Jul 13, 2015
Posttranslational Modification
Nucleus acetylation, phosphorylation
Lysosome mannose-6-phosphate labelled N-linked sugar
Mitochondria N-formyl acylation
Golgi N- and O-linked ologosaccharide, sulfation,
palimitoylation
ER N-linked oligosaccharide, GPI-anchor
Cytosol acetylation, methylation, phosphorylation,
Ribosome myristoylation
Plasma membrane N- and O-glycosylation, GPI-anchor
Extracellular fluid N- and O-glycosylation, acetylation,
phosphorylation
Extracellular matrix N- and O-glycosylation, phosphorylation,
hydroxylation
ModificationLocation
1. stability of protein
2. Biochemical activity (activity regulation)
3. Protein targeting (protein localization)
4. protein signaling (protein-protein interaction, cascade amplification)
Purpose of modification
Folding of proteins Protein folding in cells probably
involves multiple pathways. Initially,
regions of secondary structure may form,
followed by folding into super secondary
structures. Large ensembles of folding
intermediates are rapidly brought to a
single native conformation.
Not all proteins fold spontaneously as
they are synthesized in the cell. Folding
for many proteins is facilitated by the
action of specialized proteins. Molecular
chaperones are proteins that interact with
partially folded or improperly folded
polypeptides, facilitating correct folding
pathways or providing microenvironments
in which folding can occur.
Chemical modification1.Amino Group Modification:Amino group modification involves the addition of a functional group at the N terminus of the amino acid
Acetylation
N-α-acetyl transferase
Acetyaltion is an acylation
(introduction of an acyl group
to an organic compound)
process which involves the
substitution of an organic
group of acetic acid for an
active hydrogen atom at the N-
terminus.
Pyroglutamate Formation:
Pyroglutamate is formed through the
cyclization (ring formation in the chemical
compound) of glutamine.
It is commonly observed in antibodies that
contain glutamate or glutamine residues at their
N-termini.
The amino group and the glutamate or
glutamine condenses to form a five-member
ring called Pyroglutamate.
Methylation
Glycosylation• The addition of sugar residues to the protein backbone.
• Most extensive posttranslational modification.
• Carried out in the ER and Golgi apparatus prior to secretion or surface display.
• All mammalian cell surface proteins of glycoprotein.
• Most secreted proteins are glycoprotein (notable exceptions include insulin, growth hormone).
• N-Linked
• O-Linked
• Membrane anchor
• The membrane bound proteins are prevalent in most Eukaryotic
systems and serve to regulate the release of molecules from cell
surfaces and exchange of membrane molecules.
• Specifically they play a critical role in the recognition of enzymatic
and antigenic molecules as well receptor mediated signal transduction
pathways.
• Proteins destined for anchorage onto a membrane surface are first
adhered at the carboxyterminus (c-terminal region) to a
phosphodeister linkage system.
• This is comprised of the adsorption of phospothanolamine to a core of
trimannosyl-non-acetylated Glucosamine. This Glucosamine structure
is then linked to phosphatidylinositol, which is then finally adhered to
the lipid bilayer via another phosphodeister linker. Solubilization of
the membrane bound protein is achieved via cleavage of the
phosphatidynisotil bond by Phospholipase C.
GPI Anchoring
Phosphorylation
Protein phosphorylation is a reversible post-transcriptional modification which plays an important role in signal transduction, protein function and localization.
The phosphorylation of serine , threonine and tyrosine residues is the most common in mammalian cells, whereas the phopshorylation of histidine and aspartate is rare in eukaryotes.
Amidation
The amide group for C-terminal
amidation is contributed by a
Glycine residue. This Glycine is
present in a precursor C-terminal
sequence that resembles XGXX,
Amidation neutralizes negative
charges on the C-terminus of the
polypeptide.
Peptidylglycine α-hydroxylating
monooxygenase and peptidyl α-
hydroxyglycine α-amidating lyase
are two enzymes associated with
amidation,
Sumoylation• Small Ubiquitin-related Modifier proteins, or SUMO proteins,
are small proteins that covalently attach to proteins in a process known as sumoylation.
• As its name suggests, SUMO proteins and ubiquitin are similar proteins sharing many characteristics including the sequence of enzymatic reactions directing either process
• unlike ubiquitin, SUMO proteins do not act as proteolyticsignals. SUMOylation has been associated with various cellular functions and processes including cell cycle regulation, DNA repair, chromosomal maintenance, modifying cytoplasmic signal transduction, nuclear import and subnuclear compartmentalization, DNA repair, transcription regulation, and stress response
Protein-protein interactions occurbetween two proteins that are similar in size. Theinterface between the two molecules tend to beflatter and smoother than those in protein-ligandinteractions. Protein-protein interactions areusually more rigid; the interfaces of theseinteractions do not have the ability to alter theirconformation in order to improve binding andease movement. Conformational changes arelimited by steric constraint and thus are said to berigid.
Molecular docking is the technique that is used to study whether amolecule binds to another and if so how does it really bind. The term“docking” is mostly related to protein molecule interactions.