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AP Biology
Ch. 5 Macromolecules:
Lipids, Proteins, and Nucleic Acids
Lipids
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Fats store large amounts of energy.
Triacylglycerols (triglycerides) are
constructed by the joining of a glycerol
molecule to three fatty acids.
Formed by dehydration synthesis
reactions.
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Saturated vs. Unsaturated Fats
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Saturated fatty acids have the maximum
number of hydrogen atoms. These tend to
be solid at room temp., such as butter and
meat fat.
Unsaturated fatty acids have one or more
double bonds in their hydrocarbon chains.
These tend to be liquid at room temp., such
as olive oil and canola oil.
Unsaturated fats are generally considered to
be healthier when used in moderation.
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Phospholipids
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Where fats have a third fatty acid linked to
glycerol, phospholipids have a negatively
charged phosphate group.
This makes the “head” of the phospholipid
hydrophilic; the hydrocarbon “tails” are
hydrophobic.
Phospholipids are the major components of
cell membranes. In a cell membrane, the
hydrophobic tails are orientated inward, while
the hydrophilic head face outward.
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Steroids
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Steroids include
cholesterol and
certain hormones,
such as testosterone
and estrogen.
Steroids have a basic
structure of four
fused rings of carbon
atoms.
Proteins
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Proteins are macromolecules of
polypeptide chains.
Polypeptide are polymers of amino
acids arranged in a specific linear
sequence linked by peptide bonds.
Proteins are one or more polypeptide
chains folded and coiled into specific
conformations (3-D shapes).
Proteins, cont.
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Proteins make up 50% of cellular dry
weight.
Each type has a unique 3-D shape.
Vary in structure and function, but all
are made from the same 20 amino
acids.
Proteins: Cellular function
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Structural support
Storage of amino acids
Transport, such as hemoglobin
Signaling, chemical messengers
Cellular response to chemical stimuli
(receptor proteins)
Movement (contractile proteins)
Defense (antibodies)
Catalysis of biochemical reactions (enzymes)
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Amino Acids: Building blocks of proteins
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Amino acids consist of an asymmetric
carbon bonded to a hydrogen atom,
carboxyl group, amino group, and a
side chain (R-group) specific for each
amino acid.
Physical and chemical properties of the
side chain determine the uniqueness of
each amino acid.
Groups of Amino Acids
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Amino acids are put into groups based on the
side chains the molecule contains, and its
properties.
Nonpolar, hydrophobic side groups make
amino acids less soluble in water.
Polar, hydrophilic side groups make amino
acids soluble in water. These can be
uncharged polar side groups, or charged
(acidic or basic) groups.
NONPOLAR AMINO ACIDS
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POLAR AMINO ACIDS
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Peptide bonds
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Peptide bonds are covalent bonds formed by
a condensation reaction that links the
carboxyl group of one amino acid to the
amino group of another.
Has polarity with an amino group one end (Nterminus) and a carboxyl group on the other
(C-terminus).
Has a backbone of repeating N-C-C-N-C-C
Polypeptide chains range in length from a few
monomers to more than a thousand, and a
unique linear sequence of amino acids.
Protein conformation: 3-D structure
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Each protein molecule has a unique
native conformation (shape of the
protein under normal biological
conditions) that reflect its function.
Enables a protein to recognize and bind
specifically to another molecule
(enzyme/substrate, hormone/receptor,
antibody/antigen)
3-D structure, cont.
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A protein’s shape is produced when a
newly formed polypeptide chain coils
and folds spontaneously, mostly in
response to hydrophobic interactions.
Is stabilized by chemical bonds and
weak forces between neighboring
regions of the folded protein.
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Overview of Protein Structure
Four levels of protein structure
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Primary- amino acid sequence
Secondary- regular repeated coiling and
folding of a protein’s amino acid chain
Tertiary- 3-dimensional shape of a protein
due to bonding between side chains
Quaternary- results from interactions
between several polypeptide chain (protein
has subunits, like hemoglobin and collagen)
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Primary
Structure
Secondary Structure
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Coiling and folding of the polypeptide backbone
Stabilized by hydrogen bonds between peptide
linkages in the amino acid chain
Two major types:
Alpha helix- helical coil stabilized by hydrogen
bonds between every 4th peptide bond. Found in
fibrous proteins such as collagen.
