Transcript TRANSLASI - alanindra

TRANSLASI
Dr. Yekti Asih Purwestri, M.Si.
Asam amino dan ikatan peptida
• Ada 20 asam amino yang
dikode oleh DNA
• Semua mempunya gugus
amino group (-NH2) dan
gugus karboksil (-COOH).
Terikat pada C sentralnya
adalah rantai samping R.
Gugus asam pada asam
mino terikat pada gugus
amino pada asam amino
berikutnya, membentuk
ikatan peptida
Dari gen menjadi protein
DNA
Transcription
mRNA
Translation
Sequence of
a.a
Primary structure of
protein
• Translasi  merupakan proses pembacaan kodon
dan menggabungkan asam amino melalui ikatan
peptida
• Komponen proses translasi
1. mRNA  tersusun atas kode genetik
2. Ribosom
3. tRNA bersama dengan asam amino
4. Enzim2
Tahap proses translasi
• Inisiasi
• Elongasi
• Terminasi
Inisiasi
Aktivasi asam amino untuk
bergabung membentuk protein
Activation of amino acids for incorporation into
proteins.
Genetic code  Three nucleotides - codon - code for
one amino acid in a protein
Codon  sequence of three nucleotides in a
mRNA that specifies the incorporation of a
specific amino acid into a protein.
The relationship between codons and the amino acids
they code for is called the genetic code.
Not all codons are
used with equal
frequency.
There is a
considerable
amount of variation
in the patterns of
codon usage
between different
organisms.
Kode Genetik
• Masing2 kelompok yang terdiri dari 3 nukleotida pada mRNA disebut
kodon. Karena 4 basa , kodon yang terbentuk adalah 43 = 64, yang
harus mengkode 20 asam amino yang berbeda.
• Lebih dari satu kodon digunakan untuk banyak asam amino : kode
genetik bersifat “degenerate”. Hal ini berarti bahwa tidak mungkin
mengambil sekuen protein dan menterjemahkannya ke dalam sekuen
basa gen tersebut.
• Dalam banyak hal, basa ketiga dari kodon (the wobble base) dapat
diubah tanpa mengubah asam aminonya.
• AUG digunakan sebagai start kodon. Awal translasi semua protein
adalah metionin, meskipun sering dihilangkan setelah translasi. Ada
juga internal metionin yang dikode oleh kodon AUG yang sama.
• Terdapat 3 stop kodon, yang disebut “nonsense” kodon. Protein
berakhir dengan stop kodon yang tidak mengkode suatu asam amino
Kode Genetik
• Kebanyakan kode genetik bersifat universal.
Digunakan baik pada prokariot maupun eukariot.
• Namun, beberapa varian dijumpai, terutama pada
mitokondria yang hanya memiliki sedikit gen.
• Contoh: CUA umumnya mengkode, tetapi pada
mitokondria yeast mengkode treonin. AGA
umumnya mengkode arginin, tetapi pada
mitokondria manusia dan Drosophila merupakan
stop kodon.
Wobble Hypothesis
Relationships of DNA to mRNA to
polypeptide chain.
Translation is
accomplished by the
anticodon loop of tRNA
forming base pairs with
the codon of mRNA in
ribosomes
Transfer RNA (tRNA)
composed of 
a nucleic acid and
a specific amino acid
 provide the link between
the nucleic acid sequence of
mRNA and the amino acid
sequence it codes for.
An anticodon  a sequence of
3 nucleotides in a tRNA that
is complementary to a
codon of mRNA
Structure of tRNAs
Transfer RNA
•
•
•
•
Transfer RNA molecules are short RNAs that
fold into a characteristic cloverleaf pattern.
Some of the nucleotides are modified to
become things like pseudouridine and
ribothymidine.
Each tRNA has 3 bases that make up the
anticodon. These bases pair with the 3 bases
of the codon on mRNA during translation.
Each tRNA has its corresponding amino acid
attached to the 3’ end. A set of enzymes, the
“aminoacyl tRNA synthetases”, are used to
“charge” the tRNA with the proper amino acid.
Some tRNAs can pair with more than one
codon. The third base of the anticodon is
called the “wobble position”, and it can form
base pairs with several different nucleotides.
Only tRNAfMet is accepted to
form
Twothe
initiation
initiation
factors
complex.
(IF1 &IF3)
bind to a 70S ribosome.
Allpromote
further charged
the dissociation
tRNAs of
require
70S ribosomes
fully assembled
into free
(i.e.,
30S
70S)
andribosomes
50S subunits.
The
mRNA
Shine-Dalgarno
and IF2, which
sequence
carries
- GTP
help ribosomes and mRNA
aligns
- the correctly
charged tRNA
for the start of
translation.
bind to a free 30S subunit.
Ribosome
consists
of all bound,
 After these
have
the
30S
-A
site
initiation
aminoacyl complex is
- Pcomplete.
site  peptidyl
- E site  exit
Peptide bond formation

catalyzed by an enzyme
complex called
peptidyltransferase
Peptidyltransferase
consists of some
ribosomal proteins and
the ribosomal RNA 
acts as a ribozyme.
The process
is repeated until a
termination signal is
reached.
Termination of translation
occurs when one of the stop
codons (UAA, UAG, or UGA)
appears in the A site of the
ribosome.
No tRNAs correspond to those
sequences, so no tRNA
is bound during termination.
Proteins called release factors
participate in termination
Posttranslational Processing of Proteins
• Folding
• Amino acid modification (some proteins)
• Proteolytic cleavage
FOLDING
Before a newly translated polypeptide can
be active, it must be folded into the
proper 3-D structure and it may have to
associate with other subunits.
