Download Biochemistry

The First Page of Teaching Plan
No.
course
Biochemistry
specialty
clinic
teacher
Chen yan
period
8h
professional
Biochemistry associate professor
title
chapter
Protein Synthesis----Translation
class
students’
level
2015-2
undergraduate
time of
2016.12
writing
time of
using
2016-2017(1)
1.Master the concept of protein biosynthesis, function of three kinds of RNAs(mRNA,
tRNA and rRNA) in protein synthesis and the basic process of protein synthesis in
objectives and
prokaryotes.
requirements
2.Familiar with the characteristics of initiation in translation in eukaryotes.
3.Understand the modification of primary structure and conformation of newly
synthesized polypeptide chains, directed transportation of protein and the relationship
between protein synthesis and medicine.
Keys: Definition and properties of genetic code; Activation of amino acid; Function of
keys and
difficulties
rRNA, mRNA and tRNA in the process protein biosynthesis; Procedure of protein
biosynthesis( including initiation in eukaryotes)
Difficulties: initiation and Elongation
updated
no
information
review the content last class(20min); Function of rRNA, mRNA and tRNA in the process
arrangement
protein biosynthesis (50min); the genetic code (70min); Fundamental procedure of protein
biosynthesis (200min); The regulation of protein biosynthesis and the relationship
between protein biosynthesis and medicine(140min); discuss and summarize(20min).
teaching
methods
Using CAI to explain, enlightening method
Lippincott’s illustrated review :Biochemistry Pamela C. Champe
& wilkins 2009
Lippincott’s willam
books and
references
Biochemistry
the second edition
High Education Press 2002
author: Reginald H. Garrett, Charles M. Grisham
teachers’
group
discussion
According to learn the 3 RNA, then to emphasized function of RNA in the process protein
biosynthesis. The processing of protein biosynthesis in prokaryotes should be lectured
clearly(Ribosomal cycle).
about the plan
Agree apply in class.
comments
from the
department
Sign name:
（Content）
Lesson plan for page
Chapter 11
Protein Biosynthesis ----Translation
I.Teaching Goals
It is based on a mastery of function of three kinds of RNAs, then study the process of
translation and to know(understand) the relationship between protein synthesis and
medicine.
II. Teaching Demands
1.Master the concept of protein biosynthesis, function of three kinds of
RNAs(mRNA, tRNA and rRNA) in protein synthesis and the basic process of protein
synthesis in prokaryotes.
2.Familiar with the characteristics of initiation in translation in eukaryotes.
3.Understand the modification of primary structure and conformation of newly
synthesized polypeptide chains, directed transportation of protein and the relationship
between protein synthesis and medicine.
III. Teaching Contents
1. Protein biosynthesis
The concept of protein biosynthesis, function of three kinds of RNAs, the
characteristics of genetic code and the cooperation of several protein factors involved
in protein synthesis, the basic process of protein synthesis in prokaryotes. The
characteristics of initiation in translation in eukaryotes. The modification of primary
structure and conformation of newly synthesized polypeptide chains. The directed
transportation of protein (self-study).
2. The relationship between protein synthesis and medicine
The concept of molecule disease. The mechanism of antibiotics inhibition of
protein synthesis. The mechanism of interferon.
IV. Teaching period:
10h
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Lesson plan for page
Chapter 11 Protein Synthesis-Translation
REVIEW
What songs do the "notes of DNA" dictate?
So, now we've made mRNA in the nucleus. So where does this newly synthesized
molecule go from here? As you recall from the Central Dogma of Molecular
Genetics, the next step after transcription is translation, the process of making
proteins. Now that the mRNA has the DNA's instructions, the molecule must travel
OUT of the nucleus to the CYTOPLASM where protein synthesis takes place.
Before we continue with the process of translation, let's examine the "players" in this
process. The terms important for this process are:
the Ribosome
tRNA (transfer RNA)
the A site
the P site
Codons
Anticodons
Amino Acids
Now let's review what amino acids are. Amino acids are the building blocks of
proteins. Everything your DNA codes for is protein, so your DNA codes for amino
acids. There are only 20 amino acids total, but each one has a generalized
structure.
Each of the 20 different amino acids shares the amino group, the carboxyl group,
the Hydrogen atom, and the central Carbon atom. The only group which
differentiates them is the "R" group. R is simply a symbol for the side group.
