Lesson 6B Protein Structure Flashcards
Protein Conformation
A functional protein is not just a polypeptide chain but one or
more polypeptide chains twisted, folded and coiled into a
molecule of unique shape. This three-dimensional shape is
known as the protein conformation.
Protein conformation determines the function of the protein.
Four Levels of Protein Structure
When a ribosome synthesizes
polypeptide the chain folds
spontaneously to assume the functional
conformation of that protein.
This is reinforced by a variety of chemical
bonds between parts of the chain
In this, three levels of structure have
been identified; primary, secondary and
tertiary
A fourth level of structure known as
quaternary occurs when a protein
contains two or more polypeptide chains
Primary Structure
The primary structure of a protein is
its unique sequence of amino acids.
Even a slight change in this primary
structure can affect its conformation
and ability to function
Ex. Sickle-cell disease is an inherited
blood disorder in which one amino
acid is substituted for another in a
single position in the primary
sequence of hemoglobin
Secondary Structure
Most proteins have segments of their polypeptide chain
repeatedly coiled or folded in patterns that contribute to the
overall conformation of the protein.
This is a result of the hydrogen bonds at regular intervals along
the polypeptide backbone
Hydrogen Bonding in Secondary
Structure
Both the oxygen and the nitrogen atoms in the backbone are very
electronegative which leaves them with a slight negative charge.
The hydrogen atoms attached to nitrogen have a slight positive charge and
therefore have an affinity for the slightly negative oxygen atom in the
neighbouring peptide bond.
Individually these hydrogen bonds are weak but because they are repeated
many times over a long region of the polypeptide chain they can support a
particular shape.Alpha (α) Helix
A type of secondary structure
A coil held together by a hydrogen
bond every fourth peptide bond
Pleated Sheet (β sheet)
A type of secondary structure
The polypeptide chain folds back and
forth where two regions are parallel
to each other
Hydrogen bonds between the parallel
regions holds this structure together
Tertiary Structure
This is a series of irregular contortions caused by bonding between the side
chains(R-groups) of the various amino acids.
Two major factors contribute to tertiary structure;
Hydrophobic interactions
Disulfide bridges
Hydrophobic
Interactions
As the polypeptide folds into its
conformation, the amino acids with
nonpolar (hydrophobic) side chains
usually move towards the core of the
protein, out of contact with water.
This keeps the hydrophobic side chains
together in localized clusters
Caused by the behaviour of water
molecules as they hydrogen bond to one
another and hydrophilic side chains.
Disulfide Bridges
Strong covalent bonds form where two
cysteine monomers (AA with sulfhydryl
groups – SH) are brought close together
by the folding of the protein.
The sulfur of one cysteine bonds to the
sulfur of a second and the disulfide
bridge rivets parts of the protein
together.
Hydrogen bonds between side chains
and ionic bonds (salt bridges) between
positively and negatively charged side
chains also help stabilize tertiary
structure
Quaternary Structure
When two or more polypeptide chains
aggregate to form one functional
macromolecule.
This is the overall protein structure that
results from the aggregation of these
polypeptide subunits.
Collagen
A fibrous protein that has helical
subunits supercoiled into a triple helix
Hemoglobin
A globular protein that consists of two
kinds of polypeptide chains, two of each
kind for four subunits per molecule
