Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Post Translational Processing Lecture PPT
Post-translatonal modification.
Addition of group or deletion of parts to make a finished
protein.
What group--
methyl
acetyl
phospho
What purpose ----
targeting( some lipoprotein )
stability ( secreted glycoprotein )
function ( surface glycoprotein ).
Post-translational modification of individual
Aminoacids.
PHOSPHORYLATION-
Phosphorylation of serine , threonine
and tyrosine residue occurs by specific kinase. This is a common
signal transduction mechanism in hormone action and activation/
inactivation of enzyme e.g.- glycogen synthase and phosphorylase.
HYDROXYLATION---
proline and lysine in collagen undergo
hydroxylation to form hydroxoproline and hydroxylysine which are
again important in the maturation of collagen .
Modification of Amino acids.
CARBOXYLATION-
Carboxylation of glutamate residues
in coagulation factor like prothrombin is necessary for
their function.
GLYCOSYLATION-
Glycosylation occurs by addition of
oligosaccharide chains to specific amino acid residue of
the polypeptide and is seen in glycoproteins.
Attachment of gluose to hemoglobin in HBA1c is also an
example.
N AND C TERMINAL MODIFICATION.
Both formylmethionine and methionine are removed by
aminopeptidases. They are not present in most mature
proteins.
Large majority of eukaryotic proteins are acetylated at
the N terminal residues.
C terminal amino acids are also modified in many proteins.
CLEAVAGE OF NASCENT POLPEPTIDE
CHAIN
Usually the nascent proteins are larger protein than their
mature functional counterparts.
They are called pre-pro-proteins and undergo cleavages
to become smaller and act
For examples, insulin is synthesized as a much larger-pre-
pro-insulin of molecular weight 11500Da while the
mature functional insulin is about half the size.
In all 38 amino acids are removed to generate a mature
functional insulin molecule
Similarly, chymotrypsinogen and trypsinogen are also
converted into active chymotrypsin and trypsin by
cleavage.
TRIMMING-
Many proteins are originally synthesised as
much bigger molecules. They undergo proteolytic
degradation or trimming to become smaller molecules
which are functional Example- the formation of insulin
from preproinsulin, conversion of zymogens to active
digestive enzymes, e.g. trypsinogen to trypsin, pepsinogen
to pepsin.
PROTEIN FOLDING.
Unique structure of protein involves the folding of its polypeptide
chain in a characteristics three dimensional structure.
This folding is essential for the biological function of the protein.
CHAPERONS---
Proteins require other accessory protein called
chaperons to promote folding.
Chaperones are the proteins which interact with partially folded or
improperly folded nascent protein to fascilitate correct folding.
Function--
1.Helps in protein folding.
2.Prevent Aggregation.
Ex. Hsp in eukaryotes.
Protein Folding
Proper folding of protein is essential for a cell to carry outs its normal
cellular function.
Misfolded protein can result in a wide variety of a pathological condition.
Loss of Protein Function.
PROTEIN FOLDING.
These are two classes?
Hsp70 protein and chaperonins.
Hsp70--
The Hsp70 are called heat shock proteins
(molecular weight 70KDa) because they are more abundant in
cells stressed by heat.
They are found in all organisms from bacteria to human
beings.
These chaperones protect ptoteins which are
denatured by heat and from aggregates.
Chaperonins--
The Hsp60 family of proteins are referred to as
chaperonins.
Chaperonins usually act later along with Hsp70.
PROTEIN MISFOLDING DISEASES.
Protein misfolding disease can be classified in to two
categories.
Diseases caused due to the misfolding or degradation of
misfolded protein.
Disease caused due to accumulation of misfolded protein.
Reason for protein misfolding---
Mutation.
Premature termination of translation.
Fault in post- translational modification.
DISEASE CAUSED DUE TO THE ACCUMULATION
OF MISFOLDED PROTEIN.
PRION DISEASES--
The prion diseases are neuro-degenerative caused
by misfolded protein.
? They are characterised by dementia, loss of
coordination and encephalitis leading to fatal
outcome.
Prion diseases.
Diseases Due to misfolded protein.
DISEASE
PROTEIN INVOLVED.
Alzheimer's disease
Amyloid Beta ? peptide.
Spongiform encephalopaties.
Prion protein
Huntington disease
Huntington protein.
Alzheimer's Disease.
Spongiform encephalitis.
These diseases are also called spongiform encephalitis
because they produce vacuolar or spongiform
changes in brain.
The best known examples is mad-cow disease or
bovine spongiform encephalitis.
Prion infected Brain of a cattle.
Prion disease.
Creutzfeldt disease is an examples in humans.
The causative agent is a prion protein---PrP,
(moleculer weight 28 Kda) coded by PrP gene on
chromosome 20.
Prion diseases may manifest as genetic, infectious and
sporadic diseases.
PROTEIN TARGETING.