There is the specialized apparatus for making proteins called the ribosome. There
are many ribosomes in the cytoplasm of a cell, and all the ribosomes are made of a
small subunit and a large subunit. These two subunits open up like a "pac-man"
allowing the mRNA message to slide through. Once the mRNA message is in
place and protein synthesis is ready to begin, the two subunits close again so that
the mRNA is now in between the two subunits.
The next player on the list is the tRNA (transfer RNA molecule). This molecule is
responsible for bringing in the proper amino acids. Remember, the mRNA is now
held within the two subunits of the ribosome and is relatively immobile. The
amino acids (which, you remember, are the building blocks of proteins) are
floating free in the cytoplasm.
So how can we bring the amino acids down to the mRNA?
This problem is solved by the action of tRNA. The tRNA molecule acts as a "taxi"
whose job is to read the code from the mRNA and bring the corresponding amino
acid
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into place. What do I mean by "corresponding" amino acid? Every tRNA molecule
has its own set of three bases which is called an anticodon. This anticodon is
complementary to mRNA codons. The other "end" of the tRNA molecule has an
"acceptor" site where the tRNA's specific amino acid will bind. The amino acid is
carried by the tRNA while attached to the 3'-terminal OH group. (terminate at the
3'-end with the sequence 5'-CCA-3')
Even though there are only 20 amino acids that exist, there are actually 64 possible
codons:
4 X 4 X 4 = 64 possible combinations
There are four choices of bases for the first space (A, U, G, or C), the same four
choices for the second space (you can repeat the same bases), and the same four
bases as a choice for the third spot. So, 4 x 4 x 4 is 64! 61 of the codons code for
specific amino acids and 3 code for chain termination as a result of pairing up with
"stop codons", signaling the end of the mRNA message. The table shows which
codons code for which amino acids:
The Genetic Code
U
C
A
G
UUU Phe
UUC Phe
U
UUA Leu
UUG Leu
UCU Ser
UCC Ser
UCA Ser
UCG Ser
UAU Tyr
UAC Tyr
UAA End
UAG End
UGU Cys
UGC Cys
UGA End
UGG Trp
C
CUU Leu
CUC Leu
CUA Leu
CUG Leu
CCU Pro
CCC Pro
CCA Pro
CCG Pro
CAU His
CAC His
CAA Gln
CAG Gln
CGU Arg
CGC Arg
CGA Arg
CGG Arg
A
AUU Ile
AUC Ile
AUA Ile
AUG Met
ACU Thr
ACC Thr
ACA Thr
ACG Thr
AAU Asn
AAC Asn
AAA Lys
AAG Lys
AGU Ser
AGC Ser
AGA Arg
AGG Arg
G
GUU Val
GUC Val
GUA Val
GUG Val
GCU Ala
GCC Ala
GCA Ala
GCG Ala
GAU Asp
GAC Asp
GAA Glu
GAG Glu
GGU Gly
GGC Gly
GGA Gly
GGG Gly
After looking at this chart, something should strike you...why does each amino acid
have more than one codon? Isn't one codon sufficient for each amino acid? In
theory, yes, this would be correct. But cellular processes do not occur in a perfect
world! What if the coding sequence in a particular codon should be GUA, but, due
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to a mutation, the coding sequence became GUC? What would happen? Check the
chart to find out!
Codon Feature
(1)The genetic code is degenerate (redundant). Each of the 20 common amino
acids has at least one codon; many amino acids have numerous codons. In some
cases, a single tRNA can recognize two or more of these synonymous codons.
Example: phenylalanine tRNA with the anticodon 3' AAG 5' recognizes not only
UUC but also UUU.
(2) The genetic code is non-overlapping (i.e. each nucleotide is used only once),
beginning with a start codon (AUG) near the 5` end of the mRNA and ending
with a termination codon (UAA, UAG, or UGA) near the 3` end.
(3) The code is commaless (i.e. there are no breaks or markers to distinguish one
codon from the next).
The code is unpunctuated .that is to say, once the reading is commenced at a
specific codon, there is no punctuation between codons, and the message is read in a
continuing sequence of nucleotide triplets until a stop codon is reached.
(4) The code is nearly universal. The same codon specifies the same amino acid
in almost all species studied; however , some differences have been found in the
codons used in mitochondria. Lastly, the Genetic Code in the table above has also
been called "The Universal Genetic Code". It is known as "universal", because it is
used by all known organisms as a code for DNA, mRNA, and tRNA. The
universality of the genetic code encompases animals (including humans), plants,
fungi, archaea, bacteria, and viruses.