Enzymes/protein involve in folding process
1. Cis-trans isomerase for proline 
Proline is the only amino acid in proteins  forms peptide bonds
in which the trans isomer is only slightly favored (4 to 1 versus
1000 to 1 for other residues).
Thus, during folding, there is a significant chance that the
wrong proline isomer will form first. Cells have enzymes to
catalyze the cis-trans isomerization necessary to speed
correct folding.
2. disulfide bond making enzymes
3. Chaperonins (molecular chaperones) 
a protein to help keep it properly folded and non-aggregated.
Insulin is synthesized  single
polypeptide  preproinsulin
has leader sequence
(help it be transported through the cell
membrane)
Specific protease cleaves leader
sequence  proinsulin.
Proinsulin folds into specific 3D
structure and disulfide bonds form
Another protease cuts molecule
 insulin  2 polypeptide chains
Chaperones
 capable to fold
a. Some proteins
Function
keep a3-D
newly
into itsto
proper
structure
synthesized
protein any
fromhelp
either
by itself without
of
improperly
folding or
other molecules
aggregating
b. Some proteins need
chaperones
to fold
(example
After
synthesized,
protein
needs
in human
hspto70)
to fold
in order
have its
function
c. Some proteins need bigger
protein  chaperonins to be
Theable
folding
pattern
is dictated in
to fold
correctly.
the amino acid sequence of the
protein.
Chaperonins  a polysubunit
protein form “a cage” like shape
 give micro environment to
protein
Protein Targeting
Nascent proteins  contain signal sequence  determine
their ultimate destination.
Bacteria  newly synthesized protein can: stay in the cytosol,
send to the plasma membran, outer membrane,
periplasmic, extracellular.
Eukaryotes  can direct proteins to internal sites 
lysosomes, mitochondria etc.
Nascent polypeptide  E.R and glycosylated  golgi complex
and modified  sorted for delivery to lysosomes,
secretory vesicle and plasma membrane.
• Translocation 
– The protein to be translocated (called
a pro-protein) is complexed in the
cytoplasm with a chaperone
– The complex keeps the protein from
folding prematurely, which would
prevent it from passing through the
secretory porean ATPase that helps
drive the translocation
– after the pro-protein is translocated,
the leader peptide is cleaved by a
membrane-bound protease and the
protein can fold into its active 3-d
form.
Signal recognition particle (SRP) detects signal sequence and
brings ribosome to the ER membrane
Most
mitochondrial
proteins are
synthesized in the
cytosol and
imported into the
organelle
Initiation of Translation
• In prokaryotes, ribosomes bind to specific translation
initiation sites. There can be several different initiation sites
on a messenger RNA: a prokaryotic mRNA can code for
several different proteins. Translation begins at an AUG
codon, or sometimes a GUG. The modified amino acid Nformyl methionine is always the first amino acid of the new
polypeptide.
• In eukaryotes, ribosomes bind to the 5’ cap, then move down
the mRNA until they reach the first AUG, the codon for
methionine. Translation starts from this point. Eukaryotic
mRNAs code for only a single gene. (Although there are a few
exceptions, mainly among the eukaryotic viruses).
• Note that translation does not start at the first base of the
mRNA. There is an untranslated region at the beginning of
the mRNA, the 5’ untranslated region (5’ UTR).
More Initiation
• The initiation process
involves first joining the
mRNA, the initiator
methionine-tRNA, and the
small ribosomal subunit.
Several “initiation factors”-additional proteins--are also
involved. The large
ribosomal subunit then
joins the complex.
Elongation
• The ribosome has 2 sites for tRNAs, called P and A. The initial tRNA with
attached amino acid is in the P site. A new tRNA, corresponding to the
next codon on the mRNA, binds to the A site. The ribosome catalyzes a
transfer of the amino acid from the P site onto the amino acid at the A
site, forming a new peptide bond.
• The ribosome then moves down one codon. The now-empty tRNA at the
P site is displaced off the ribosome, and the tRNA that has the growing
peptide chain on it is moved from the A site to the P site.
• The process is then repeated:
– the tRNA at the P site holds the peptide chain, and a new tRNA binds to the A
site.
– the peptide chain is transferred onto the amino acid attached to the A site
tRNA.
– the ribosome moves down one codon, displacing the empty P site tRNA and
moving the tRNA with the peptide chain from the A site to the P site.
Elongation
Termination
•
•
•
Three codons are called “stop
codons”. They code for no amino
acid, and all protein-coding regions
end in a stop codon.
When the ribosome reaches a stop
codon, there is no tRNA that binds to
it. Instead, proteins called “release
factors” bind, and cause the
ribosome, the mRNA, and the new
polypeptide to separate. The new
polypeptide is completed.
Note that the mRNA continues on
past the stop codon. The remaining
portion is not translated: it is the 3’
untranslated region (3’ UTR).
Post-Translational Modification
• New polypeptides usually fold themselves spontaneously into
their active conformation. However, some proteins are
helped and guided in the folding process by chaperone
proteins
• Many proteins have sugars, phosphate groups, fatty acids, and
other molecules covalently attached to certain amino acids.
Most of this is done in the endoplasmic reticulum.
• Many proteins are targeted to specific organelles within the
cell. Targeting is accomplished through “signal sequences”
on the polypeptide. In the case of proteins that go into the
endoplasmic reticulum, the signal seqeunce is a group of
amino acids at the N terminal of the polypeptide, which are
removed from the final protein after translation.