Biovet 2 - Kuliah - Translasi - Slide Kuliah Translasi (PDF)

8/17/2019 Biovet 2 - Kuliah - Translasi - Slide Kuliah Translasi (PDF)

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T R A N S L A T I O N


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Degenerate DNA Code

Nucleotides read 3 at a time meaning

that there are 64 combinations for a

codon (set of 3 nucleotides)

Only 20 amino acids


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Reading

Frames

Translation can occur in 1 of 3 possiblereading frames, dependent on wheredecoding starts in the mRNA


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tRNA

Molecules

Translation requires anadaptor molecule thatrecognizes the codonon mRNA and at adistant site carries theappropriate amino acid

Intra-strand basepairing allows for thischaracteristic shape

Anticodon is oppositefrom where the aminoacid is attached


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Wobble

Base Pairing

Due to degenerate code for amino acidssome tRNA can recognize several codons

because the 3rd spot can wobble or be

mismatched

Allows for there only being 31 tRNA for the

61 codons


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Attachment of amino-acid to tRNA

Aminoacyl-tRNA synthase is the enzyme

responsible for linking the amino acid to

the tRNA

A specific enzyme for each amino-acid

and not for the tRNA


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Two ‘Adaptors’ Translate

Genetic Code to Protein

1

2


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Ribosomal Subunits

One large subunit – catalyzes the formation of thepeptide bond

One small subunit – matches the tRNA to the mRNA

Moves along the mRNA adding amino acids to growing

protein chain


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Ribosomal Movement

Four binding sites

mRNA binding site

Peptidyl-tRNA binding site (P-site) Holds tRNA attached to growing end of the peptide

Aminoacyl-tRNA binding site ( A-site)

Holds the incoming amino-acid

Exit site (E-site)

E-site


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Three Steps

Elongation Phase Elongation is a cycle of events

Step 1: aminoacyl-tRNA comes into empty A-site

next to the occupied P-site; pairs with the codon


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Three Steps Elongation Phase Step 2: C’ end of peptide chain uncouples from

tRNA in P-site and links to amino-acids in A-site

Peptidyl transferase responsible for bond formation

Each amino-acid added carries the energy for the

addition of the next amino-acid


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Three Steps Elongation Phase

Step 3: peptidyl-tRNA moves to the P-site;

requires hydrolysis of GTP

tRNA released back to the cytoplasmic pool


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Initiation Process

Determines whether mRNA is synthesizedand sets the reading frame that is used

to make the protein

Initiation process brings the ribosomalsubunits together at the site where the

peptide should begin

Initiator-tRNA brings in Met Initiator tRNA is different than the tRNA that

adds other Met


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Ribosomal Assembly

Initiation Phase

Initiation factors (IFs) catalyze the steps not well defined

Step 1: small

ribosomal subunit withthe IF finds the startcodon (AUG) Moves 5’ to 3’ on mRNA

Initiator tRNA brings in

the first amino-acid whichis always Met and thencan bind the mRNA


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Ribosomal Assembly

Initiation Phase

Step 2: IF leavesand then largesubunit can bind protein synthesis

continues Met is at the start of

every protein untilpost-translational

modification takesplace


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Eukaryotic

vs

Procaryotic

Procaryotic

No CAP and have specific ribosome binding siteupstream of AUG

Polycistronic: multiple proteins from same mRNA

Eucaryotic

Monocistronic: only one polypeptide for each mRNA


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Protein Release

Protein released when aSTOP codon isencountered (UAG, UAA,UGA)

Cytoplasmic release factors bind to the stop codonthat gets to the A-site;alters the peptidyl

transferase and adds H2Oinstead of an amino-acid


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Protein Release

Protein released andthe ribosome breaksinto the 2 subunitsto move on to

another mRNA


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Polyribosomes As the ribosomemoves down the

mRNA, it allows

for the addition of

another ribosome

and the start ofanother protein

Each mRNA has

multiple ribosomes

attached,polyribosome or

polysome


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Regulation of Protein Synthesis Lifespan of proteins vary,

need method to remove oldor damaged proteins

Enzymes that degradeproteins are called

proteases process iscalled proteolysis

In the cytosol there arelarge complexes of

proteolytic enzymes thatremove damaged proteins

Ubiquitin, small protein, isadded as a tag for disposal

of protein

Biovet 2 - Kuliah - Translasi - Slide Kuliah Translasi (PDF)

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