Transcript Chapter 1

Chapter 1
Protein
Contents
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1.
2.
3.
4.
5.
Chemical components
Molecular structures
Structure-function relationship
Physical and chemical properties
Exploration of proteins
What are proteins?
Proteins are macromolecules
composed of amino acids linked
together through peptide bonds.
Section 1
Chemical Components of
Proteins
Element components of
proteins
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major elements
C, H, O, N, S.
trace elements
P, Fe, Cu, Zn, I, …
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The average nitrogen content in
proteins is about 16%.
The protein quantity can be estimated.
protein in 100g sample = N per gram x 6.25 x 100
The basic building blocks of proteins
Amino Acids
only 20 types of amino acids
are used for protein synthesis
in biological systems.
L-α-Amino acid
L-α-Amino acid
COOH
H2N
C
¦Á
¹² ͬ²¿ ·Ö
H
R
²à Á´
A Classification of
Amino Acids
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Amino acids are grouped as
(1) non-polar, hydrophobic;
(2) polar, neutral;
(3) acidic;
(4) basic.
Special amino acids
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Gly
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Pro
Having a ring structure and imino group
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Cys
active thiol groups to form disulfide bond
optically inactive
Peptide
A peptide is a compound of
amino acids linked together
by peptide bonds.
peptide bond
A peptide bond is a covalent bond
formed between the carboxyl
group of one AA and the amino
group of its next AA with the
elimination of one H2O molecule.
Biologically active peptides
Glutathione (GSH)
As a reductant to protect nucleic
acids and proteins
Peptide hormones
Neuropeptides responsible for signal
transduction
Section 2
Molecular Structures of Proteins
Primary Structure
Secondary Structure
Tertiary Structure
Quaternary Structure
Spatial structure
Primary Structure
The primary structure of proteins is
defined as a linear sequence of amino
acids joined together by peptide
bonds.
Peptide bonds and disulfide bonds are
responsible for maintaining the
primary structure.
Secondary Structure
The secondary structure of a protein is
defined as a local spatial structure of a
certain peptide segment, that is, the
relative positions of backbone atoms
of this peptide segment.
H-bonds are responsible for stabilizing
the secondary structure.
Repeating units of Ca-C(=O)-N(-H)Ca constitute the backbone of
peptide chain.
Six atoms, Ca-C(=O)-N(-H)-Ca,
constitute a planer peptide unit.
Four common types of
secondary structure
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α-helix
β-pleated sheet
β-turn
random coil
Motif
When several local peptides of
defined secondary structures are
close enough in space, they are able
to form a particular structure--Motif.
Zinc finger
HLH (helix-loop-helix)
HTH (helix-turn-helix)
Tertiary Structure
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The tertiary structure is defined as
the three-dimensional arrangement
of all atoms of a protein.
Five types of interactions
stabilize the protein tertiary
structure.
hydrophobic interaction
ionic interaction
hydrogen bond
van der Waals interaction
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• disulfide bond
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Domain
Large polypeptides may be organized
into structurally close but functionally
independent units---Domain
Chaperon
Chaperones are large, multisubunit
proteins that promote protein
foldings
Quaternary Structure
The quaternary structure is defined
as the s p a t i a l a r r a n g e m e n t o f
multiple subunits of a protein.
These subunits are associated
through H-bonds, ionic interactions,
and hydrophobic interactions.
From primary to quaternary structure
Protein classification
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Constituents
simple protein
conjugated protein = protein +
prosthetic groups
Overall shape
Globular protein
Fibrous protein
long/short < 10
long/short > 10
Section 3 Structure-Function
Relationship of Proteins
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Relationship between primary
structure and function
Primary structure is the fundamental
to the spatial structures and
biological functions of proteins.
Example
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Proteins having similar amino
acid sequences demonstrate the
functional similarity.
The alternation of key AAs in a
protein will cause the lose of its
biological functions.
Relationship between spatial
structure and function
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A particular spatial structure of a
protein is strongly correlated with
its specific biological functions.
Example
1.The denatured protein remains its
primary structure, but no
biological function.
2. Allosteric change of hemoglobin
by O2
Section 4 Physical and Chemical
Properties of Proteins
1.
Amphoteric
isoelectric point (pI)
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The pH at which the protein has
zero net-charge is referred to as
isoelectric point (pI)
2. Colloid property
Hydration shell and electric repulsion
make proteins stable in solution.
3 Protein denaturation
renaturation, precipitation
and coagulation
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The process in which a protein loses
its native conformation under the
treatment of denaturants is
referred to as protein denaturation.
• Applications
sterilization, lyophilization
4 UV absorption
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Trp, Tyr, and Phe have aromatic
groups of resonance double bonds.
Proteins have a strong absorption
at 280nm
5 Coloring reactions
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Biuret reaction
Ninhydrin reaction
Section 5 Exploration of
Protein
Isolation and purification
• Centrifugation
• Dialysis
• Precipitation
• Chromatography
• Electrophoresis
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Protein Sequence Determination
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Edman degradation
Deduction from DNA sequence
Structure Determination
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Circular dichroism spectroscopy
X-ray crystallography
Nuclear magnetic resonance
spectroscopy
Computer simulation