Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
0
Post translational modifications, Protein Folding
and misfolding
Objectives:
1. To understand the post translational process that
occurs after protein synthesis.
2. To understand the process of protein folding
3. To know about the diseases that occur due to
Protein misfolding
Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
3
Post-Translational Modifications
Some proteins must be modified in one or more of a number of ways before
they realize their final functional form. The following are some of the
modifications that have been found to occur to proteins after they have been
synthesized:
1.
Dealing with the N-terminal residue
In bacteria, the N-terminal residue of the newly-synthesized protein is
modified in bacteria to remove the formyl group. The N-terminal methionine
may also be removed.
In eukaryotes, the methionine is also subject to removal.
2.
Amino Acid Modifications
a.
Acetylation
b.
Phosphorylation
c.
Methylation
d.
Carboxylation
e.
Hydroxylation
f.
Glycosylation
g.
Nucleotidylation
h.
Lipid Addition
Others
The protein, thyroglobin, is
iodinated
during the synthesis of thyroxine.
i.
Adding Prosthetic Groups
Proteins that require a prosthetic group for activity must have this group
added. For example, the haem (heme) group must be added to globins and
cytochromes; Fe-S clusters must be added to ferredoxins.
Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
2
j.
Forming Disulfide Bonds
Many extracellular proteins contain disulfide cross-links (intracellular proteins
almost never do). The cross-links can only be established after the protein
has folded up into the correct shape.
Proteolytic Processing
Some proteins are synthesized as inactive precursor polypeptides which
become activated only after proteolytic cleavage of the precursor polypeptide
chain. Two well-known examples are:
Chymotrypsin & Trypsin
Chymotrypsin and trypsin are both synthesized as
zymogens
. Cleavage of
chymotrypsinogen between Arg15 and Ile 16 by trypsin yields the
enzymatically active pi-chymotrypsin. Two further proteolytic cleavages
catalyzed by chymotrypsin removes the dipeptides Ser
14
-Arg
15
and Thr
147
-
Asn
148
to yield alpha-chymotrypsin.
Trypsin is activated by the removal of the N-terminal seven amino acids.
Insulin
Insulin is synthesized as a precursor polypeptide. The initial
preproinsulin
contains a signal sequence since the protein is targeted for secretion.
Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
4
Protein Folding
As they are being synthesized, proteins must adopt the correct conformation
for their function.
Protein folding is the process by which a string of amino acids (the chemical
building blocks of protein) interacts with itself to form a stable three-
dimensional structure during production of the protein within the cell.
The process is roughly analogous to the ways in which a length of wire may
be twisted onto or against itself to form various functional entities, for example
a spring, a paperclip or a coat hanger.
Folding occurs very rapidly, probably within milliseconds of production of the
string of amino acids, and results in 3-D conformations which usually are
quite stable, with specific biological functions.
The folding of proteins thus facilitates the production of discrete functional
entities, including enzymes and structural proteins, which allow the various
processes associated with life to occur.
Importantly, folding not only
1. allows the production of structures which can perform particular
functions in the cellular milieu, but also
2. it prevents inappropriate interactions between proteins, in that folding
hides elements of the amino acid sequence which if exposed would
react non-specifically with other proteins.
Proteins may either fold spontaneously or they may need the assistance of
chaperone proteins so that they do not get trapped in stable folding
intermediates but rather fold into the correct final conformation.
Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
5
There are 3 major classes of chaperones:
The Hsp70 family
The Hsp 60 family
The Hsp 90 family
Protein misfolding diseases
In many cases, misfolded proteins are recognized to be undesirable by a
group of proteins called heat shock proteins, and consequently directed to
protein degradation machinery in the cell. This involves conjugation to the
protein ubiquitin, which acts as a tag that directs the proteins to proteasomes,
where they are degraded into their constituent amino acids. Hence many
protein misfolding diseases are characterized by absence of a key protein, as
it has been
recognised as dysfunctional and eliminated by the cell’s own
machinery.
Diseases caused by lack of a particular functioning protein, due to its
degradation as a consequence of misfolding, include:
cystic fibrosis (misfolded CFTR protein),
Marfan syndrome (misfolded fibrillin),
Fabry disease (misfolded alpha galactosidase),
Gaucher’s disease (misfolded beta glucocerebrosidase) and
retinitis pigmentosa 3 (misfolded rhodopsin).
In addition, some cancers may be associated with misfolding, and hence
ineffective functioning, of tumour suppressor proteins such as p53.
Many protein misfolding diseases are characterized not by disappearance of
a protein but by its deposition in insoluble aggregates within the cell.
Lecture 5: Post translational processes and protein folding and misfolding
3102
Prof. Dr.Hedef D. El-Yassin
6
Diseases caused by protein aggregation include:
Alzheimer’s disease (deposits of amyloid beta and tau),
Type II diabetes (depositis of amylin),
Parkinson’s disease (deposits of alpha synuclein), and
the spongiform encephalopathies such as Creutzfeldt-Jakob disease
(deposits of prion protein).
Protein misfolding appears at least in some cases to be due to mutations
(missing or incorrect amino acids) in the protein which destabilise it such that
it is more likely to fold incorrectly.
Alternatively, the misfolding could occur due to progressively lower levels of
chaperone proteins in ageing neurons. It may also be that mutations or other
changes in the chaperone proteins themselves cause them to actually
promote misfolding, rather than guard against it.
Conclusions:
1. Modification of amino acid residues in a protein occurs during or after
translation.
2. Folding of the polypeptide into its three-dimensional configuration
occurs as the polypeptide is being translated. This process involves
proteins called chaperones.
3. Misfolding processes cause certain diseases