(5) The start codon (AUG) determines the reading frame. Subsequent
nucleotides are read in sets of three, sequentially following this codon. The codon
AUG serves two related functions
It begins every message; that is, it signals the start of translation placing the amino
acid methionine at the amino terminal of the polypeptide to be synthesized.
When it occurs within a message, it guides the incorporation of methionine.
(6) The codon has the degeneracy and wobble properties .
The violation of the usual rules of base pairing at the third nucleotide of a codon is
called "wobble".
Most cells contain isoaccepting tRNAs, different tRNAs that are specific for the
same amino acid, however, many tRNAs bind to two or three codons specifying their
cognate amino acids. As an example yeast tRNAphe has the anticodon 5'-GmAA-3'
and can recognize the codons 5'-UUC-3' and 5'-UUU-3'. It is, therefore, possible for
non-Watson-Crick base pairing to occur at the third codon position, i.e. the 3'
nucleotide of the mRNA codon and the 5' nucleotide of the tRNA anticodon. This has
phenomenon been termed the wobble hypothesis.
OK, so your next question might be..
HOW IN THE WORLD CAN ONLY 20 AMINO ACIDS CREATE THE
PRACTICALLY INFINITE NUMBER OF PROTEINS PRESENT IN THE
BODY?!!??
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It seems impossible, doesn't it? The key to all the variety is that the 20 amino acids
can be linked in different combinations and in different numbers. For example,
alanine-valine-tryptophan........serine
is a different protein than
valine-serine-tryptophan........alanine
because the sequence is different, even though the same amino acids are represented.
Similarly, a protein made of 200 amino acids is quite different than a protein that is
2000 amino acids. The reason for this is because a protein's function is directly
related to its shape (which is related to its amino acid sequence). Thus, if you change
a protein's amino acid sequence, then you change its shape; and if you change the
protein's shape, you change its function!
So, the key to remember here is that the FUNCTION OF THE PROTEIN IS
DIRECTLY RELATED TO THE SEQUENCE OF AMINO ACIDS ! To go one step
further, the sequence of amino acids is related to the code on the mRNA molecule,
which is determined by the code on the DNA molecule itself! This is how DNA
eventually codes for proteins!!
Now you know WHY it's so important that the DNA code stays intact (no
mutations) because if you change the DNA, you change the mRNA, you change the
amino acids coded for, and thus, you change the protein! The problem is if you
change the protein, it usually renders the protein biologically inactive (in other
words, it won't work properly!).
As the term "anticodon" on tRNA implies, it is complementary to the codon on
mRNA. The codon is ALSO a set of three bases, but because the codon is found on
the mRNA molecule, it is called something different. So, let's review this…
A series of three nucleotide bases on a DNA molecule is called a triplet;
A set of three nucleotide bases on an mRNA molecule is called a codon; and
A set of three nucleotide bases on a tRNA molecule is called an anticodon.
You might be saying to yourself, "Isn't this just a case of the same thing being called
a different name depending on where it is?" YES, YOU ARE CORRECT! Try to
compare yourself to this example: You may be called by your first name here at
school, by a nick-name by someone you know well, and Mr. or Ms. on a job
interview. So, you are still the same person, you're just called a different name
depending on where you are!
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Activation of Amino Acids
Activation of amino acids is carried out by a two step process catalyzed by
aminoacyl-tRNA synthetases. Each tRNA, and the amino acid it carries, are
recognized by individual aminoacyl-tRNA synthetases. This means there exists at
least 20 different aminoacyl-tRNA synthetases, there are actually at least 21 since the
initiator met-tRNA of both prokaryotes and eukaryotes is distinct from non-initiator
met-tRNAs.
Activation of amino acids requires energy in the form of ATP and occurs in a two
step reaction catalyzed by the aminoacyl-tRNA synthetases. First the enzyme
attaches the amino acid to the a-phosphate of ATP with the concomitant release of
pyrophosphate. This is termed an aminoacyl-adenylate intermediate. In the second
step the enzyme catalyzes transfer of the amino acid to either the 2'- or 3'-OH of the
ribose portion of the 3'-terminal adenosine residue of the tRNA generating the
activated aminoacyl-tRNA. Although these reaction are freely reversible, the forward
reaction is favored by the coupled hydrolysis of PPi.