PROTEIN TARGETING-
PROTEIN FOR EXTERNAL SECRETION
The process is also called
as "protein sorting'' or ``protein localization''. The
secreted proteins, plasma membrane integral proteins,
lysosomal enzymes and membrane proteins of ER are
synthesized on rough endoplasmic reticulum by
membrane bound polyribosomes. The newly synthesized
protein is then delivered to the destined compartment.
Blobel and sabatini proposed the signal hypothesis to
explain the different destination of proteins. Guenter
Blobel was awarded Nobel Price in 1999.
Protein targeting.
Protein targeting or protein sorting is the mechanism by
which a cell transport to the appropriate positions in the
cell or outside of it.
Protein targeting is necessory for protein that are
destined to work outside the cytoplasm.
The delivery process is carried out based on information
contained in the protein itself.
Correct sorting is crucial for the cell, errors can lead to
disease.
Synthesized protein is transferred to an SRP receptor on
endoplasmic reticulum, nascent protein is inserted into
translocation complex.
Targeting signal.
Targeting signal are the piece of information that enable
the cellular transport machinery to correctly position a
protein inside or outside the cell.
This information is contained in the polypeptide chain or
in the folded protein.
In abscence of targeting signal, a protein will remain in the
cytoplasm.
Protein targeting.
Protein has to correctly localised to perform proper
function.
Receptor---plasma membrane.
DNA polymerase--nucleus.
Catalase-----peroxisomes.
Insulin-------outside.
All protein begins to be synthesized on cytosolic
ribosomes.
Sorting or translocation can occur
Co- tranlational.
Post-translational.
If protein is for cytosolic function, the synthesis will be
finished on free ribosomes and the peptide is released
into the cytosol.
Protein targeting.
The nascent protein is passed through the membrane into the
channels of ER. Then protein synthesis is completed, and the
protein molecule is now inside the endoplasmic membrane. As
the nascent protein is traversing the inner membrane of ER,
carbohydrate moieties are added at particular regions by
specific enzymes; this is called co-translational glycosylation.
CORRECT ADDRESS of DESTINATION is LABELED
The protiens carry an "address" that is specific for its correct
destination inside the cell. This is present in the carboxy
terminal end of proteins. Diseases due to defective protein
targeting.
Protein targeting.
Protein Targeting
Zellweger syndrome is due to defective oxidation of very long chain
fatty acids (VLCFA). Here the correct "address" is not printed on
the protein packet; so that it could not be delivered to the correct
location. Peroxisomal enzymes are produced; but their entry into
peroxisomal is denied. This leads to insufficient oxidation of VLCFA.
Accumulation of VLCFA in CNS causes neurological impairment and
death in childhood.
Another example is primary hyperoxaluria, which causes
kidney stones at an early age. The defect is due to protein trageting
defect and the enzyme alanine glyoxylate amino transferase seen in
mitochondria, instead of its normal peroxisomal location.
Iysosomes cell diseases due to non-entry of normal enzymes into
Iysosomes. Mannose-6-phosphate is the marker to target enzymes
to Iysosomes;this is absent.
MITOCHONDRIAL DNA and RNA.
Some of mitochondrial protein synthesis is under the control
of mitochondrial DNA;
but important proteins of the outer membrane of the
mitochondria are synthesized under the influence of nuclear
DNA.
Shows that mitochondria are similar to bacteria more than
mammalian cells.
This fact supports the theory that mitochondria are derived
from prokaryotes symbiotically adapted to multicellular
organisms.
Maternal inheritance:
Since, the mitochondria are inherited cytoplasmically,
the mtDNA is inherited from the mother.
Mother transmits mtDNA through oocyte.
There are hunderds of copies of mtDNA in each cell
(nuclear DNA has only 2 copies).
During cell division, mtDNA replicates and they segregate
to the daughter cells.
If a mutation occurs in mtDNA, the daughter cells may
inherit the mutant or normal mtDNA.
Heteroplasmy is defined as the presence of normal and
mutant mtDNA in different proportions in different cells.
Defects in mitochondrial genome will lead to mitochondrial
myopathies.
Leber`s hereditary optic neuropathy is caused by a single base
mutation which alters one argrinine to histidine in the NADH
Coenzyme Q reductase. OXPHOS (oxidative
phosphorylation).
OXPHOS DISEASES
Syndrome
Features
Leber`s Hereditary
Complex l defect; blindness,cardiac
Neuropathy(LHON)
conduction defects
Myoclonic epilepsy ragged
Myoclonic epilepsy, myopathy,
Red fiber diseases (MERRF)
dementia
Leigh`s syndrome
Complex l defect ; movement
disorders
GENOME AND PROTEOME.
All the DNA contained in an organism or a cell which
includes both the chrosomes within the nucleus and the
DNA in mitochondria----GENOME.
PROTEOME--Is the some of all proteins expressed by
the genome of an organism.
This post was last modified on 30 November 2021