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PROTEIN
STRUCTURE AND FOLDING
© 2012 Pearson Education, Inc.
Proteins are made from amino acids
 Proteins are
– involved in nearly every
function in your body and
– very diverse,
– 50,000 - 100,000 proteins, each
with a specific structure and
function, in the human body.
 Proteins are composed of
differing arrangements of a
common set of just 20
amino acids.
© 2012 Pearson Education, Inc.
Amino
group
Carboxyl
group
Proteins are made from amino acids
 Amino acids are classified as either
– hydrophobic or hydrophilic.
Hydrophobic
Leucine (Leu)
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Hydrophilic
Serine (Ser)
Aspartic acid (Asp)
Figure 3.11C_s2
Amino acids are joined together by a dehydration reaction
Carboxyl
group
Amino acid
Amino
group
Amino acid
Peptide
bond
Dehydration
reaction
Dipeptide
A protein’s specific shape determines its function
 Proteins are distinguished by:
– The combination of sequence of amino acids
– The linear sequence of amino acids in a protein is called the PRIMARY
STRUCTURE of the protein
– 3D structure
 The amino acid sequence causes the protein to assume a particular
shape.
 The shape of a protein determines its specific function.
 If a protein’s shape is altered, it can no longer function.
 In the process of denaturation, a polypeptide chain
– unravels,
– loses its shape, and
– loses its function.
© 2012 Pearson Education, Inc.
Primary structure
Amino
acid
DNA Sequence (gene) determines the amino acid sequence
of the protein
Gene
DNA
Nucleic acids
Transcription
RNA
Translation
Amino
acid
Protein
Four Levels of Protein Structure
Primary structure
determines final
3D structure!!!
Secondary structure =
Folding of protein
Backbone;
H-bond interactions
Amino
acids
Amino acids
Hydrogen
bond
Beta pleated
sheet
Alpha helix
Tertiary structure =
3D shape protein adopts
Involves interactions between R
Groups and water
Transthyretin
polypeptide
Quaternary structure =
Interaction of multiple proteins
Thru surface R groups
(not all proteins have quaternary
structure)
Transthyretin, with four
identical polypeptides
3.13 A protein’s shape depends on four levels of
structure
 The primary structure of a protein is its unique
amino acid sequence.
– The correct amino acid sequence is determined by the
cell’s genetic information.
– The slightest change in this sequence may affect the
protein’s ability to function.
© 2012 Pearson Education, Inc.
3.13 A protein’s shape depends on four levels of
structure
 Protein secondary structure results from coiling
or folding of the protein into:.
– alpha helix or beta-pleated sheet
– Secondary structure is maintained by H-bonds between
N-H and C=O groups along protein’s backbone.
© 2012 Pearson Education, Inc.
3.13 A protein’s shape depends on four levels of
structure
 The overall three-dimensional shape of a
polypeptide is called its tertiary structure.
– Tertiary structure generally results from interactions
between the R groups of the various amino acids and R
groups with water
– Hydrophobic R groups pushed inside protein
– Hydrophilic generally on protein surface
© 2012 Pearson Education, Inc.
3.13 A protein’s shape depends on four levels of
structure
 Two or more protein chains (subunits) associate
Protein chain
providing quaternary structure.
– Examples:
– Collagen has three subunits
– Hemoglobin has four subunits
Collagen
© 2012 Pearson Education, Inc.