Exam 5 Flashcards
Carbohydrates
Named so because many have formula Cn(H2O)n
Produced from CO2 and H2O via photosynthesis in plants
Range from as small as glyceraldehyde (90 g/mol) to as large as amylopectin (200mil g/mol)
Carbohydrate functions
–energy source and energy storage
–structural component of cell walls and exoskeletons
–informational molecules in cell-cell signaling
Basic nomenclature of carbs:
number of carbon atoms in the carbohydrate + “-ose”
All carbohydrates initially had a carbonyl functional group.
–aldehydes = aldose
–ketones = ketose
EG: aldotriose
Stereoisomers
Isomers that differ in the orientation of their atoms in space
Enantiomers
–Stereoisomers that are nonsuperimposable mirror images
–Same physical properties
Isomers
Molecules that have the same numbers of the same kinds of atoms and hence the same formula but differ in chemical and physical properties
D and L Conformations
–D and L sugar conformations are enantiomers based on the final hydroxy group position
–D: right side
–L: Left side
–Most hexoses in living organisms are D stereoisomers
Diastereomers
stereoisomers that are not mirror images
different physical properties
Epimers
–stereoisomers that differ at only one chiral center
–they are not enantiomers (mirror image)
–they are diastereomers with different physical properties
Reactivity of Carbohydrates: Hemiacetals
-Aldehyde carbons are electrophilic.
-Alcohol oxygen atom is nucleophilic.
-When aldehydes are attacked by alcohols, a hemiacetal forms (the double bonded oxygen is now -OH and -OR joins the original aldehyde carbon)
They form the basis of cyclization of sugars
Reactivity of Carbohydrates: Hemiketals
-Ketone carbons are electrophilic.
-Alcohol oxygen atom is nucleophilic.
-When ketones are attacked by alcohols, a hemiketal forms (the double bonded oxygen is now -OH and -OR joins the original ketone carbon)
They form the basis of cyclization of sugars
Cyclization of Monosaccharides
The nucleophilic alcohol attacks the electrophilic carbonyl carbon, allowing formation of a hemiacetal.
A ring forms.
The carbonyl carbon is reduced to an alcohol and that alcohol is variable/transient
anomeric carbon
The former carbonyl carbon that becomes a new chiral center
The new hydroxyl group either makes the anomer alpha or beta
The ring may be cis or trans as well
pyranoses
Six-membered oxygen-containing rings after the pyran ring structure
favor “chair” conformations
Multiple “chair” conformations are possible but require energy for interconversion (~46 kJ/mole)
furanoses
Five-membered oxygen-containing rings after the furan ring structure
Reducing Sugars
These sugars have a free anomeric carbon, typically on aldehydes with the H+
Reducing sugar tests
Aldehyde can reduce Cu2+ to Cu+ (Fehling’s test).
Aldehyde can reduce Ag+ to Ag0 (Tollens’ test).
It allows detection of reducing sugars, such as glucose.
Colorimetric Glucose Analysis
–The enzyme glucose oxidase catalyzes the conversion of glucose to glucono-delta-lactone and hydrogen peroxide.
–Hydrogen peroxide oxidizes organic molecules into highly colored compounds.
–Concentrations of such compounds is measured colorimetrically.
The Glycosidic Bonds (Nonreducing)
Two sugar molecules can be also joined between two anomeric carbons.
The product has two acetal groups and no hemiacetals.
There are no reducing ends; this is a nonreducing sugar.
The Glycosidic Bond (Reducing)
–Two sugar molecules can be joined between an anomeric carbon (nonreducing) and a hydroxyl carbon
–The glycosidic bond (an acetal) between monomers is more stable and less reactive than the hemiacetal (reducing) at the second monomer.
Disacharides can be named by the organization and linkage or a common name.
heteropolysaccharides
(multiple monomer units)
Four Types of Polysaccharides
homopolysaccharides
heteropolysaccharides
linear
branched
homopolysaccharides
(one monomer unit)
linear polysaccharides
(one type of glycosidic bond)
branched polysaccharides
(multiple types of glycosidic bonds)
Polysaccharide standard weight
Polysaccharides do not have a defined molecular weight.
Polysaccharides are often in a state of flux; monomer units are added and removed as needed by the organism.
Glycogen
Branched homopolymers of glucose
Glucose monomers form (a1 - 4) linked chains.
There are branch points with (a1 - 6) linkers every 8–12 residues.
It functions as the main storage polysaccharide in animals.
Starch
mixture of two homopolysaccharides of glucose.
Amylose is an unbranched polymer of (a1 - 4) linked residues.
Amylopectin is branched like glycogen, but the branch points with (a1 - 6) linkers occur every 24–30 residues.
main storage polysaccharide in plants.
Metabolism of Glycogen and Starch
Insoluble due to their high molecular weight and often form granules in cells.
Granules contain enzymes that synthesize and degrade these polymers.
Glycogen and amylopectin have one reducing end but many nonreducing ends.