Now that we have charged aminoacyl-tRNAs and the mRNAs to convert
nucleotide sequences to amino acid sequences we need to bring the two together
accurately and efficiently. This is the job of the ribosomes. Ribosomes are
composed of proteins and rRNAs. All living organisms need to synthesis proteins
and all cells of an organism need to synthesize proteins, therefore, it is not hard to
imagine that ribosomes are a major constituent of all cells of all organisms. The
make up of the ribosomes, both rRNA and associated proteins are slightly different
between prokaryotes and eukaryotes.
Order of Events in Translation
The ability to begin to identify the roles of the various ribosomal proteins in the
processes of ribosome assembly and translation was aided by the discovery that the
ribosomal subunits will self assemble in vitro from their constituent parts.
Following assembly of both the small and large subunits onto the mRNA, and given
the presence of charged tRNAs, protein synthesis can take place. To reiterate the
process of protein synthesis:
1. synthesis proceeds from the N-terminus to the C-terminus of the protein.
2. the ribosomes "read" the mRNA in the 5' to 3' direction.
3. active translation occurs on polyribosomes (also termed polysomes). This means
that more than one ribosome can be bound to and translate a given mRNA at any one
time.
4. chain elongation occurs by sequential addition of amino acids to the C-terminal
end of the ribosome bound polypeptide.
Translation proceeds in an ordered process. First accurate and efficient initiation
occurs, then chain elongation and finally accurate and efficient termination must
occur. All three of these processes require specific proteins, some of which are
ribosome associated and some of which are separate from the ribosome, but may be
temporarily associated with it.
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Protein synthesis occurs in three stages: Initiation, Elongation and Termination.
INITIATION
Protein synthesis is initiated when an mRNA, a ribosome, and the first tRNA
molecule (carrying its Methionine amino acid) come together.
The ribosome is inactive when it exists as two subunits (a large one and a small
one) before it contacts an mRNA. The small unit of the ribosome will initiate the
process of translation when it encounters an mRNA in the cytoplasm.
The first AUG codon on the 5' end of the mRNA acts as a "start" signal for the
translation machinery and codes for the introduction of a methionine amino acid.
THIS CODON AND, THUS, AMINO ACID WILL ALWAYS BE THE FIRST IN
ANY AND ALL mRNA MOLECULES!!
Initiation is complete when the methionine tRNA occupies one of the two
binding sites on the ribosome. Since this first site is the site where the growing
peptide (another word for protein) will reside, it's known as the P site. This is where
the growing Protein will be. There is another site just to the 3' direction of the P
site; it is known as the A site. This is where the incoming tRNA will Attach itself.
Initiation of translation in both prokaryotes and eukaryotes requires a specific
initiator tRNA, tRNAmeti, that is used to incorporate the initial methionine residue
into all proteins. In E. coli a specific version of tRNAmeti is required to initiate
translation, [tRNAfmeti]. The methionine attached to this initiator tRNA is
formylated. Formylation requires N10-formy-THF and is carried out after the
methionine is attached to the tRNA. The fmet-tRNAfmeti still recognizes the same
codon, AUG, as regular tRNAmet. Although tRNAmeti is specific for initiation in
eukaryotes it is not a formylated tRNAmet.
Specific Steps in Translational Initiation
Initiation of translation requires 4 specific steps:
1. A ribosome must dissociate into its' 30S(small) and 50S(large) subunits.
2. The mRNA is bound to the small subunits.
The initiation of translation requires recognition of an AUG codon. In the
polycistronic prokaryotic RNAs this AUG codon is located adjacent to a
Shine-Delgarno element in the mRNA. The Shine-Delgarno element is recognized by
complimentary sequences in the small subunit rRNA (16S in E. coli). In eukaryotes
initiator AUGs are generally, but not always, the first encountered by the ribosome.
A specific sequence context, surrounding the initiator AUG, aids ribosomal
discrimination. This context is AGGAGG in most mRNAs.
A ternary complex termed the preinitiation complex is formed consisting of the
initiator, GTP, IF-2 and the 30S subunit.
3. The initiator tRNA can then bind to the complex at the P site paired with AUG
codon.
4. The 50S subunits can now bind. GTP is then hydrolyzed and IFs are released to
give the 70S initiation complex.
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ELONGATION
The incoming tRNA will bind to the A site (next to where the tRNA with the
methionine attached is on the P site). ALL available tRNAs will approach the site
and try to attach, but the only tRNA which will successfully attach is the one whose
anticodon IS COMPLEMENTARY to the codon of the A site on the mRNA.
