Ch 19: Protein Synthesis

______ is the first & most important part of protein synthesis

Translation

______ catalyze the joining of the amino acid monomers directed by the mRNA sequence

Ribosomes

________ attach amino acids to the appropriate tRNAs.

Amino-acyl tRNA synthetases

The________ act as adaptors in the translation of the nucleic acid sequence of the mRNA into the amino acid sequence of the protein.

amino-acyl tRNA

Translation occurs in three stages... 

molecule (a tRNA carrying the first amino acid binds to the start codon). 
2) In elongation, amino acids are brought to the mRNA as amino-acyl tRNAs and are added one at a time to a growing polypeptide chain. 
3) In termination, a stop codon in the mRNA is recognized by a protein release factor and the translational apparatus comes apart to release a completed polypeptide.

The model of ribosome structure shows the __ and ___ sites as cavities on the ribosome where charged ___ (carrying ____) molecules bind during __________

A (aminoacyl) and P (peptidyl)

tRNA

an amino acid

polypeptide synthesis.

The recently postulated_____ site is the site from which discharged tRNAs leave the ribosome.

E (exit)

tRNA molecules contain 4 Things:

1) three major loops,

2) four base-paired regions,

3) an anticodon triplet and

4) a 3 prime terminal sequence of CCA (where the appropriate amino acid can be attached by an ester bond).

The 3D structure of ___ molecules is similar to a hockey stick (the L shaped tertiary structure) which has the ______site is at one end (tip of the handle) and the ____ at the other (blade of the stick)

tRNA

amino acid attachment

anticodon

The modified nucleotides in the tRNA structure are:

inosine (I), methylinosine (mI), dihydrouridine (D), ribothymidine (T), pseudouridine (¥) and methylguanosine (G^m)

Although there are __ possible codons, there are far fewer tRNAs. 

61

A number of codons that encode the same amino acid differ only in the __ position of the codon. 

third

A slight shift or “wobble” in the position of the base ___ in a tRNA anticodon would permit it to pair with ___ instead of its normal complementary base (___) and other similar unusual pairings.

guanine

uracil

cytosine

In two main chemical steps, aminoacyl-tRNA synthetases catalyzes the formation of an ester bond between the carboxyl group of an amino acid and the 3 prime hydroxyl (OH) group of the appropriate tRNA

Step 1 (Activation):

Amino acid + ATP → aminoacyl-AMP (activated amino acid) + pyrophosphate (PPi → 2 Pi)

Step 2 (Transfer):

Activated amino acid is transferred to tRNA → aminoacyl-tRNA + AMP released

3 Steps of Translation

Consists of the stages initiation, elongation and termination

The leader contains a ribosome binding site (or ______) sequence.

Shine-Dalgarno

The rest of the mRNA contains a coding sequence that starts with an _____ start codon and ends with a stop codon (_____)and a 3 prime non-coding trailer sequence.

AUG

UAA, UAG or UGA)

eukaryotic mRNAs lack a ____________

ribosome binding site (SD site).

The formation of the 70S translation initiation complex occurs in three steps:

1) Three initiation factors (IF) and GTP bind to the small ribosomal subunit. 
2) The initiator aminoacyl tRNA and mRNA are attached. 
The mRNA-binding site is composed, at least in part, of a portion of the 16S rRNA of the small ribosomal subunit. 
The 3 prime end of the 16S rRNA bears a pyrimidine-rich stretch that base pairs with the Shine-Dalgarno sequence of the mRNA (in prokaryotes). 
3) The large ribosomal subunit joins the complex. 

The resulting 70S initiation complex has fMet-tRNA^fMet residing in the ribosome's P site. 

Translation Elongation Step 1: Elongation begins when the second __________ __________ binds to the __________ site, escorted by __________ carrying __________ GTP; GTP is __________, releasing __________, and __________ recycles it.
-- aminoacyl tRNA

A site
EF-Tu
2
hydrolyzed
EF-Tu
EF-Ts

Translation Elongation Step 2: A peptide bond forms between the __________ group (P site) and __________ group (A site), catalyzed by __________ activity of __________ rRNA in the __________ subunit.

carboxyl
amino
peptidyl transferase
23S
large ribosomal

Translation Elongation Step 3: __________-GTP drives translocation: peptidyl tRNA moves from __________ to __________, empty tRNA moves from __________ to __________ and exits; __________ shifts, bringing the next __________ into the __________ site.

EF-G
A site
P site
P site
E site
mRNA
codon
A site

The heat shock proteins, _____, are molecular chaperones. 

Hsp70 and Hsp60

Prion diseases, such as_____, may "self-propagate" based upon a misfolded protein that can, in turn, mis-fold other versions of the same protein.

"mad cow" disease`

After the amino acid chain is made, proteins often undergo __________ __________, including removal of __________ of amino acids.

posttranslational processing
stretches