bio 98 - lecture 11 carbohydrates a.k.a. sugars, carbs…
TRANSCRIPT
Bio 98 - Lecture 11
Carbohydrates
a.k.a. Sugars,
Carbs…
I. Definition of a carbohydrate
General formula: (CH2O)n , “hydrated carbon”
Example: C6H12O6 is glucose
Many carbohydrates have more complex formulas & contain amino, phosphate, sulfate & other groups
II. Functions
1. Fundamental source of metabolic energy for most life forms.
2. Component of many important biomolecules.
animals, plants respiration
ATP ADP
sunlight
CO2 + H2O
plants photosynthesis
carbohydrate + O2
1. Carbohydrates as an energy source
bacteria
2. Biological molecules
Cellulose - wood, plant fiber, etc.Chitin - exoskeleton of arthropodsCell walls of bacteria & yeastGlycoproteins, glycolipids - cell membranesDNA, RNA - deoxyribose and ribose
Carbohydrates are often polymers
Monosaccharides: glucose, ribose, fructose, etc. Oligosaccharides: di-, tri-, tetra-, etc. Sucrose is a disaccharide: glucose + fructose
Polysaccharides: can be linear or branched (i.e. starch)
Monosaccharide nomenclature
1. Carbon number: triose, .., pentose, hexose, .., octose
2. Aldoses and ketoses
HC=O
HC-OH
HO-CH
H2C-OH
HC-OH
HC-OH
C#
1
2
3
4
5
6
D-glucose
an aldohexose
H2C-OH
C=O
HO-CH
H2C-OH
HC-OH
HC-OH
D-fructose
a ketohexose
Asymmetric (chiral) carbons generate many distinct monosaccharides; compare mannose, glucose, and galactose, which are all aldohexoses.
How many chiral carbons are in glucose?
O-chem terms relevant to monosaccharide structure
2. epimers - differ in chirality at only one carbon. Glucose and galactose are epimers at carbon 4.
1. diastereomers - identical structures except for configuration (chirality) at one or more carbons; e.g., all aldohexoses are diastereomers of each other.
3. enantiomers - mirror images; designated as D- & L-; no name change (like amino acids)!
How many aldohexose names are possible?
2 configuration choices at each of 4 asymmetric carbons; however, half of these represent enantiomers (3. above).
Number of unique names = 24 / 2 = 8.
Ring structures
Most pentoses and hexoses spontaneously form ring (cyclized) structures in solution.
5-member ring: furanose
6-member ring: pyranose
When forming a ring, a new chiral center is created, giving rise to 2 possible anomers.
HC=O
HC-OH
HO-CH
H2C-OHHC-OHHC-OH..
+
D-glucose(linear
form)
and anomers
hemiacetal
Linear form
Haworth projection
-D-glucopyranose
LinearD-glucose
-D-glucopyranose
-D-glucopyranose
In solution rapid mutarotation occurs
(1%)
(~66%)
(~33%)
Disaccharides
1. Mainly found in plants
2. Three common disaccharides• sucrose - sugar cane, sugar beets• lactose - milk sugar• maltose - malted (germinating) barley, wheat
3. 2 monosaccharides joined covalently by an O-glycosidic bond
Common disaccharides are produced by enzyme- catalyzed condensation/dehydration reactions
Can be either or due to mutarotation
Notice: there is nohemiacetal
Polysaccharides - aka Glycans
1. homopolysaccharides vs heteropolysaccharides
2. can be branched or unbranched
3. used by animals and plants as a compact storage form of CHOs
4. common examples• starch - plants, roots and seeds• glycogen - liver of mammals• cellulose - plant fiber, wood
Starch = amylose + amylopectin
found in corn, rice, potato, wheat and barley
a linear polysaccharide of glucose units
Amylopectin - a branched form of amylose
Structure of starch
glycogen (animal starch) is like starch, but more highly branched
High-fructose corn syrup
High-fructose corn syrup is produced by milling corn to produce corn starch, then enzymatically processing that starch to yield corn syrup, which is almost entirely glucose, followed by adding other enzymes that change a portion of the glucose into fructose.
1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.2. Glucoamylase - which is produced by Aspergillus, a fungus, in a fermentation vat — breaks the sugar chains down even further to yield the simple monosaccharaide glucose.3. The enzyme xylose isomerase (aka glucose isomerase) then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.
http://en.wikipedia.org/wiki/High-fructose_corn_syruphttp://en.wikipedia.org/wiki/High-fructose_corn_syrup_and_health
High-fructose corn syrup
1. Corn starch is treated with alpha-amylase to produce shorter chains of sugars called oligosaccharides.
High-fructose corn syrup
3. The enzyme glucose isomerase then converts some of the glucose to a mixture of about 42% fructose and 50–52% glucose with some other sugars mixed in.
https://wiki.geneseo.edu/display/food/Group+7-+High+Fructose+Corn+Syrup+-+Societal+Beliefs+vs.+Science
• Sugars covalently attached to the polypeptide as oligosaccharide chains containing 4 to 15 sugars
• Sugars frequently comprise 50% or more of the total molecular weight of a glycoprotein
• Most glycosylated proteins are either secreted or remain membrane-bound
• Glycosylation is the most abundant form of post-translational modification
• Glycosylation confers resistance to protease digestion by steric protection
• Important in cell-cell recognition
Protein glycosylation:A post-translational modification
Blood group antigens on the erythrocyte surface
• The O substance is a tetrasaccharide which is missing the 5th residue and does not elicit an antibody response (non-antigenic).
• The A antigen and B antigen are pentasaccharides which differ in composition of the 5th sugar residue