Chapter 8.3: Translation: From mRNA to Protein Flashcards
What are posttranslational modifications?
o Enzymatic addition of chemical constituent or even other small peptides to specific amino acids may also modify a polypeptide after translation Such changes to polypeptides are known as posttranslational modifications
Describe initiation of translation
A special signal indicated where along the mRNA translation should begin. In prokaryotes, this signal is called the ribosome binding site, and it has two important elements. The first, is a short sequence of six nucleotides—usually 5’…AGGAGG…3’—named the Shine-Dalgarno box after its discoverers. The second element in an mRNA’s ribosome binding site is the triplet 5’AUG 3’, which serves as the initiation codon. A special initiator tRNA, whose 5’ CAU 3’ anticodon is complementary to AUG, recognizes an AUG preceded by the Shine-Dalgarno box of a ribosome binding site. The initiator carries N-formylmethionine (fMet), a modified methionine whose amino end is blocked by a formyl group. The specialized fMet tRNA functions only at an initiation site. During initiation, the 3’ end of the 16S rRNA in the 30S ribosomal subunit binds to the mRNA’s Shine-Dalgarno box, the fMet tRNA binds to the mRNA’s initiation codon, and a large 50S ribosomal subunit associates with the small subunit to round out the ribosome. At the end of initiation, the fMet tRNA sits in the P site of the completed ribosome. Proteins known as initiation factors play a transient rile in the initiation process. In eukaryotes, the small ribosomal subunits bind first to the methylated cap at the 5’ end of the mature mRNA. It then migrates to the initiation site—usually the first AUG it encounters as it scans the mRNA in the 5’ to 3’ direction. The initiator tRNA in eukaryotes carries unmodified methionine (Met) instead of fMEt.
What are aminoacyl-tRNA synthetases?
o At the other end of the L, where the 5’ and 3’ ends of the tRNA strand are found, enzymes known as aminoacyl-tRNA synthetases connect the tRNA to the amino acid that corresponds to the anticodon
What is a charged tRNA?
o A tRNA covalently couple to its amino acid is called a charged tRNA
What is the function of the 30S ribosome subunit?
The small 30S subunit is the part of the ribosome that initially binds to mRNA
Define wobble
o The flexibility in base pairing between the 3’ nucleotide in the codon and the 5’ nucleotide in the anticodon is known as wobble
Define polyprotein
o The larger polypeptide made before it is cleared into smaller polypeptides is called a polyprotein
What is a polyribosome?
A complex of several ribosomes translating from the same mRNA is called a polyribosome
What is the anticodon?
o At one end of the L, the tRNA carries an anticodon: 3 nucleotides complementary to an mRNA codon specifying the amino acid carried by the tRNA
Both ribosomal subunits contribute to what?
Both subunits contribute to 3 distinct tRNA binding areas known as the aminoacyl (or A) site, the peptidyl (or P) site, and the exit (or E) site
What does the A site stand for?
aminoacyl site
What does the E site stand for?
exit site
What are zymogens?
Some proteins are synthesized in inactive forms called zymogens that are activated by enzymatic cleavage that removes an N-terminal prosegment
Describe the structure of ribosomes
Two different ribosomal subunits called the 30S subunit and the 50S subunit (with S designating a coefficient of sedimentation related to the size and shape of the subunit; the 30S subunit is smaller than the 50S subunit)
What does the P site stand for?
peptidyl site
Describe elongation of translation
Proteins known as elongation factors usher the appropriate tRNA into the A site of the ribosome. The ribosome simultaneously holds the initiating tRNA at its P site and the second tRNA at it’s a site so the peptidyl transferase can catalyze formation of a peptide bond between the amino acids carried by two tRNAs. As a result, the tRNA at the A site now carries two amino acids. The N terminus of this dipeptide is fMet; the C terminus is the second amino acid, whose carboxyl group remains covalently linked to its tRNA. Following formation of the first peptide bond, the ribosome moves exposing the next mRNA codon. The ribosome’s movement requires the help of elongation factors and an input of energy. As the ribosome moves, the initiating tRNA, which no longer carries an amino acid, is transferred to the E site, and the other tRNA carrying the dipeptide shifts from the A site to the P site. The empty A site now receives another tRNA, whose identity is determined by the next codon in the mRNA. The uncharged initiating tRNA is bumped off the E site and leaves the ribosome. Peptidyl transferase then catalyzes formation of the second peptide bond, generating a chain of three amino acids connected at its C terminus to the tRNA currently in the A site. With each subsequent round of ribosome of ribosome movement and peptide bond formation, the peptide chain grows one amino acid longer. Note that each tRNA moves from A to P to E (except initiating tRNA which first enters the P site) Polypeptide synthesis proceeds from the N terminus to the C terminis. The ribosome must move along the mRNA in the 5’-to-3’ direction so that the polypeptide cab grow in the N-to-C direction Once a ribosome has moved far enough away from the mRNA’s binding site, that site becomes accessible to other ribosomes. In fact, several ribosomes can work on the same mRNA at one time. A complex of several ribosomes translating from the same mRNA is called a polyribosome. This complex allows the simultaneous synthesis of many copies of a polypeptide from a single mRNA.
Describe termination of translation
When movement of the ribosome brings a nonsense codon into the ribosome’s A site, no tRNAs can bind to that codon. Instead, proteins called release factors recognize the termination codons and bring polypeptide synthesis to a halt. The tRNA specifying the C-terminal amino acid releases the completed polypeptide, the same tRNA as well as the mRNA separate from the ribosome, and the ribosome dissociates into its large and small subunits
