Download Protein Synthesis II

PROTEIN SYNTHESIS
! Translation of nucleic acid information
RNA components of protein synthesis
! Messenger RNA (mRNA):
gene-specific information
! Transfer RNA (tRNA):
codon decoders
! Ribosomal RNA (rRNA):
peptide bond synthesis
Transfer RNA (tRNA)
! Transcribed from special tRNA genes (about 40;
one for each codon, one or more for each amino
acid).
! Matches codons with amino acids (called
“adaptor” or “translator” molecule).
! Generally a cloverleaf with secondary and
tertiary structure; has “anticodon” at one end (3
bases complementary to codon), corresponding
amino acid hooked onto the other end.
Three bases make a “codon.”
(Degenerate, but not ambiguous)
Charging the transfer RNAs
! Activating enzyme; aa-tRNA synthetase
! Joins amino acids to tRNAs with the correct
anticodons
! Provides some of the energy to form the peptide
bond
How many activating enzymes would you expect a cell to have?
Ribosome structure
Each ribosome consists of a large and small subunit
Each subunit consists of 1 or 2 RNA
molecules and 35-45 proteins
Ribosome structure
Each ribosome consists of a large and small subunit
Ribosome structure
On an exam, you will probably
See the ribosome with only two
binding sites--the A and P sites
Initiation
Translation begins
with the formation
of an initiation
complex
Elongation
The growing polypeptide chain is added to
each new amino acid; then the ribosome
moves one codon forward.
Two steps skipped here
Crystal structures of the
70S ribosome with
translation elongation
factors EF-Tu and EF-G.
During protein synthesis,
EF-Tu (periwinkle blue)
delivers an aminoacyl
tRNA (green)
to the ribosome for each
amino acid indicated by
the mRNA. EF-G helps
move the mRNA and
tRNAs through the
ribosome.
What enzyme forms
the peptide bond
between the two
amino acids?
The RNA of the large
subunit is the catalyst!
Termination
When the ribosome reaches a stop codon,
a release factor disconnects the complex.
Polysomes
Simultaneous protein synthesis by
several ribosomes using one mRNA
Many proteins are modified after translation
Common modifications:
•Proteolysis: ER import, enzyme activation
•Glycosylation: of secreted proteins
•Phosphorylation: enzyme activation or inactivation
Compartmentation
Summary
•mRNA provides the information (code) for a protein
•tRNA translates the code, codon by codon, amino acid
by amino acid
•rRNA (in ribosomes) holds the mRNA and tRNAs
together and forms the peptide bonds
•Post-translational processing may be needed to produce
a finished protein
•Where do the proteins go?
Compartmentation is an
another--and important--issue, especially in eukaryotes