Transcript Chp 5 Structure and Function of Macromolecules

Structure and Function of
Macromolecules
Chapter 5
Most macromolecules are polymers
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Polymer -- (Poly = many; mer = part) Large molecule
consisting of many similar subunits connected
together.
Monomer -- Subunit or building block molecule of a
polymer.
Macromolecule -- (Macro = large) Large organic
polymer.
Four classes of macromolecules in living organisms:
1. Carbohydrates.
2. Lipids.
3. Proteins.
4. Nucleic acids.
Building Polymers
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Most polymerization reactions in living organisms are
condensation reactions.
Polymerization reactions -- Chemical reactions that
link two or more small molecules to form larger
molecules.
Condensation reactions -- Monomers are covalently
linked, resulting in removal of a water molecule.
• One monomer loses a hydroxyl (OH), and the other
monomer loses a hydrogen (H).
Hydrolysis -- (Hydro = water; lysis = break) A
reaction process that breaks covalent bonds between
monomers by the addition of water molecules.
Organisms use carbohydrates for fuel and
building material
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Carbohydrates -- Organic molecules made of sugars
and their polymers.
A. Monosaccharides (“single sugar”) Simple sugars
with C, H and O in the ratio of 1:2:1.
B. Disaccharides (“two sugars”) Consists of two
monosaccharides joined by a glycosidic linkage.
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Glycosidic linkage = Covalent bond formed by a
condensation reaction between two sugar monomers.
C. Polysaccharides (“many sugars”) Polymers of a
few hundred or thousand monosaccharides.
Monosaccharides
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Are major nutrients for cells (glucose).
Store energy in their chemical bonds which is harvested by
cellular respiration.
Can be incorporated as monomers into disaccharides and
polysaccharides.
Characteristics of a sugar:
1. An -OH group is attached to each carbon except one, which
is double bonded to an oxygen (carbonyl).
Terminal carbon forms a double bond with oxygen --aldehyde
sugar or aldose (glucose).
Inside carbon bonds with oxygen -- ketone sugar or ketose
(fructose).
2. Size of the carbon skeleton varies from 3 to 7 carbons.
3. In aqueous solutions, many monosaccharides form rings.
Disaccharides
Glycosidic linkage -- Covalent bond
formed by a condensation reaction
between two sugar monomers.
 Examples of Disaccharides:
 Maltose (malt sugar) glucose + glucose.
 Lactose (milk sugar) glucose + galactose.
 Sucrose (table sugar) glucose + fructose.
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Polysaccharides
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Have two important biological
functions:
1. Energy storage (starch and
glycogen).
2. Structural support (cellulose and
chitin).
Storage Polysaccharides
Cells hydrolyze storage
polysaccharides into sugars as
needed. Two most common storage
polysaccharides are starch and
glycogen.
Starch -- Glucose polymer that is a
storage polysaccharide in plants.
 Helical glucose polymer with a 1-4
linkages (amylose and amylopectin).
 Most animals have digestive enzymes to
hydrolyze starch (amylase in saliva).
 Glycogen -- Glucose polymer that is a
storage polysaccharide in animals.
 Stored in the muscle and liver of humans
and other vertebrates.
 More branched than amylopectin.
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Polysaccharides (cont)
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Structural Polysaccharides
Include cellulose and chitin.
Cellulose -- Linear unbranched polymer of β (beta)
glucose in 1-4 linkages (-OH group on carbon one is
above the ring's plane).
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A major structural component of plant cell walls.
Starch has α (alpha) glucose configuration (-OH group on
carbon one is below the ring's plane).
Hydrogen bonds hold together parallel cellulose
molecules in bundles of microfibrils.
Cellulose cannot be digested by most organisms
because they lack an enzyme to hydrolyze the β 1-4
linkage.
Chitin -- Polymer of an amino sugar.
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Forms exoskeletons of arthropods.
In cell walls of some fungi.
Amino sugar similar to beta glucose with a nitrogencontaining group replacing the hydroxyl on carbon 2.
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Alpha and Beta Glucose
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Cellulose Cartoon
Lipids are hydrophobic molecules with
diverse functions
Insoluble in water, but will dissolve in
nonpolar solvents (e.g. ether, chloroform,
benzene).