Let's say for example that the second tRNA that lands next to the
methionine-tRNA is for leucine. The two tRNAs (holding methionine on the P site
and leucine on the A site) are now next to each other. What happens next is crucial
for the building of proteins.
In order for a protein chain to form, the amino acids must be attached, linked
together. The link between amino acids is called a PEPTIDE BOND.
Amino acids continue to be linked until the protein is finished. This special type
of bond is formed by the enzyme PEPTIDASE. Once the bond has formed between
the two amino acids, the tRNA on the P site leaves and passes its amino acid on to
the tRNA on the A site. Now something interesting occurs!
The tRNA with the two amino acids on it is now sitting on the P site (because it
is holding the growing protein!). The ribosome slides down three bases (1 codon on
the mRNA) exposing a new A site by the action of a TRANSLOCASE! The next
appropriate tRNA molecule "lands" bringing its amino acid right next to the tRNA
holding the two amino acids. At this point, the process repeats itself: a peptide bond
forms between the two amino acid molecules already joined together and the newly
brought in amino acid; the tRNA on the P site leaves and the chain of amino acids is
passed to the tRNA on the A site by the action of translocase (now this site is called
the P site because this tRNA now has the growing protein chains). The ribosome
slides down another codon and the procedure repeats itself until the termination
event occurs. This hyperlink leads to an overview of the process, and try this
hyperlink to see an electron micrograph of the process of translation!
Entrance
Peptide bond formation
Translocation
TERMINATION
The elongation procedure continues until the proper protein is completed. A "stop"
codon (UAA, UGA, or UAG) signals the end of the process. There is no tRNA
that is complementary to the Stop Codon, so the process of building the protein
stops. An enzyme called the releasing factor then frees the newly made
polypeptide chain, also known as the PROTEIN, from the last tRNA. In E. coli the
termination codons UAA and UAG are recognized by RF-1, whereas RF-2
recognizes the termination codons UAA and UGA.The mRNA molecule is
released from the ribosome as the small and large subunits fall apart. The mRNA
can then be re-translated or it may be degraded, depending on how much of that
particular protein is needed. All mRNA messages are eventually degraded when
the protein no longer needs to be made.
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Most of the differences in the mechanism of protein between prokaryotes and
eukaryotes occur in the initiation stage, where a greater numbers of eIFs and
a scanning process are involed in eukaryotes.
The eukaryotic initiator tRNA does not become N-formylated.
Initiation of translation requires 4 specific steps:
1. A ribosome must dissociate into its' 40S and 60S subunits.
2. A ternary complex termed the preinitiation complex is formed consisting of
the initiator, GTP, eIF-2 and the 40S subunit.
3. The mRNA is bound to the preinitiation complex.
4. The 60S subunit associates with the preinitiation complex to form the 80S
initiation complex.
The initiation factors eIF-1 and eIF-3 bind to the 40S ribosomal subunit
favoring antiassociation to the 60S subunit. The prevention of subunit
reassociation allows the preinitiation complex to form.
The first step in the formation of the preinitiation complex is the binding of
GTP to eIF-2 to form a binary complex. eIF-2 is composed of three subunits,
,  and . The binary complex then binds to the activated initiator tRNA,
met-tRNAmet forming a ternary complex that then binds to the 40S subunit
forming the 43S preinitiation complex. The preinitiation complex is stabilized
by the earlier association of eIF-3 and eIF-1 to the 40S subunit.
Phosphorylation of eIf2, which delivers the initiation tRNA, is an important
control point.
The cap structure of eukaryotic mRNAs is bound by specific eIFs prior to
association with the preinitiation complex. Cap binding is accomplished by
the initiation factor eIF-4F.
Eukaryotes use only one release factors eRF, which requires
GTP,recognize all three termination codons.
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Protein Synthesis: Folding, Modification, Targeting and Degradation
Protein Folding
As they are being synthesized, proteins must adopt the correct conformation for
their function.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.There are 3 major classes of
chaperones:
1.The Hsp70 family
DnaK is the best known member of this family in E. coli.
2.The Hsp 60 family
The E. coli member of this family, GroEL forms a double-layered cylindrical
structure which provides a protected environment in which proteins can fold. The
structure is capped at one end by another heat-shock protein: GroES, a member of the
Hsp 10 family.
