Post translational modification of proteinsPresentation Transcript
The proteins are basic constituents of any living
system and plays a very significant role in the
structural and functional organization of any cell.
The protein molecules necessary for the cell are
synthesized by the protein synthetic machinery of the
This process involves the central dogma concept in
DNA mRNA Protein
Proteins are the active players in most cell processes,
implementing the myriad tasks that are directed by
the information encoded in genomic DNA.
Protein synthesis is thus the final stage of gene
However, the translation of mRNA is only the first
step in the formation of a functional protein.
After a cell has completed stringing together a bunch
of amino acids to make a protein, many modifications
have to happen within the protein to make it fully
The non-functional protein made right after
translation is called immature protein or simply a
Post translational modification (PTM) is the chemical
modification of a protein after its translation.
It is one of the later steps in protein biosynthesis, and
thus gene expression, for many proteins.
The PTM of amino acids extend the range of functions
of the protein by attaching it to other biochemical
functional groups (such as acetate, phosphate, various
lipids and carbohydrates), changing the chemical
nature of an amino acid (e.g., citrullination) or making
structural changes (e.g., formation of disulfide
The polypeptides must fold into distinct three–
dimensional conformations, and in many cases multiple
polypeptide chains must assemble into a functional
Polypeptides should be cleaved properly which are
critical for the functioning and correct localization of
proteins within the cell.
PTM of proteins can be detected by mass spectrometry
or Eastern blotting.
The functional protein thus formed after proper folding
and biochemical changes have to be transported to
different sites within or outside the cell according to
A signal sequence is an amino acid sequence which
tags the proteins and directs the cell where to deliver
If the protein has no signal sequence, then it stays in
Some signal sequences direct new proteins to go to
nucleus, mitochondria or chloroplast.
Other signal sequences cause protein synthesis to
pause for a while to be finished in ER.
Amino acid sequence is added causes
protein to stay inside ER.
Protein becomes glycosylated goes either
lysosome or plasma membrane.
Nothing is added protein travels to Golgi
modification protein is
secreted from cell.
Protein folding is a physical process leading from an
unstructured polypeptide chain to a functional
protein with a definite structure.
Any polypeptide chain that containing ‘n’ residue
could, in principle ,fold into 8n conformations .
This value is based on the fact that only eight bond
angles are stereo chemically allowed in the
In general ,however any molecule of any polypeptide
chain adopt a single conformation called the native
For the vast majority of proteins, the native state is
the most stably folded form of the molecule.
Protein fold to their native conformations via directed
pathway rather than stumbling on them through
random conformational searches, protein folding is a
self directed process.
Proteins that facilitate the folding of other proteins
are called molecular chaperones.
Chaperones thus act as catalysts that facilitate
assembly without being part of the assembled
Chaperones do not convey additional information
required for the folding of polypeptides, as it is
determined solely by its amino acid sequence.
Chaperone binding stabilizes the amino-terminal
portion in an unfolded conformation until the rest of
the polypeptide chain is synthesized and the completed
protein can fold correctly.
Chaperones also stabilize unfolded polypeptide chains
during their transport into subcellular organelles.
Many of the proteins now known to function as
chaperones were initially identified as heat-shock
proteins, a group of proteins expressed in cells that
have been subjected to elevated temperatures.
Two families of chaperone proteins, the Hsp70
chaperones and chaperonins, act in a general
pathway of protein folding in both prokaryotic and
They are found in the cytosol and in subcellular
These proteins bind to short hydrophobic segments of
unfolded polypeptides, maintaining it in an unfolded
configuration and preventing aggregation.