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Proteins
AP Biology
Proteins
Multipurpose
molecules
AP Biology
2006-2007
Proteins
 Most structurally & functionally diverse

group of biomolecules
Function:

involved in almost everything
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enzymes (pepsin, polymerase, etc.)
structure (keratin, collagen)
carriers & transport (membrane channels)
receptors & binding (defense: antibodies)
contraction (actin & myosin)
signaling (hormones: insulin)
storage (bean seed proteins)
Proteins
 Structure:

monomer = amino acids
 20 different amino acids

polymer = polypeptide
 protein can be one or more polypeptide
chains folded & bonded together
 large & complex molecules
 complex 3-D shape
hemoglobin
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Rubisco
growth
hormones
Amino acids
 Structure:
central carbon ( α carbon)
 amino group
 carboxyl group (acid)
 R group (side chain)

 variable group
 confers unique
chemical properties
of the amino acid
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H O
H
| ||
—C— C—OH
—N—
|
H
R
Nonpolar amino acids
 nonpolar & hydrophobic
Why are these nonpolar & hydrophobic?
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Polar amino acids
 polar or charged & hydrophilic
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Why are these polar & hydrophilic?
Ionizing in cellular waters
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H+ donors
Ionizing in cellular waters
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H+ acceptors
Sulfur containing amino acids
 Form disulfide bridges

cross links between sulfur atoms in cysteine
H-S – S-H
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Building proteins
 Peptide bonds
linking NH2 of one amino acid to
COOH of another
 C–N bond

dehydration synthesis
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peptide
bond
Building proteins
 Polypeptide chains
N-terminus = NH2 end
 C-terminus = COOH end
 repeated sequence (N-C-C) is the
polypeptide backbone

 can only grow in one direction
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Protein structure & function
 Function depends on structure

3-D structure
 twisted, folded, coiled into unique shape
pepsin
hemoglobin
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collagen
Primary (1°) structure
 Order of amino acids in chain
amino acid sequence
determined by gene (DNA)
 slight change in amino acid
sequence can affect protein’s
structure & it’s function

 even just one amino acid change
can make all the difference!
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lysozyme: enzyme
in tears & mucus
that kills bacteria
Sickle cell anemia
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Secondary (2°) structure
 “Local folding”

folding along short
sections of
polypeptide
backbone
 interaction between
adjacent amino
acids
 H bonds between
R groups
 -helix
 -pleated sheet
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Secondary (2°) structure
“Let’s
go to the video tape!”
AP Biology
(play movie here)
Tertiary (3°) structure
 “Whole molecule
folding”

determined by
interactions
between R groups
 hydrophobic
interactions
 effect of water
in cell
 anchored by
disulfide bridges
(H & ionic bonds)
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Quaternary (4°) structure
 More than one polypeptide chain
joined together

only then is it a functional protein
 hydrophobic interactions
collagen =
skin & tendons
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hemoglobin
Protein structure (review)
R groups
hydrophobic interactions,
disulfide bridges
3°
multiple
polypeptides
hydrophobic
interactions
1°
aa sequence
peptide bonds
determined
by DNA
AP Biology
2°
R groups
H bonds
4°
Denature a protein
 Unfolding a protein

disrupt 3° structure
 pH

salt

temperature
unravels or denatures protein
 disrupts H bonds, ionic bonds &
disulfide bridges


destroys functionality
 Some proteins can
return to their
functional shape
after denaturation,
many cannot
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Chaperonin proteins
 Guide protein folding


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provide shelter for folding polypeptides
keep the new protein segregated from
cytoplasmic influences
Protein models
 Protein structure visualized by
X-ray crystallography
 extrapolating from amino acid sequence
 computer modelling

lysozyme
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