3.The Hsp 90 family
All are named because these proteins were first identified as Heat Shock Proteins their synthesis increased in response to a sudden rise in temperature. In this role,
chaperones protect important cell proteins from denaturation as a result of a sudden
rise in temperature.
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 have been synthesized:
1.Cleavage
To remove signal peptide
To release mature fragments from polyproteins
To remove internal peptide as well as trimming both N-and C-termini
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.Covalent modification
Many of the amino-acid side-chains can be modified. Here are some examples:
(1)Acetylation
The amino-terminal residues of some proteins are acetylated. For example, the
N-terminal serine of histone H4 is invariably acetylated as are a number of lysine
residues.
(2)Hydroxylation
The conversion of proline to hydroxyproline in collagen is the classical example of a
post-translational modification.
This conversion is catalyzed by prolyl-4-hydroxylase which is a tetramer of two a
and two b subunits; the b subunits are multifunctional and also carry disulfide
isomerase activity.
(3)Phosphorylation
Phosphorylation of proteins (at Ser, Thr, Tyr and His residues) is an important
regulatory mechanism. For example, the activity of glycogen phosphorylase is
regulated by phosphorylation of Serine 14.Phosphorylation of tyrosine residues (in
particular) is an important aspect of signal transduction pathways.
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(4)Methylation
The activity of histones can be modified by methylation. Lysine 20 of histone H4
can be mono- or di- methylated.
(5)Glycosylation
Many extracellular (but not intracellular) proteins are glycosylated. Mono- or
Oligo-saccharides can be attached to asparagine (N-linked) or to serine/threonine
(O-linked) residues.
(6)Nucleotidylation
Mononucleotide addition is used to regulate the activity of some enzymes.
Glutamine synthetase is adenylylated (i.e. AMP is added) at a specific tyrosine
residue. The enzyme is inactive when it is adenylylated.
(7)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.
The prototype is insulin, which is a low-molecular-weight protein having two
polypeptide chains with interchain and intrachain disulfide bridges. The molecule is
synthesized as a single chain precursor, or prohormone, which folds to allow the
disulfide bridges to form. A specific protease then clips out the segment that connects
the two chains which form the functional insulin molecule.
(8)Trimming
Large precursor molecules of secretory proteins are acted upon by endoproteases
in secretory vesicles or after secretory (e.g. zymogens) to release an active molecule.
Protein targeting
Both in prokaryotes and eukaryotes, newly synthesized proteins must be delivered
to a specific subcellular location or exported from the cell for correct activity. This
phenomenon is called protein targeting.
Protein degradation
Different proteins have very different half-lives. Regulatory proteins tend to turn
over rapidly and cells must be able to dispose of faulty and damaged proteins.
Protein Synthesis Inhibitors
Ribosomes in bacteria and in the mitochondria of higher eukaryotic cells differ
from the mammalian ribosome. This difference is exploited for clinical purposes
because many effective antibiotics interact specifically with the proteins and RNAs
of prokaryotic ribosomes and thus inhibit protein synthesis. This results in growth
arrest or death of the bacterium.
Many of the antibiotics utilized for the treatment of bacterial infections as well as
certain toxins function through the inhibition of translation. Inhibition can be effected
at all stages of translation from initiation to elongation to termination.
1.Several Antibiotic and Toxin inhibitors of Translation：
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Inhibitor
Comments
Chloramphenicol
inhibits prokaryotic peptidyl transferase
Streptomycin
inhibits prokaryotic peptide chain initiation, also
induces mRNA misreading
Tetracycline
inhibits prokaryotic aminoacyl-tRNA binding to
the ribosome small subunit
Neomycin
similar in activity to streptomycin
Erythromycin
inhibits prokaryotic translocation through the
ribosome large subunit
Fusidic acid
similar to erythromycin only by preventing EF-G
from dissociating from the large subunit
Puromycin
resembles an aminoacyl-tRNA, interferes with
peptide transfer resulting in premature termination
in both prokaryotes and eukaryotes
Diptheria toxin
catalyzes ADP-ribosylation of and inactivation of
eEF-2
Ricin
Cycloheximide
found in castor beans, catalyzes cleavage of the
eukaryotic large subunit rRNA
inhibits eukaryotic peptidyltransferase
2.Diphtheria toxin, an exotoxin of Corynebacterium diphtheriae infected with a
specific lysogenic phage, catalyzes the ADP-ribosylationof EF-2 on the unique
amino acid diphthamide in mammalian cells. This modification inactivates EF-2 and
thereby specifically inhibits mammalian protein synthesis. Many animals
(eg, mice) are resistant to diphtheria toxin. This resistance is due to inability of
diphtheria toxin to cross the cell membrane rather than to insensitivity of mouse EF-2
to diphtheria toxin-catalyzed ADP-ribosylation by NAD.