 Important groups are:
 A. Fats -- Constructed from glycerol and
fatty acid.
 B. Phospholipids -- Glycerol, two fatty
acids, phosphate group.
 C. Steroids -- Four fused carbon rings
with various functional groups attached.
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Fats
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Glycerol (a three-carbon alcohol) + Fatty acid
(carboxylic acid).
Fatty acids composed of a carboxyl group at one
end and an attached hydrocarbon chain ("tail"),
usually 16-18 carbons.
Nonpolar C-H bonds make the tail hydrophobic
and not water soluble.
Three fatty acids can bond to one glycerol
(triglyceride).
Ester linkage -- condensation reaction linking
glycerol to fatty acids; bond formed between
hydroxyl group and carboxyl group.
Fats (cont)
SATURATED FAT
 No double bonds between carbons in fatty
acid tail.
 Carbon skeleton of fatty acid is bonded to
maximum number of hydrogens (saturated
with hydrogens).
 Usually a solid at room temperature.
 Most animal fats.
 Bacon grease, lard and butter.
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Fats (cont)
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UNSATURATED FAT
One or more double bonds between carbons in fatty acid
tail.
Tail kinks at each C=C, so molecules do not pack closely
enough to solidify at room temperature.
Usually a liquid at room temperature.
Most plant fats.
Corn, peanut and olive oils.
In many commercially prepared food products,
unsaturated fats are artificially hydrogenated to prevent
them from separating out as oil (e.g. peanut butter and
margarine).
Fats (cont)
Functions:
 Energy storage -- One gram of fat stores
twice as much energy as a gram of
polysaccharide.
 Animals store more energy with less
weight than plants which use starch.
 Cushions vital organs in mammals
(kidney).
 Insulates against heat loss (mammals
such as whales and seals).
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Phospholipids
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Differ from fat in that the third carbon of glycerol is
joined to a negatively charged phosphate group.
Hydrocarbon tails are hydrophobic.
Polar head (glycerol/phosphate) is hydrophilic.
Cluster in water as their hydrophobic tails turn away
from water (micelle).
Major constituents of cell membranes. Phospholipids
form a bilayer held together by hydrophobic
interactions among the hydrocarbon tails.
Hydrophilic heads -- point towards exterior of bilayer.
Hydrophobic tails -- point towards interior of bilayer.
Steroids
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Cholesterol, an
important steroid:
Is the precursor to
many other steroids
including vertebrate
sex hormones and
bile acids.
Is a common
component of animal
cell membranes.
Nucleic acids store and transmit hereditary
information
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Two types of nucleic acids.
1. Deoxyribonucleic Acid (DNA)
• Contains coded information that programs all cell
activity.
• Is copied and passed from one generation of cells to
another.
• In eukaryotic cells, is found primarily in the nucleus.
2. Ribonucleic Acid (RNA)
• Functions in the actual synthesis of proteins coded for
by DNA.
• Messenger RNA (mRNA) carries encoded genetic
message from the nucleus to the ribosomes in the
cytoplasm.
The flow of genetic information goes from DNA —> RNA
—> protein.
DNA strand is a polymer made up of
nucleotides
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Nucleic acid -- Polymer of nucleotides linked together by
condensation reactions.
Nucleotide -- Building block molecule of a nucleic acid
made of:
1. Pentose (5-Carbon Sugar): ribose or deoxyribose.
2. Phosphate group attached to the number 5 carbon of
the sugar.
3. Nitrogenous Base: Adenine, Guanine, Cytosine,
Thymine (DNA only), Uracil (RNA only).
Nucleotides are joined into a polymer by phosphodiester
linkages between the phosphate of one nucleotide and
the sugar of the next.
Pyrimidine -- Nitrogenous base characterized by a sixmembered ring made up of carbon and nitrogen atoms
(C, T, U).
Purine -- Nitrogenous base characterized by a fivemembered ring fused to a six-membered ring (A, G).
Inheritance is based on precise
replication of DNA
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In 1953, J. Watson and F. Crick (with help of M. Wilkins
and R. Franklin) proposed the double helix as the three
dimensional structure of DNA.
Consists of two nucleotide chains wound in a double
helix.
Sugar-phosphate backbones are on the outside of the
helix.
Nitrogenous bases are paired in the interior of the helix
and are held together by hydrogen bonds.
