Nafith Abu Tarboush
DDS, MSc, PhD
natarboush@ju.edu.jo
www.facebook.com/natarboush
Protein conformation
 Many conformations are possible for
proteins due to flexibility of amino
acids linked by peptide bonds
 At least one major conformations has
biological activity, and hence is
considered the protein’s native
conformation or the native protein
Levels of Protein Structure
 1°structure: sequence and number, from N to C
 2°structure: the ordered 3-dimensional arrangements
(conformations) in localized regions of a polypeptide
chain, backbone interactions through hydrogen
bonding;
 e. g., -helix and -pleated sheet
 3˚ structure: 3-D arrangement of all atoms
 4˚ structure: arrangement of monomer subunits with
respect to each other
Primary Structure
of Proteins
 Zigzag arrangement
 R-groups
 Determination?
 1o Sequence & 3-D conformation;
relation to functional properties
(MW& genetic mutations)
 Site-directed mutagenesis and
structure function relationship
Shape-Determining
Interactions in Proteins
 Is it ordered or spaghetti?
 Hydrogen, ionic, covalent, & hydrophobic
Secondary Structure of Proteins
 What is the secondary structure of
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proteins? “folding of the backbone”
What are the bonds that have a free
rotation? What is the implication?
These angles repeat themselves in
regular secondary structures
Two main kinds: α-helix and βpleated sheet
They are periodic; their features repeat
at regular intervals
Stability of secondary structure
The α-helix
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H-bonds are parallel to the helix axis, same segment
C=O binds N—H four residues away
linear arrangement of H-bonds (maximum strength and stability)
Turns occur every 3.6 residues, right handed, clockwise
The pitch (linear distance between corresponding points on successive
turns) is 5.4 Å
 Proteins have varying amounts of α-helical structures
 What factors affect the helix (specific amino acids, electrostatic
repulsion, steric repulsion)
The β-sheets
 Backbone is almost completely extended
 R groups extending above and below the sheet
 H-bonds are intra-chain or inter-chain bonds
 Perpendicular to the direction of the protein chain
 Parallel vs. anti-parallel
 Zigzag structure
Others regular ones
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Why do we have them?
Loops vs. turns
Also known as β-bends
Involve 4 amino acids
(H-bond: C=O of 1 & N–
H of 4)
 A motif is a repetitive
super-secondary
structure. Regardless of
function
 Domains; protein
conformations with
similar functions (leucine
zipper)
-Helices and -Sheets
 Supersecondary structures: a
combination of - and -sections
  unit: (parallel)
  unit: (helix-turn-helix),
anti-parallel
 -meander: an anti-parallel
sheet formed by a series of
tight reverse turns connecting
stretches of a polypeptide
chain
 Greek key: a repetitive supersecondary structure formed
when an anti-parallel sheet
doubles back on itself
 -barrel: created when sheets are extensive enough to
fold back on themselves
Fibrous Proteins
 Contain polypeptide chains
organized approximately
parallel along a single axis:
 Consist of long fibers or
large sheets
 Mechanically strong
 Insoluble
 play an important
structural role
 Examples are
 Keratin
 Collagen
 fibroin
Fibroin, β-sheets,
α-keratins,
alternating glycine bundles of αand alanine
helices
Globular Proteins
 Folded to, a more or less, spherical shape
 Soluble
 Polar vs. non-polar, exterior vs. interior
 Most of them have substantial sections of -helix and -
sheet
3˚ Structure
 The 3-dimensional arrangement of all the atoms in the
molecule
 Simple vs. conjugated
 Backbone hydrogen bonding, side chains hydrogen
bonding, hydrophobic interactions, electrostatic attraction,
electrostatic repulsion, metal coordination
 Not every protein have all kinds of interactions (myoglobin
& hemoglobin; no disulfides) (trypsin & chymotrypsin, no
metal complexes)
 Interactions between side chains also plays a role
Forces That Stabilize Protein Structure
3°& 4°Structure
 Tertiary (3°) structure: the arrangement in space of all
atoms in a polypeptide chain
 It is not always possible to draw a clear distinction
between 2°and 3°structure
 Quaternary (4°) structure: the association of
polypeptide chains into aggregations (dimers, trimers,
tetramers,…etc). Non-covalent interactions. Simple or
conjugated
Determination of 3°Structure
 X-ray crystallography
 uses a perfect crystal; that is, one in which all individual
protein molecules have the same 3D structure and
orientation
 exposure to a beam of x-rays gives a series diffraction
patterns
 information on molecular coordinates is extracted by a
mathematical analysis called a Fourier series
 2-D Nuclear magnetic resonance
 can be done on protein samples in aqueous solution
X-Ray and NMR Data
• High resolution method to determine 3˚
structure of proteins (from crystal)
• Determines solution structure
• Diffraction pattern produced by
electrons scattering X-rays
• Structural info. Gained from
determining distances between nuclei
that aid in structure determination
• Series of patterns taken at different
angles gives structural information
• Results are independent of X-ray
crystallography
Complex Protein Structures
 Covalent conjugation to
carbohydrates (glyco-proteins): posttranslational, function, N-linked (-N
of Asn) & O-linked (-OH of Ser or
Thr) & occasionally to the –OH of
hydroxy-lysine
 Lipoproteins: Non-covalent, store &
transport lipids and cholesterol
 Extremely important: ex. Erythrocytes
& blood groups (glycoshpingolipids)
 Phosphoproteins: contain phosphate
groups esterified to Ser, Thr, or Tyr.;
the phosphate group usually regulates
protein function
Chemical Properties of Proteins
1. Protein Hydrolysis
 The reverse of protein synthesis
 Digestion of proteins is hydrolyzing peptide bonds
 Takes place in the stomach and small intestine
2. Protein Denaturation
 How the protein preserve its shape?
 What is denaturation? It affects physical, chemical, and biological
properties, such as enzymes
 Solubility decreased, ex. eggs
 Heat (≈≥50 οC)
 Mechanical agitation
 Detergents
 Organic compounds: acetone, ethanol, bacterial proteins
 pH change: disrupt salt bridges, milk turns sour because it has
become acidic
 Most denaturation is
irreversible
Protein Folding & prediction
 The integration of biochemistry and computing has led to
bioinformatics
 Predicting the 3˚ structure (limitations)
 First step is to search for sequence homology
Hydrophobic Interactions & Chaperons
 Hydrophobic interactions are
spontaneous
 Chaperones aid in correct &
timely folding of many proteins
 hsp70 were the first chaperone
proteins discovered
 Chaperones exist in organisms
from prokaryotes to humans
Structure & diseases (prion, Alzheimer)
 Mad-cow disease (bovine spongiform
encephalopathy), and human
spongiform encephalopathy
(Creutzfeldt-Jakob disease)
 The cause is a small (28-kDa) protein
called a prion (Met129)
 Alzheimer's Disease: buildups of
plaques and fibers of hard insoluble
material
 Amyloid protein mis-folded and
broken down (≈40 a.a, Ab protein) and
aggregates