Enzymatic processing occurs simultaneously in many nonreducing ends.
Cellulose
linear homopolysaccharide of glucose.
Glucose monomers form (B1 - 4) linked chains.
Hydrogen bonds form between adjacent monomers and there are additional H-bonds between chains.
Structure is now tough and water insoluble.
It is the most abundant polysaccharide in nature. Cotton is nearly pure fibrous cellulose.
Cellulose Metabolism
Most animals cannot use cellulose as a fuel source because they lack the enzyme to hydrolyze (B1- 4) linkages.
Fungi, bacteria, and protozoa secrete cellulase, which allows them to use wood as source of glucose.
Ruminants and termites live symbiotically with microorganisms that produce cellulase
Chitin
linear homopolysaccharide of N-acetylglucosamine.
N-acetylglucosamine monomers form (B1- 4)-linked chains.
forms extended fibers that are similar to those of cellulose so they are hard, insoluble, cannot be digested by vertebrates
found in cell walls in mushrooms and in exoskeletons of insects, spiders, crabs, and other arthropods
Agar and Agarose
branched heteropolysaccharide composed of agarose and agaropectin.
Agar serves as a component of cell wall in some seaweeds.
Agar solutions form gels that are commonly used in the laboratory as a surface for growing bacteria.
Agarose solutions form gels that are commonly used in the laboratory for separation DNA by electrophoresis.
Glycosaminoglycans
Linear polymers of repeating disaccharide units
One monomer is either:
–N-acetyl-glucosamine or
–N-acetyl-galactosamine
Negatively charged
–uronic acids (C6 oxidation)
–sulfate esters
Forms meshwork with fibrous proteins to form extracellular matrix
–connective tissue
–lubrication of joints
Heparin and Heparan Sulfate
Heparin is linear polymer, 3–40 kDa.
Heparan sulfate is heparin-like polysaccharide but attached to proteins.
Highest negative-charge density biomolecules
Prevent blood clotting by activating protease inhibitor antithrombin
Binding to various cells regulates development and formation of blood vessels.
Can also bind to viruses and bacteria and decrease their virulence
Glycoconjugates: Glycolipids
Lipids with covalently bound oligosaccharide
They are parts of plant and animal cell membranes.
In vertebrates, ganglioside carbohydrate composition determines blood groups.
In gram-negative bacteria, lipopolysaccharides cover the peptidoglycan layer.
Glycoconjugates: Proteoglycans
Sulfated glucosaminoglycans attached to a large rod-shaped protein in cell membrane
–syndecans: protein has a single transmembrane domain
–glypicans: protein is anchored to a lipid membrane
–interact with a variety of receptors from neighboring cells and regulate cell growth
Linkage from anomeric carbon of xylose to serine hydroxyl
Lipids
Structurally diverse
Two types:
1. Contain fatty acids
2. Do not contain fatty acids
Biological Functions of Lipids
Storage of energy
Insulation from environment (high heat capacity)
Water repellant (feathers)
Buoyancy control and acoustics in marine mammals
Membrane structure
Cofactors for enzymes
Signaling molecules
Pigments
Antioxidants
Fatty acids
Carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons (unbranched typically)
Saturated Fatty Acids
no double bonds between carbons in the chain
Monounsaturated Fatty Acids
one double bond between carbons in the alkyl chain
Polyunsaturated Fatty Acids
more than one double bond in the alkyl chain
Fatty Acid Nomenclature
delta numbering of carbon skeleton: 18:1Δ9
describes location of the first carbon of the alkene in relationship to the carbonyl carbon
omega numbering of carbon skeleton: 18:1ω9
describes location of the first carbon of the alkene in relationship to the terminal methyl
Omega-3 fatty acids
Essential nutrients.
Humans need them but cannot synthesize them.
They including ALA, DHA, and EPA.
although DHA and EPA can be synthesized from ALA
Solubility and Melting Point of Fatty Acids
Solubility
-decreases as the chain length increases
Melting Point
–decreases as the chain length decreases
–decreases as the number of double bonds increases
Conformation of Fatty Acids
The saturated chain tends to adopt extended conformations.
The double bonds in natural unsaturated fatty acids are commonly in cis configuration, which kinks the chain.
Melting Point and Double Bonds
Unsaturated cis fatty acids pack less orderly due to the kink.
less-extensive favorable interactions
It takes less thermal energy to disrupt disordered packing of unsaturated fatty acids.
Unsaturated cis fatty acids have a lower melting point.
Trans Fatty Acids
form by partial dehydrogenation of unsaturated fatty acids.
trans double bond allows a given fatty acid to adopt an extended conformation.
Trans fatty acids can pack more regularly and show higher melting points than cis forms.
The body cannot break down
Triacylglycerols (Nonpolar)
Solid ones are called fats.
Liquid ones are called oils.
The primary storage form of lipids (body fat)
Less soluble in water than fatty acids due to the esterification of the carboxylate group
Less dense than water: fats and oils float.