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3.Interferon
Regulation of translation can also be induced in virally infected cells. It would
benefit a virally infected cell to turn off protein synthesis to prevent propagation of
the viruses. This is accomplished by the induced synthesis of interferons (IFs).
Interferons (IFNs) are natural proteins produced by the cells of the immune system of
most vertebrates in response to challenges by foreign agents such as viruses, bacteria,
parasites and tumor cells.There are 3 classes of IFs. The leukocyte or α-IFs, the
fibroblast or β-IFs and the lymphocyte or γ-IFs. IFs are induced by dsRNAs and
themselves induce a specific kinase termed RNA-dependent protein kinase (PKR)
that phosphorylates eIF-2 thereby shutting off translation in a similar manner to that
of heme control of translation. Additionally, IFs induce the synthesis of
2'-5'-oligoadenylate, pppA(2'p5'A)n, that activates a pre-existing ribonuclease,
RNase L. RNase L degrades all classes of mRNAs thereby shutting off translation.
Summary
Translation is the RNA directed synthesis of polypeptides. This process
requires all three classes of RNA. Although the chemistry of peptide bond
formation is relatively simple, the processes leading to the ability to form a
peptide bond are exceedingly complex. The template for correct addition of
individual amino acids is the mRNA, yet both tRNAs and rRNAs are involved
in the process. The tRNAs carry activated amino acids into the ribosome
which is composed of rRNA and ribosomal proteins. The ribosome is
associated with the mRNA ensuring correct access of activated tRNAs and
containing the necessary enzymatic activities to catalyze peptide bond
formation. Activation of amino acids requires energy in the form of ATP and
occurs in a two step reaction catalyzed by the aminoacyl-tRNA synthetases.
First the enzyme attaches the amino acid to the a-phosphate of ATP with the
concomitant release of pyrophosphate. The ability to begin to identify the
roles of the various ribosomal proteins in the processes of ribosome
assembly and translation was aided by the discovery that the ribosomal
subunits will self assemble in vitro from their constituent parts. Following
assembly of both the small and large subunits onto the mRNA, and given the
presence of charged tRNAs, protein synthesis can take place. To reiterate the
process of protein synthesis:
1. synthesis proceeds from the N-terminus to the C-terminus of the protein.
2. the ribosomes "read" the mRNA in the 5' to 3' direction.
3. active translation occurs on polyribosomes (also termed polysomes). This
means that more than one ribosome can be bound to and translate a given
mRNA at any one time.
4. chain elongation occurs by sequential addition of amino acids to the
C-terminal end of the ribosome bound polypeptide.
Lesson plan for page
（Content）
Translation proceeds in an ordered process. First accurate and efficient
initiation occurs, then chain elongation and finally accurate and efficient
termination must occur. All three of these processes require specific proteins,
some of which are ribosome associated and some of which are separate
from the ribosome, but may be temporarily associated with it. Protein
synthesis occurs in three stages: Initiation, Elongation and Termination.
As they are being synthesized, proteins must adopt the correct
conformation for their function.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. 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
have been synthesized.
Ribosomes in bacteria and in the mitochondria of higher eukaryotic cells
differ from the mammalian ribosome. This difference is exploited for clinical
purposes because many effective antibiotics interact specifically with the
proteins and RNAs of prokaryotic ribosomes and thus inhibit protein
synthesis. This results in growth arrest or death of the bacterium.
Many of the antibiotics utilized for the treatment of bacterial infections as
well as certain toxins function through the inhibition of translation. Inhibition
can be effected at all stages of translation from initiation to elongation to
termination.
Definition:
1 Central dogma
2 Reverse transcription
3 Exon
4 Intron
5 Open reading frame
6 holoenzyme and corn enzyme
Short essay question:
1.What kind of enzymes and proteins involve in the DNA replication?
2.Listing and deseribing the function and structure character of three kinds
of RNA.
3.What kinds of properties do the genetic codons have?
4. Comparing with replication, transcription and translation from the
following parts: synthetic direction, manner, template, precursor and end
product.