chapter 7 Flashcards
(45 cards)
Carbohydrates
-Named so because many have formula
-Produced from
-Range from as small as
-Fulfill a variety of functions including:
-Can be covalently linked with
-Named so because many have formula Cn(H2O)n
-Produced from CO2 and H2O via photosynthesis in plants
-Range from as small as glyceraldehyde (Mw = 90 g/mol) to as large as amylopectin (Mw = 200,000,000 g/mol)
-Fulfill a variety 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
Glycoprotein
Proteoglycan
Glycoprotein
-Primarily protein
-CHO chains may be negatively charged
-Present in cell surface
Proteoglycan
-Primarily carbo
-CHO chains always negatively charged
-Present in connective tissue
Aldoses and Ketoses
An aldose contains an aldehyde functionality
A ketose contains a ketone functionality
Trioses
Two common hexoses
Glucose, Fructose
The pentose components of nucleic acids.D-Ribose is a component of ribonucleic acid (RNA), and 2-deoxy-D-ribose is a component of deoxyribonucleic acid (DNA)
Three types of carbohydrates
Monosaccharides
Single polyhydroxy aldehydes/ketone (e.g. glucose)
Disaccharides
Two monosaccharides linked by glycosidic bond; their name ends with suffix “_OSE” (e.g. sucrose)
Polysaccharides
Thousands/millions of mono- units (e.g. cellulose/glycogen)
Enantiomers
Enantiomers
–Stereoisomers that are nonsuperimposable mirror images
In sugars that contain many chiral centers, only the one that is most distant from the carbonyl carbon is designated as D (right) or L (left)
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
Some simple sugars occur in the L-form, such as L-arabinose
Diastereomers? Seterisomers that are not mirrorimages of each other
Drawing Monosaccharides
-Chiral compounds can be drawn using perspective formulas
-However, chiral carbohydrates are usually represented by Fischer projections
-Horizontal bonds are pointing toward you; vertical bonds are projecting away from you
Diastereomers
-Diastereomers: stereoisomers that are not mirror images
-Diastereomers have different physical properties
–For example, water solubilities of threose and erythrose are different
Epimers
-Epimers are two sugars that differ only in the configuration around one carbon atom
Structures to Know
-Ribose is the standard five-carbon sugar
-Glucose is the standard six-carbon sugar
-Galactose is an epimer of glucose
-Mannose is an epimer of glucose
-Fructose is the ketose form of glucose
Hemiacetals and Hemiketals
-Aldehyde and ketone carbons are electrophilic
-Alcohol oxygen atom is nucleophilic
-When aldehydes are attacked by alcohols, hemiacetals form
-When ketones are attacked by alcohols, hemiketals form
Cyclization of Monosaccharides
-Pentoses and hexoses readily undergo
-The former carbonyl carbon becomes a
-The former carbonyl oxygen becomes a
-If the hydroxyl group is on the opposite side
-If the hydroxyl group is on the same side
-Pentoses and hexoses readily undergo intramolecular cyclization
-The former carbonyl carbon becomes a new chiral center, called the anomeric carbon
-The former carbonyl oxygen becomes a hydroxyl group; the position of this group determines if the anomer is α or β
-If the hydroxyl group is on the opposite side (trans) of the ring as the CH2OH moiety the configuration is α
-If the hydroxyl group is on the same side (cis) of the ring as the CH2OH moiety, the configuration is β
Mutarotation
α β interconvertible BUT with breakage of covalent bonds
Pyranoses and Furanoses
-Six-membered oxygen-containing rings are called pyranoses
-Five-membered oxygen-containing rings are called furanoses
-The anomeric carbon is usually drawn on the right side
Chain-Ring Equilibrium and Reducing Sugars
-The ring forms exist in equilibrium with the open-chain forms
-Aldehyde can reduce Cu2+ to Cu+ (Fehling’s test)
-Aldehyde can reduce Ag+ to Ag0 (Tollens’ test)
-Allows detection of reducing sugars, such as glucose
Basis of Fehling’s Reaction = Glucose is Reducing
This is the more sensitive and specific test for glucose
Colorimetric Glucose Analysis
Nowadays, enzymatic methods are used to quantify reducing sugars such as glucose
–The enzyme glucose oxidase catalyzes the conversion of glucose to glucono-δ-lactone and hydrogen peroxide
–Hydrogen peroxide oxidizes organic molecules into highly colored compounds
–Concentrations of such compounds is measured colorimetrically
Electrochemical detection is used in portable glucose sensors
The nonenzymatic reaction of glucose with a primary amino group in hemoglobin
Concentration of GHB is dangerous damage to kidneys, retinas, cardiovascular system
The Glycosidic Bond
Two sugar molecules can be joined via a
The glycosidic bond (an acetal) between monomers is
The disaccharide formed upon
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 hemiacetal at the second monomer
Second monomer, with the hemiacetal, is reducing
Anomeric carbon involved in the glycosidic linkage is nonreducing
The disaccharide formed upon condensation of two glucose molecules via 1 → 4 bond is called maltose
Nonreducing Disaccharides
-Two sugar molecules can be also joined via a glycosidic bond between
-The product has two
-There are no
-Trehalose is a
-Two sugar molecules can be also joined via a glycosidic bond between two anomeric carbons
-The product has two acetal groups and no hemiacetals
-There are no reducing ends, this is a nonreducing sugar
-Trehalose is a constituent of hemolymph of insects
–Provides protection from drying
–Resurrection plant (> 15 yrs)
Polysaccharides
-Natural carbohydrates are usually found as
-These polysaccharides can be
-Polysaccharides do not have a defined
-Natural carbohydrates are usually found as polymers
-These polysaccharides can be
–homopolysaccharides (storage forms of monosacc as fuel (starch and glycogen); structural elements in plant cells (cellulose, chitin))
–heteropolysaccharides (provide extracellular support for organism (protection, shape, support to cells, tissues, etc.))
–linear
–branched
-Polysaccharides do not have a defined molecular weight.
–This is in contrast to proteins because unlike proteins, no template is used to make polysaccharides
homopolysaccharides heteropolysaccharides
differ in
Differ in:
mono- units,
chain length
linking bond types
degree of branching
homopolysaccharides heteropolysaccharides
Glycogen
Glycogen is a
-Glucose monomers form
-Branch-points with
-Molecular weight reaches
-Functions as the main
WHY: store glycogen instead of glucose.
Glycogen is a branched homopolysaccharide of glucose
-Glucose monomers form (α1 → 4) linked chains
-Branch-points with (α1 → 6) linkers every 8–12 residues
-Molecular weight reaches several millions
-Functions as the main storage polysaccharide in animals
WHY: store glycogen instead of glucose. The glycogen is insoluble and does not affect the osmolarity of the cell. Glucose is soluble and will affect osmolarity
Glycosidic Linkages of glycogen
Starch
Starch is a mixture of
–Molecular weight of amylopectin is up
-Starch is the main
Starch is a mixture of two homopolysaccharides of glucose
–Amylose is an unbranched polymer of (α1 → 4) linked residues
–Amylopectin is branched like glycogen but the branch-points with (α1 → 6) linkers occur every 24–30 residues
–Molecular weight of amylopectin is up to 200 million
-Starch is the main storage polysaccharide in plants
