Topic 19: Translation Elongation

Elongation factors

• catalyze formation of peptide bonds

• Use energy from GTP

• Guanosine Triphosphate (GTP) can have a phosphate group cleaved off becoming Guanosine Diphosphate (GDP)

• This releases an inorganic phosphate (Pi) and energy

• The energy can be used to power other chemical reactions such as creating a peptide bond between amino acids.

Elongation (I)

A tRNA complementary to the next codon binds to the A site

Elongation (II)

• (3) A reaction transfers the Met to the amino acid on the tRNA in the A site, forming a peptide bond

• (4) The ribosome moves down one codon, which puts the tRNA carrying the polypeptide into the P site and the now uncharged tRNA into the E site, where it is released. A new tRNA complementary to the next codon binds to the A site

Elongation (III)

The polypeptide transfers to the amino acid on the tRNA in the A site. The polypeptide is elongated by repeating steps 3 and 4.

Name each tRNA binding site found on the large ribosomal subunit in order from the one closest to the 5’ end of the mRNA to the one closest to the 3’ end of the mRNA. 

For each, explain what happens to the tRNA and amino acid it carries when the tRNA is bound to that site.

Note: order of the sites is important

• E site - tRNA without amino acid (okay to say uncharged) exits

• P site - tRNA with growing polypeptide (okay to say protein, chain of amino acids or similar) binds here and will be transferred to the amino acid in the A site.

• A site - tRNA with complimentary anticodon and charged with corresponding amino acid binds. Growing polypeptide on tRNA in P site is transferred to the amino acid. _This does NOT occur at the P site

Where does the energy for the peptide bonds in a growing protein come from?

The energy comes from GTP.

Explain the function of elongation factors.

Elongation factors catalyze formation of peptide bonds using energy from GTP.

Elongation involves the formation of polymer from subunits. What type of macromolecule is the polymer? What are the subunits? What is the bond that is formed between the subunits?

Elongation is the formation of a protein (can say polypeptide instead) from amino acid subunits through the formation of peptide bonds.