Download Lecture - Ch 25-7

Chapters 25-7
Biomolecules
CHE2202, Chapter 25-7
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Importance of Carbohydrates
• Widely distributed in nature
• Key intermediates of metabolism (sugars)
• Found in the structural components of
plants (cellulose)
• Important in manufacturing materials of
industrial products: Paper, lumber, fibers
• Key component in food sources
– Sugars, flour, vegetable fiber
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Chemical Formula and Name
• Term carbohydrate comes from the
historical belief that glucose was a hydrate
of carbon
– Carbohydrate generally refers to
polyhydroxylated aldehydes and ketones
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Classification of Carbohydrates
• Simple sugars(monosaccharides) cannot be
converted into smaller sugars by hydrolysis
• Complex carbohydrates consist of two or
more simple sugars connected by acetal bonds
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Naturally Occurring D Sugars
• D sugars occur naturally and have the
hydroxyl group at the bottom of the chirality
center pointing to the right
• L sugars are the mirror image of
corresponding D sugars and have the
opposite configuration from the D sugar at
all chirality centers
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Anomers
• Anomers are produced when an openchain monosacchride cyclizes to a
pyranose form
– Cyclization of the open-chain monosacchride
results in generation of a new chirality center
at the former carbonyl atom
– The hemiacetal carbon is called the anomeric
center
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Glucose in its Cyclic Pyranose
Forms
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Glycoprotein Formation
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The Eight Essential
Monosaccharides
• Cells need eight monosaccharides for
proper functioning
– It is more energetically efficient to obtain these
from environment than through biosynthesis
• They include L-fucose, D-galactose, Dglucose, D-mannose, N-acetyl-Dglucosamine, N-acetyl-D-galactosamine, Dxylose, and N-acetyl-D-neuraminic acid
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The Eight Essential
Monosaccharides
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Maltose and Cellobiose
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Lactose
• Disaccharide that occurs naturally in milk
• Contains two different monosaccharides
joined by a -glycosidic bond
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Sucrose
• Also known as table sugar
• Yields 1 equivalent of glucose and 1
equivalent of fructose on hydrolysis
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Polysaccharides and Their
Synthesis
• Polysacchrides are complex carbohydrates
in which many simple sugars are linked
• Cellulose and starch are the two most
widely occurring polysaccharides
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Cellulose
• Comprises thousands of D-glucopyranosyl
1→4--glucopyranosides as in cellobiose
• Cellulose molecules form a large aggregate
structures held together by hydrogen bonds
• Main component of wood and plant fiber
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Starch and Glycogen
– Starch comprises monosaccharide units linked by
1→4--glycoside bonds
• Digested into glucose
• Components of starch
– Amylose, which comprises 20% of starch
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Starch and Glycogen
– Amylopectin, which comprises 80% of starch
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Glycogen
• Polysaccharide that serves the same
energy storage function in animals that
starch serves in plants
• Highly branched and larger than
amylopectin-up to 100,000 glucose units
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Biomolecules
• Amino acids contain a basic amino group
and an acidic carboxyl group
– They are joined as amides between the –NH2
of one amino acid and the –CO2H to the next
amino acid
• Chains with fewer than 50 units are called
peptides
• A protein comprises of large chains that
have structural or catalytic functions in
biology
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Structures of Amino Acids
• Amino acids occur primarily in the form of a
dipolar ion, or zwitterion, in an aqueous
solution
• Since amino acid zwitterions are internal salts,
most properties are associated with salts
• Amino acids are amphiprotic
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Common Amino Acids
• 20 amino acids form amides in proteins
• All are -amino acids - Amino and carboxyl
are connected to the same C
• They differ by the other substituent attached
to the  carbon, called the side chain
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The 20 Common Amino Acids in
Proteins
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The 20 Common Amino Acids in
Proteins
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The 20 Common Amino Acids in
Proteins
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The 20 Common Amino Acids in
Proteins
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Chirality of Amino Acids
• The  carbons of the amino acids are
centers of chirality
– Glycine is achiral
• The stereochemical attributes of naturally
occurring -amino acids are similar to that
of L sugars
– -amino acids are also called L amino acids
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Classification Based on Side
Chains
• Common amino acids can be classified as:
– Neutral - Fifteen amino acids
– Acidic - Two amino acids
• Aspartic and glutamic acid
– Basic - Three amino acids
• Lysine, arginine, and histidine
• Cysteine and tyrosine possess weakly acidic
chains that can be protonated in a basic
solution
– Classified as neutral amino acids
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Essential Amino Acids
• Humans can synthesize only 11 of the 20
amino acids
– Called non-essential amino acids
• Rest are obtained from food
– Found in plants and microorganisms
– Called essential amino acids
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Isoelectric Point (pI) of Amino Acids
• It is the pH at which the amino acid is
balanced between anionic and cationic forms
– Exists primarily as the neutral, dipolar
zwitterion
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Peptides and Proteins
• Proteins and peptides are amino acid
polymers in which the individual amino acid
units, called residues, are linked together by
amide bonds or peptide bonds
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Peptides and Proteins
• The long repetitive sequence of –N–CH–CO–
atoms that make up a continuous chain is
called the protein’s backbone
• Peptides are always written with the Nterminal amino acid (the one with the free –
NH2 group) on the left and the C-terminal
amino acid (the one with the free –CO2H
group) on the right
• Alanylserine is abbreviated Ala-Ser or A-S,
and serylalanine is abbreviated Ser-Ala or SA
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Peptide Linkages
• Amino acids in peptides are linked together
by amide bonds
– Amide nitrogen interacts with the carbonyl group,
causing delocalization of its unshared electron
pair
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Peptide Linkages
• Disulfide linkages in peptides occur
between two cysteine residues
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Protein Structure
• Classified as fibrous or globular
– Fibrous proteins comprise polypeptide
chains arranged side by side in long filaments
• Tough and insoluble in water
– Globular proteins are coiled into compact,
roughly spherical shapes
• Soluble in water and are capable of motion within
the cells
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Protein Structure
• The primary structure of a protein is simply
the amino acid sequence.
