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Lecture 5

NUTR 3210 Lecture Notes - Lecture 5: Anomer, Aldohexose, Glycosidic Bond

7 pages97 viewsFall 2016

Department
Nutrition
Course Code
NUTR 3210
Professor
Jennifer Monk
Lecture
5

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NUTR 3210 Carbohydrates
Carbohydrates
Composed of carbon, hydrogen and oxygen
General formula: Cn(H2O)n
Major source of fuel or energy in the body (50% of calories in the American diet) but
contains no essential nutrients
Glucose is an essential metabolite in blood and tissues but can be derived from dietary
amino acids, glycerol and more not an essential nutrient
CHO are found in the form of polyhydroxy aldehydes and polyhydroxy ketones (not
hydrates of carbon)
Classification:
1. Simple Sugars (mono- and disaccharides)
Monosaccharides:
o Simpliest forms
o Most common are 6C (hexoses) especially glucose
o Trioses (3C) have an important role in metabolism pyruvate
o Main 6C monosaccharides = glucose, fructose and Galactose
o *see linear and cyclic structures
Disaccharides:
o Form through glycosidic bond linking 2 monosaccharides
o Includes natural sugars like sucrose and lactose
o Maltose is also considered a disaccharide, although it is a partial breakdown
product of starch or glycogen (formed in food processing or during digestion)
o Common Disaccharides:
Glucose + Fructose = Sucrose digested by sucrose
Glucose + Galactose = Lactose digested by lactase
Glucose + Glucose = Maltose digested by maltase
o Sugars have two functional groups:
Aldose at the end of molecule
Ketose within the molecule
2. Complex
Olgiosaccharides (typically digestion intermediates)
o Consists of short monosaccharide chains (typically 3-9) joined together by
glycosidic bonds
Polysaccharides
o Longer chains (10+) monosaccharides; often have 100s-1000s of units
o Includes starches like amylose and amylopectin (chains of glucose) from
plants and glycogen from animal sources
o From plants, there are insoluble fibers (including cellulose, hemicellulose and
soluble fibers) within different mixtures of sugars and bond types
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Monosaccharide Nomenclature:
-ose = suffix used to designate sugars
Names = functional group + #C + -ose
Ex. Triose + metabolites of glucose + aldehyde functional group aldotriose
(glyceraldehyde)
Ex. Pentose + components of DNA/RNA + aldehyde aldopentose (ribose)
Ex. Hexose + major form in diet/blood/tissue storage + aldehyde aldohexose (glucose)
Functional Groups:
Linear and cyclic forms exist in an equilibrium
To form a ring, second lowest OH bends around and attacks the carbonyl carbon (this is a
spontaneous, non-enzymatic reaction)
D-glucose an aldohexose
D-fructose a ketohexose
Anomeric carbon C=O ; carbon of the carbonyl group (aldehyde or ketone)
Stereoisomerism:
Same molecular formula but different 3D structure
Types:
o Enantiomers (mirror images) opposite at every chiral carbon
o Diasteromers cis/trans isomers
Must have at least one chiral carbon (with four different groups)
o Glucose contains four chiral carbons
Biological system are sterospecific enzymes recognize specific isomers
Fischer Projection of Glucose (linear, 2D)
o Anomeric carbon is close to the top (and is the lowest numbered C)
o D-glucose is natural OH on C5 is to the right
o L-glucose is synthetic OH on C5 is on the left
There are 2n enantiomers where n=# chiral C glucose has 16 enantiomers
D-Glucose: from Fischer Projection (linear) to Haworth Model (ring)
When glucose is put into a solution, the majority of the molecules will take on a cyclic
configuration
o There are 3 forms of D-glucose linear, alpha and beta
Cyclization occurs with the 5-position OH attacking the aldehyde carbon chiral
carbon; it loses C=O to make a new OH group
o Alpha: -OH is pointing down
o Beta: -OH is pointing up
A monosaccharide ring that was derived from an aldehyde is called a hemiacetal
(alpha/beta-D-glucose are examples of hemicetals)
o Hemicetal: monosaccharide in a ring derived from aldehyde
Enzymes are specific for alpha and beta forms (e.g. sucrase is specific for alpha-D-
glucose)
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