1. Step 2 of Protein Synthesis
Translation
Step 2 of Protein Synthesis

2. Molecular Components
Transfer RNA: takes amino acids from the cytoplasm to a ribosome
Each tRNA carries a specific amino acid at one end and has an anticodon on the other end.
Anticodon: a nucleotide triplet which base- pairs with a complementary codon on mRNA.
tRNA is a translator because it reads a nucleic acid word (mRNA codon) and interprets it as a protein word (amino acid).

3. Molecular Components The structure and function of tRNA:
Job – to take a specific amino acid to the ribosome, then go pick up another from the cytosol
Looks like a cloverleaf if flattened into a 2D structure due to the H-bonding between bases
Its 3D shape is roughly an “L.”
An enzyme called aminoacyl-tRNA synthetase attaches a given amino acid to the appropriate tRNA. There is a different synthetase for each amino acid.
tRNAs can sometimes match multiple codons because of the third base pair not needing to match exactly (“wobble”)

5. Molecular Components Ribosomes
Facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis
2 subunits: large and small
Made of proteins and rRNA (ribosomal RNA)
Cells contain thousands of ribosomes, so rRNA is the most abundant type of RNA.
Since prokaryotic ribosomes are different from eukaryotic ribosomes, antibiotic drugs are able to inactivate them without inhibiting the ability of eukaryotic cells to make proteins.

6. Molecular Components Ribosomes Each has 3 binding sites:
P site: (peptidyl-tRNA site) holds the tRNA carrying the growing polypeptide chain
A site: (aminoacyl-tRNA site) holds the tRNA carrying the next amino acid to be added to the chain
E site: (exit site) where discharged tRNAs leave the ribosome
The ribosome forms peptide bonds between the amino acids.

8. Building a Polypeptide
3 stages:
Initiation –
The small ribosomal unit binds to an initiator tRNA (carrying Methionine) and mRNA, and then finds the start codon (AUG);
Next, proteins called initiation factors bring this complex and the large ribosomal unit together;
Then, the initiator tRNA is in the P site.

9. Building a Polypeptide
Elongation
Involves proteins called elongation factors
Uses energy from GTP (like ATP)
mRNA is moved through the ribosome in the 5’ to 3’ direction only
Many ribosomes may trail along one mRNA at a time (polyribosomes)

10. Building a Polypeptide
Termination
Stop codon reaches the A site (triplets do not code for amino acids)
A protein called a release factor binds to the stop codon and causes the addition of a water molecule instead of an amino acid
The polypeptide is released
Translation assembly comes apart

11. Completing and Targeting the Functional Protein
Protein Folding and Post-translational modifications:
A protein may begin to coil and fold spontaneously.
Chaperone proteins (“chaperonins”) usually help the polypeptide fold correctly.
Examples of post-translational modifications before the protein can do its job… attachment of sugars, lipids, phosphate groups; removal of amino acids from the leading end; enzymatic cleaving into 2 or more pieces

12. Completing and Targeting the Functional Protein
Targeting Polypeptides to Specific Locations:
2 kinds of ribosomes –
Free: suspended in cytosol and mostly synthesize proteins that dissolve in the cytosol and function there
Bound: attached to the cytosolic side of the endoplasmic reticulum or nuclear envelope; make proteins of the endomembrane system as well as proteins secreted from the cell

13. Completing and Targeting the Functional Protein
Targeting Polypeptides to Specific Locations:
The above ribosomes are identical and can switch their status from free to bound.
Polypeptide synthesis always starts in the cytosol with a free ribosome.
If a protein that are destined for the endomembrane system are marked by a signal peptide.
As it emerges from the ribosome, a signal- recognition particle (SRP) recognizes it and brings the ribosome to a receptor protein built into the ER.