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BCH210H1 (500)
Lecture 4

BCH210H1 Lecture Notes - Lecture 4: Anomer, Enantiomer, Good Energy


Department
Biochemistry
Course Code
BCH210H1
Professor
Michael Baker
Lecture
4

Page:
of 6
Lecture 4
we will see how glucose is a good energy source
we will talk about:
Monosacchrides
cyclization reactions of common forms of sugars
sugar derivatives
and ultimately higher forms that have two or more monosacchrides linked in chain
any glucose that you eat that your body doesn't eat will change into fat
Roles of carbohydrates
1. Energy source – fuels (simple sugars like glucose)
- too little sugar in blood= hypoglycemia
2. Storage form of chemical energy (high molecular weight sugar polymers
3. source of C in synthesis of other molecules (sugar into fat)
4. structural elements in cells and tissues- paper; cellulose, cotton
5. components of nucleotides, nucleic acids, glycoproteins and glycolipids
Classes of carbohydrates
Monosacchrides
simple sugar with one polyhydroxy aldehyde or ketone
most sugars end in ose – lactose, sucrose, amylose, galactose, glucose (most sugar endings were
ase)
general formula monosacchrise: (CH2O)n where n could be 3 -7
minimum 3 carbons and maximum 7 carbons in the monosacchride sugar
Carbohydrate – for each carbon there is a hydrate (H2O)- for most cases
C6H12O6 – glucose – above 2 mm to be in consciousness
each carbon on a straight chain has a hydroxy group (OH) except for one which is either an
aldehyde (first carbon) or a ketone group (second carbon)
aldehyde group – aldose sugar
ketone group- ketose sugar
mono sacchride – white crystalline solids – freely water soluble with a sweet tast
sugars can be defined by a number of carbons it has – triose= 3 carbon; pentose - 5; hexose=6
there could be aldotriose and ketotriose (dihydroxyacetone)
two forms of aldo triose ( glyceraldehyde)
L and D form- the OH group on the middle carbon
if left side = L
if right side= D
they can vary with respect to sweetness- fructose much sweeter so used by manufacturers
if too much sugar in the blood – proteins and hemoglobin reacts with it- interaction of high
sugar levels with tissues = diabetes
Stereoisomers
same number of functional groups
cannot be super imposed
ALL monosacchrides have one or more chiral centres except for dihydroxyacetone ( has no
single carbon with 4 carbons around it)
tetrahedral geometry for stereocentres (fisher projection)
Rotation of plane polarized light
chirality produces interesting optical projections
chiral centres rotate plane polarized light
if you wear sunglasses, you realize that it only allows light vibrating in one plane to come in
your eye hence reducing glare – light usually come in a number of different planes
if you take a polarized light in one plane and shine it down in a solution containing
stereoisomer- it cranks the plane to the right or left
if you have d-glucose- cranks the light to the right
Aldohexoses
6 carbons with aldehyde group 6 carbon 1
4 chiral carbons in a straight chain
4 chiral centres -each exist in 2 – L or D
2*2*2*2 = 16
memorize three of them
8 D stereoisomers come from carbon 5 (furthest chiral centre away from the aldehyde)
last asymetric centre – the hydroxyl on the right
D sugars are the ones biologically active
3 most common D-glucose; D- mannose; D-galactose
D-glucose = biologically active form of glucose
D-glucose and D-mannose are epimers at C-2
epimers = stereoisomer that differ only at one chiral carbon
glucose has the OH on the right side; mannose has it on the left side
D glucose and D galactose – are epimers at carbon 4
glucose has the OH on the right side; galactose on the left side
Enantiomers
complete mirror images
D and L form are mirror images of each other
Riboses- aldo pentose
of the 5 carbons,3 are chiral
8 stereoisomers (2^3)
4 Ds and 4Ls
Rememember
Ketohexose
3 chiral carbons again
8 stereoisomers
Aldehyde – very reactive
Too much blood sugar- hyperglycemia (prolonged blood sugar) aldehyde react with amino
groups- problem
aldehyde interacts with hydroxyl groups (alcohol) as well
after the aldehyde group interacts it alcohol it forms a prochiral carbon 1- then C 1 interacts
with carbon 5 to form a cyclic structure
depending on what side you go at you can form an alpha or a beta structure- this is only
possible because carbon will now become chiral
Hemiacetal
you form a new chiral carbon as a result of this cyclization reaction
it forms a pentose (6 is a stretch)
hemicacetal comes and defines a pyranose ring
pyrane ring – 5 carbons and an oxygen
new chiral centre – C1 = anomeric isomers
Recall:
Enantiomers – 2 mirror images of each other – L and D form
Epimers – differ only at one carbon D-glucose and D- galactose
Anomers – carbon 1 could be axial (below the plane) or equitorial (up)- alpha and beta form
respectively
Hayworth projections
rings are drawn flat- 2D structure
alpha D glucose (glucopyronose) and beta D glucose (glucopyronose) = hemiacetals
Potential experiment
if you have alpha D glucose – dissolve in water – you do have alpha D glucose
very common form used anobalically? to give you energy
alpha D glucose has a rotation of polarized light of 112 degrees
Do optical rotation to confirm
put in a polarimeter- which shines the polarized light by grading one plane – it cranks it to the
right direction (+) 112 degrees
you come back in a couple of hours – repeat the experiment
it's 88 degrees now- getting lower
It's because the cyclic structure can open up
and go to the other side
it passes in between through a straigh chain form
equilibrium between alpha and beta form if dissolved in water
this is called mutarotation
Beta is more stable than alpha–2/3 will appear to be in beta form in the solution and 1/3 in alpha
Hemiketals
fructose = ketohexose
ketone group at C2
C2 and C5 come together
makes C2 a chiral carbon as a result of cyclization
which is attacted by carbon 5 hydroxy group
in this case both C1 and C 6 are outside the ring (only C6 is outside the ring for hemiacetal)
again two different ways of cyclization-
furane ring- 4 carbons and 2 oxygens
looks like an organic molecule called furane
the smell coming from corn boiling = furane
again 5 membered ring
Alpha and Beta fructofuranose
carbon 2 if below the plane = alpha; above the plane – beta
How does ribose form a ring structure – look it up?
Higher forms of carbohydrates
glucose is in the blood; but not a way carbohydrates are stored
carbohydrates are stored in the form of polymers or disaccharides (oligosacchrides)
oligosacchrides = 2 to 7 monosacchrides joined by oligosacchride links
2 hydroxyls comes – release water – and produce coupling between two monosacchrides=
glycosidic linkage
2 hydroxyl groups ( 1 from carbon 4 and another one from carbon 1) from two sugars come