Beta pleated sheet- antiparallel chains fold into
accordian-like pleats, held by hydrogen bonds. Form
the dense core of globular proteins (lysozyme) and
some fibrous proteins (silk)
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Secondary
Structure:
Alpha helix or
Beta-pleated sheet
Spider silk: a structural protein
Tertiary structure
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3-D shape of a protein due to bonding between side
chains, and interactions with the aqueous
environment.
Protein shape is stabilized by:
Weak interactions such as hydrogen bonding
between side chains, ionic bonds between charged
side chains, and hydrophobic interactions between
nonpolar side chains
Covalent linkages such as disulfide bridges
between two cysteine monomers brought together by
protein folding
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Quaternary structure
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Occurs in proteins made up of two or more
polypeptides, resulting from the interactions
between them.
Collagen is a fibrous protein with three helical
polypeptides supercoiled into a triple helix;
makes it very strong connective tissue in
animals
Hemoglobin is a globular protein with four
subunits that fit together.
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Protein conformation
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Protein’s 3-D shape is a consequence of
the interactions responsible for
secondary and tertiary structure.
Protein conformation is influenced by
the physical and chemical environment
If a protein’s environment is changed, it
may become denatured and lose its
shape.
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Denaturation
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2)
3)
Process that changes a protein’s structure,
therefore affecting its biological function.
Can be caused by:
Heat- disrupts weak interactions
Chemical agents that disrupt H-bonds, ionic
bonds, and disulfide bridges (pH for
example)
Transfer to an organic solvent causes
hydrophobic chains to move toward the
outside, while the hydrophilic side chains
turn toward the interior.
Protein folding
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3-D shape is hard to predict from amino acid
sequence alone.
Protein’s native conformation may alternate
between several shapes with folding
occurring in stages.
Biochemists can now track a protein as it
goes through the folding process.
Chaperone proteins temporarily brace a
folding protein.
Nucleic Acids: Informational Polymers
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Nucleic acids store and transmit
hereditary information.
DNA stores information for the
synthesis of specific proteins.
RNA, specifically mRNA, carries this
genetic information to the protein
synthesizing machinery (ribosomes).
Nucleic acids, cont.
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Nucleic acids are polymers of nucleotides.
Each nucleotide monomer consists of a
pentose (5-C sugar) covalently bonded to a
phosphate group and to one of four
nitrogenous bases (A,G,C, T or U).
In making a chain, nucleotides join to form a
sugar-phosphate backbone from which the
nitrogenous bases project.
The sequence of bases along a gene specifies
the amino acid sequence of a particular
protein.
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DNA: Deoxyribonucleic acid
DNA is a helical, double-stranded
macromolecule with bases projecting into the
interior of the molecule.
DNA has the pentose, deoxyribose. Adenine
hydrogen bonds to thymine, and cytosine to
guanine (Chargaff’s rule).
One DNA strand serves a template for a new
strand.
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X-ray evidence of DNA
structure
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In the early 1950’s, Rosalind Franklin
studied DNA using X-ray diffraction.
The patterns in her pictures showed the
DNA formed a coil shape (helix).
Her studies indicated that there were
two strands, and that the nucleotides
were toward the center of the molecule.
Franklin’s X-ray diffraction of DNA
Watson and Crick
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James Watson and Frances Crick were
working on the structure of DNA in the 1950’s
also.
Using information from Chargaff, Franklin,
and other scientists, they put together a 3-D
model of DNA.
Their model was a double helix, with Hbonded nitrogenous bases holding the
strands together.
The won the Nobel Prize for their work.
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Ribonucleic Acid (RNA)
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RNA is a single-stranded
macromolecule with the 5-C sugar
ribose.
In RNA, adenine binds to uracil instead
of thymine, and guanine binds to
cytosine.
RNA uses the information from DNA to
assemble protein.
Types of RNA
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Messenger RNA (mRNA)-carries messages
from DNA for assembling amino acids into
proteins (made during transcription).
Ribosomal RNA (rRNA)-Proteins and rRNA
make up ribosomes, the site of protein
synthesis.
Transfer RNA (tRNA)- transfers each amino
acid to the ribosome as specified by codes in
the mRNA.
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