The advantage of fats over polysaccharides:
Fatty acids carry more energy per carbon because they are more reduced.
Fatty acids carry less water per gram because they are nonpolar.
Fats are for long-term (months) energy needs, good storage, and slow delivery.
Waxes
esters of long-chain saturated and unsaturated fatty acids with long-chain alcohols.
Insoluble and have high melting points
Wax functions
storage of metabolic fuel in plankton
protection and pliability for hair and skin in vertebrates
waterproofing of feathers in birds
protection from evaporation in tropical plants and ivy
used by people in lotions, ointments, and polishes
Beeswax
mixture of a large number of lipids, including esters of triacontanol, and a long-chain alkane hentiacontane
Structural Lipids in Membranes (Polar)
Contain polar head groups and nonpolar tails (usually attached fatty acids)
The properties of head groups determine the surface properties of membranes.
Diversification of Structural Lipids in Membranes (Polar)
modifying a different backbone
changing the fatty acids
modifying the head groups
Different organisms have different membrane lipid head group compositions.
Different tissues have different membrane lipid head group compositions.
Glycerophospholipids
Primary constituents of cell membranes
Two fatty acids form ester linkages with the first and second hydroxyl groups of l-glycerol-3-phosphate.
The phosphate group is charged at physiological pH.
General Structure of Glycerophospholipids
Unsaturated fatty acids are commonly found connected to C2 of glycerol-3-phosphate.
The highly polar phosphate group may be further esterified by an alcohol; such substituent groups are called the head groups
Phosphatidylcholine
Phosphatidylcholine is the major component of most eukaryotic cell membranes.
Many prokaryotes, including E. coli, cannot synthesize this lipid; their membranes do not contain phosphatidylcholine.
Ether Lipids: Plasmalogen
Common in vertebrate heart tissue
Also found in some protozoa and anaerobic bacteria
Function is not well understood
-Increase membrane rigidity?
-Sources of signaling lipids?
-resistant to cleavage by common lipases but cleaved by few specific lipases
-May be antioxidant?
Ether Lipids: Platelets-Activating Factor
First signaling lipid to be identified
Stimulates aggregation of blood platelets
Plays role in mediation of inflammation
Acetic acid has esterified position C2
Sphingolipids
The backbone of sphingolipids is a long-chain amino alcohol sphingosine.
A fatty acid is joined to sphingosine via an amide linkage, rather than an ester linkage as usually seen in lipids.
A polar head group is connected to sphingosine by a glycosidic or phosphodiester linkage.
Sphingomyelin
Ceramide (sphingosine + amide-linked fatty acid) + phosphocholine attached to the alcohol
Sphingomyelin is abundant in myelin sheath that surrounds some nerve cells in animals.
Glycosphingolipids and Blood Groups
The blood groups are determined in part by the type of sugars located on the head groups in glycosphingolipids.
The structure of sugar is determined by an expression of specific glycosyltransferases.
Blood types
Individuals with no active glycosyltransferase will have the O antigen.
Individuals with a glycosyltransferase that transfers an N-acetylgalactosamine group have A blood group.
Individuals with a glycosyltransferase that transfers a galactose group have B blood group.
Lysosome and Lipids
Most cells continually degrade and replace their membrane lipids.
Phospholipids are degraded by phospholipases A−D.
Each phospholipase cleaves a specific bond.
Gangliosides are degraded via a series of enzymatic cleavages
Sterols and Cholesterol
steroid nucleus: four fused rings
hydroxyl group (polar head) in the A-ring
various nonpolar side chains
The tetracycle structure of sterols is almost planar.
Physiological Role of Sterols
present in the membranes of most eukaryotic cells:
-modulate fluidity and permeability
-thicken the plasma membrane
-no sterols in most bacteria
Mammals obtain cholesterol from food or synthesize it de novo in the liver.
Steroid Hormones
Steroids are oxidized derivatives of sterols.
Steroids have the sterol nucleus but lack the alkyl chain found in cholesterol.
They are more polar than cholesterol.
Steroid hormones are synthesized from cholesterol in gonads and adrenal glands.
They are carried through the body in the bloodstream, usually attached to carrier proteins.
Many of the steroid hormones are male and female sex hormones.
Biologically Active Lipids
present in much smaller amounts than storage or structural lipids
Play vital roles as signaling molecules between nearby cells
Lipid-soluble vitamins (A, D, E, and K)
Arachidonic Acid Derivatives as Signaling Lipids
Enzymatic oxidation of arachidonic acid yields:
prostaglandins (inflammation and fever)
thromboxanes (formation of blood clots)
leukotrienes (smooth muscle contraction in lungs)
Vitamin D
Synthesized from Cholesterol and Regulates Calcium Metabolism
Vitamin A (Retinol)
Derived from B-carotene
Vitamins E, K, and Other Lipid Quinones
Antioxidants
Polyketides
Biologically Active Lipids with Medicinal Uses (erythromycin)
