Download Tertiary Structure

Supersecondary structures
Supersecondary structures
motifs
• motifs or folds, are particularly stable
arrangements of several elements of the
secondary structure.
• Supersecondary structures are usually
produced by packing side chains from
adjacent secondary structural elements close
to each other.
Rules for secondary structure.
• Hydrophobic side groups must be buried inside the
folds, therefore, layers must be created (b-a-b; aa).
• a-helix and b-sheet, if occur together, are found in
different structural layers.
• Adjacent polypeptide segments are stacked together.
• Connections between secondary structures do not
form knots.
• The b-sheet is the most stable.
Motif
• Secondary structure composition,
e.g. all a, all b, segregated a+b,
mixed a/b
• Motif = small, specific combinations
of secondary structure elements,
e.g. b-a-b loop
Supersecodary Structures (Motifs)
α-amylase inhibitor
Serum albumin
Ferritin
Pilin
Immunoglobulin
Tertiary protein structure
• Secondary structures fold and pack together to form
tertiary structure
– Usually globular shape
• Tertiary structure stabilised by bonds between R groups
(i.e. side chains)
• Intracellular protein tertiary structures mostly held
together by weak forces. Extracellular tertiary
structures stabilised by disulfide (covalent) bonds.
Three-dimensional structure of
proteins
• Three-dimensional structure of proteins is
determined by it amino acid sequence.
• Function of the protein depends on its structure.
• Each protein has a unique or nearly unique structure.
• Non-covalent interactions are the most important
forces stabilizing the three dimensional structure of
the protein.
• There common structural patterns in vast protein
architecture.
• Native structure is the natural 3-dimensional structure
of a protein.
Domains
• Domains are the fundamental functional and threedimensional structural units of polypeptides
• Polypeptide chains that are greater than 200 amino
acids in length generally consist of two or more
domains
• The core of a domain is built from combinations of
supersecondary structural elements (motifs)
• Folding of the peptide chain within a domain usually
occurs independently of folding in other domains
• Therefore, each domain has the characteristics of a
small, compact globular protein that is structurally
independent of the other domains in the polypeptide
chain.
Interactions stabilizing tertiary structure :
1.Disulfide bonds: These strong, covalent bonds help
stabilize the structure of proteins, and prevent them
from becoming denatured in the extracellular
environment.
2.Hydrophobic interactions
3.Hydrogen bonds
4. Ionic interactions
Tertiary structure - disulfide bond
– Covalent bond
between sulfur
atoms on two
cysteine amino
acids
Tertiary structure - H bond
Hydrogen bond
• H bonds weak
allowing to be
broken and
reformed easily
– Allows structural
change
• produces ‘functional’
molecules
• Ions on R groups
form salt bridges
through ionic
bonds
Tertiary structure - hydrophobic forces
• Close attraction of non-polar
R groups through dispersion
forces
• Very weak but collective
interactions over large area
stabilise structure
• Repel polar and charged
molecules/particles
Tertiary Structure
Tertiary Structure
• The interactions
of the R groups
give a protein its
specific threedimensional
tertiary structure.
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Tertiary Structure
• non-linear
• 3 dimensional
• global but restricted to the amino
acid polymer
• formed and stabilized by
hydrogen bonding, covalent (e.g.
disulfide) bonding, hydrophobic
packing toward core and
hydrophilic exposure to solvent
• A globular amino acid polymer
folded and compacted is
somewhat functional (catalytic)
and energetically favorable 
interaction!
Quaternary Structure of Proteins

Many proteins consist of a single polypeptide chain,
and are defined as monomeric proteins.
 others may consist of two or more polypeptide
chains that may be structurally identical or totally
unrelated.
 The arrangement of these polypeptide subunits is
called the quaternary structure of the protein.
Quaternary Structure of Proteins
• The biological function of some molecules is
determined by multiple polypeptide chains –
multimeric proteins
• Two kinds of quaternary structures: both are
multi-subunit proteins.
• Homotypic: association between identical polypeptide
chains.
• Heterotypic: interactions between subunits of very different
structures.
• The interactions within multimers is the same as
that found in tertiary and secondary structures
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Quarternary Structure
•
•
This structure for proteins that have more than one
polypeptide chains.
•
It is the arrangement of protein subunits (protein that
has more than one polypeptide chain) in three
dimensional complex.
•
The interaction between subunits are stabilized by:
•
hydrogen bonds
•
electrostatic bonds
•
hydrophobic bonds
e.g. of proteins having quaternary
structure:
•
Lactate dehydrogenase enzyme: (4 subunits),
hemoglobin (4 subunits)
Summary of Structural Levels
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