• The secondary structure of a protein
describes how segments of the peptide
backbone orient into a regular pattern.
• The tertiary structure describes how the
entire protein molecule coils into an overall
three-dimensional shape.
• The quaternary structure describes how
different protein molecules come together to
yield large aggregate structures
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-Helix
• Right-handed coil of the protein backbone
• Each spiral contains 3.6 amino acid residues
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-Pleated Sheet
• -pleated sheet contains a fully extended
peptide chain
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Tertiary Proteins
• Structure of tertiary proteins is influenced
by
– Hydrophilic interactions of the polar side
chains
– Hydrophobic interactions of the nonpolar side
chains
– Formation of disulphide bridges between
cysteine residues
– Formation of hydrogen bonds between
nearby amino acid residues
– Salt bridges
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Denaturation of Proteins
• The tertiary structure of a globular protein is
the result of weak intramolecular attractions
that can be disrupted by a change in
temperature or pH, causing the protein to
become denatured
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Enzymes and Coenzymes
• An enzyme is a protein that acts as a
catalyst for a biological reaction.
– Increases the rate of reaction by decreasing
the activation energy
• Most enzymes are specific for substrates
while enzymes involved in digestion, such
as papain, attack many substrates
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Enzymes and Coenzymes
• Factors that contribute to the accelerating
action of enzymes:
– Ability to adjust their shape
– Wrapping action
– Ability to lower the rate-limiting transition state
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Energy Diagrams for Uncatalyzed and
Enzyme-Catalyzed Processes
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Classification of Enzymes
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Enzymes and Coenzymes
• Most enzymes contain a non-protein
component called a cofactor
– It is either an inorganic ion or a small organic
molecule called a coenzyme
– Coenzymes are reactants that undergo a
chemical change during the reaction and
require an additional step to revert to their
initial state
• Derived from vitamins
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Lipids
• Natural molecules that can be preferentially
extracted using nonpolar organic solvents
• Includes fats, oils, waxes, vitamins and
hormones, some components of membrane
• Classified based on hydrolysis
– Fats and waxes
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Waxes, Fats, and Oils
• Waxes contain various esters formed from
long-chain (C16-C36) carboxylic acids and
long-chain alcohols (C24-C36)
• Triacontyl hexadecanoate is found in
beeswax
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Triacylglycerol
• Triesters of glycerol with three long-chain
carboxylic acids called fatty acids
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Structures of Some Common
Fatty Acids
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Fatty Acids
• When hydrolysed with NaOH, a fat or oil
yields glycerol and three fatty acids
– Fatty acids contain an even number of carbon
atoms and are unbranched
• Of the 100 different known fatty acids, 40
are common
– Palmitic acid (C16) and stearic acid (C18) are
the most common saturated fatty acids
– Oleic (C18) and linoleic (C18) acids are the
most common unsaturated fatty acids CHE2202, Chapter 25-7
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Fatty Acids
• Monosaturated fatty acids have one double
bond
– Oleic acid
• Polyunsaturated fatty acids have more than
one double bond
– Linoleic, linolenic, and arachinodic acid
• Humans require linoleic and linolenic acid
– Deprivation leads to stunted growth and skin
lesions in infants
– Linolenic acid has shown to lower blood
tryglyceride levels
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Unsaturated and Polyunsaturated
Fatty Acids
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Double Bonds in Fatty Acids
• Melting points of unsaturated fatty acids are
generally lower than those of saturated fatty
acids
– Number of double bonds influences the melting
point of oil
• Catalytic hydrogenation reduces the number
of double bonds
– Reaction is carried at a high temperature using a
nickel catalyst
– Remaining double bonds undergo cis-trans
isomerization, resulting in trans unsaturated fatty
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acids
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Soap
• Mixture of sodium or potassium salts of longchain fatty acids produced by alkaline
hydrolysis (saponification) of animal fat with
alkali
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Cleansing Action of Soap
• Carboxylate end of the long-chain molecule is
ionic and therefore is hydrophilic
• Hydrocarbon tail is nonpolar and dissolves in
grease and oil
• Soaps enable grease to be dissolved into
water
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Phospholipids
• Phospholipids are diesters of H3PO4,
phosphoric acid
• Classified into glycerophospholipids and
sphingomyelins
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Glycerophospholipids
• Based on phosphatidic acid, which contains a
glycerol backbone linked by ester bonds to
two fatty acids and phosphoric acid
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Phosphoglyceride Membranes
• Phosphoglycerides are the major lipid
component of cell membranes
• Their nonpolar tails aggregate in the center of
a lipid bilayer
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Steroids
• Molecules derived from the triterpenoid
lanosterol
• Has four fused rings A, B, C, and D,
beginning at the lower left
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Steroids
• Two cyclohexane rings can be joined in either
a cis or trans manner
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Steroid Conformations
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Male Sex Hormones
• Testosterone and androsterone are the two
most important male sex hormones, or
androgens
• Have a significant influence on the development
of secondary sex characteristics and muscle
growth
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Female Sex Hormones
• Estrone and estradiol are two most important
female sex hormones, or estrogens
• Progesterone is the most important progestin; it
that functions during pregnancy
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