Base-pairing rules are that adenine (A) always pairs with
thymine (T); guanine (G) always pairs with cytosine (C).
Two strands of DNA are complimentary and thus can
serve as templates to make new complementary
strands. It is this mechanism of precise copying that
makes inheritance possible.
Most DNA molecules are long — with thousands or
millions of base pairs.
DNA structure
Replication
Proteins are the molecular tools for most
cellular functions
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Polypeptide chains -- Polymers of amino acids arranged in a
specific sequence linked by peptide bonds.
Protein -- Consists of one or more polypeptide chains folded
and coiled into specific conformations or shapes.
Functions in the cell:
1. Structural support (collagen: connective tissue, keratin: nails
and skin; elastin: skin).
2. Storage of amino acids (casein: milk).
3. Transport (hemoglobin: binds to oxygen in blood).
4. Chemical messengers (insulin: regulates blood sugar).
5. Chemical receptors (membrane proteins: neural functions).
6. Movement (actin and myosin: muscles).
7. Immune defense (antibodies).
8. Catalysts (enzymes).
Amino acids are the building blocks
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Amino acid – there are 20; most (except glycine) consist of
an asymmetric carbon covalently bonded to:
1. Hydrogen atom.
2. Carboxyl group (COOH); acidic – donates H+.
3. Amino group (NH2); basic – accepts H+.
4. R group (side chain) different in each amino acid.
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Physical and chemical properties of the protein determined
by the R groups (acid, base, polar, nonpolar).
Minerals (positive metal ions) are often important parts of R
groups.
Only left-handed isomers are synthesized naturally.
Amino Acids
Peptide Bonds
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Peptide bond -- Covalent bond formed by a
condensation reaction that links the carboxyl
group of one amino acid to the amino group of
another.
Polypeptide chain has polarity with a positive
amino group on one end (N-terminus) and a
negative carboxyl group on the other (Cterminus).
Backbone has repeating sequence -N-C-C-N-CC-.
Polypeptide chains range in length from a few
monomers to more than a thousand.
Each has a unique linear sequence of amino
acids.
A protein's function depends on its
specific conformation
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Protein conformation -- Three-dimensional
shape of a protein.
• Enables a protein to recognize and
bind specifically to another molecule
(hormone/receptor; enzyme/substrate;
antibody/antigen) – link between form and
function.
Four Levels of Protein Structure:
1. Primary structure.
2. Secondary structure.
3. Tertiary structure.
4. Quaternary structure.
Primary Structure
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Unique sequence of amino acids in a protein.
• Determined by genes.
• Slight change can affect a protein's
conformation and function (sickle-cell anemia).
Secondary Structure
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Coiling and folding of the polypeptide backbone.
Hydrogen bonding with nitrogens and oxygens
in the molecule.
Two types of secondary structure:
1. Alpha (α) Helix -- a coil with hydrogen bonding
between every fourth peptide bond; found in
fibrous proteins.
2. Beta (β) Pleated Sheet -- a sheet of parallel
chains folded into accordion pleats; hydrogen
bonding between parallel regions; found in
globular proteins.
Alpha Helix
Beta Sheet
Tertiary Structure
Bends and loops in a polypeptide due to
bonding between side chains (R groups).
 A. Hydrogen bonding between polar side
chains.
 B. Hydrophobic interactions of nonpolar
side chains.
 C. Disulfide bridges form between two
sulfhydryl groups (-SH) on the same
polypeptide.
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Quaternary Structure
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Interaction among several polypeptides
(subunits) in a single protein.
Due to same forces involved in tertiary
structures.
Examples:
Collagen has three polypeptides supercoiled into
a triple helix; gives it strength.
Hemoglobin has four subunits which are tightly
packed
Levels of Structure
Protein Denaturation
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Denaturation -- A process that alters a protein's
shape and biological activity. Proteins can be
denatured by:
Organic solvents: Hydrophobic side chains move
from the inside of the protein towards the
outside.
Chemical agents: Disrupt hydrogen bonds, ionic
bonds, and disulfide bridges.
Also: excessive heat and changes in pH or salt
concentration.
Some denatured proteins return to their native
conformation when conditions return to normal;
evidence that the amino acid sequence (primary
structure) determines conformation.
Denaturation