Transcript supersecondar, tertiary and quaternary structure

+
Super secondary
structures
+
Super secondary structures
motifs

motifs or folds, are particularly stable arrangements of
several elements of the secondary structure.

Super secondary 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; a-a).
 a-helix
and b-sheet, if occur together, are
found in different structural layers.
 Adjacent
polypeptide segments are
stacked together.
 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
+ Super secondary Structures (Motifs)
+
+
Tertiary protein structure

Secondary structures fold and pack together to form tertiary
structure

Usually globular shape

Tertiary structure stabilized by bonds between R groups (i.e.
side chains)

Intracellular protein tertiary structures mostly held together by weak
forces. Extracellular tertiary structures stabilized by disulfide
(covalent) bonds.
+
Three-dimensional structure of
proteins
 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.
+
+
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
H
Hydrogen
bond
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 stabilize structure
 Repel
polar and charged
molecules/particles
Tertiary Structure
+
19
Tertiary Structure
 The
interactions
of the R groups
give a protein its
specific threedimensional
tertiary
structure.
+
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. (Dimeric)

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.
23
The interactions within multi subunits are the same as that found in
tertiary and secondary structures
+
Quaternary 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:
•
Hemoglobin (4 subunits)
+
28
Summary of Structural Levels