycosidic bonds to release glucose
*cows are herbivores so form this symbiotic relationship
peptidoglycan
- exclusive to bacteria
- alternating beta 1-4 linked GlcNac and MurNac sugars that are short peptides
- forms a 3 dimensional mesh like cell wall structure that provides rigidity to the bacteria and protects it from osmotic rupture
- peptide crosslinks(create perpendicular reinforcements to provide strength and prevent osmotic rupture
Lysozyme
-degrade bacterial cell walls by catalyzing the hydrolysis of Beta 1-4 linkages(linear linkage) between Nam and Nag
-part of the non specific host defense system
-found in secretions such as tears saliva milk and mucus
found in egg white
