ch 5:Protein Synthesis – Amino Acids, Translation & Protein Sorting

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Amino group

–NH₂

Acts as a base; binds to carboxyl of another amino acid

Carboxyl group

–COOH

Acts as an acid; forms peptide bond

R group

Variable

Determines chemical behavior and protein folding

Peptide bond

covalent bond between amino group (–NH₂) of one amino acid and carboxyl group (–COOH) of another.

dehydration synthesis

removal of H₂O

Polypeptide chain

long chain of amino acids.

Codon:

3-base sequence on mRNA that corresponds to a specific amino acid.

• Example: AUG = Methionine (start codon)

Start codon:

AUG (begins translation)

Stop codons:

UAA, UAG, UGA (end translation)

Amino acid attachment site (3′ end)

Attaches to specific amino acid by an enzyme called aminoacyl-tRNA synthetase.

Anticodon

3-base sequence complementary to mRNA codon; ensures correct amino acid is added.

mRNA

Carries codon sequence from DNA.

tRNA

Brings correct amino acids to ribosome.

Ribosome

Site of translation; made of rRNA and proteins.

Amino acids:

Building blocks of proteins.

Enzymes & energy (GTP):

Help in bond formation and movement.

A site (Aminoacyl)

tRNA enters with new amino acid.

P site (Peptidyl):

Holds tRNA with growing polypeptide.

E site (Exit)

Empty tRNA leaves ribosome.

1. Initiation

• Small ribosomal subunit binds to mRNA at the start codon (AUG).

• Initiator tRNA carrying methionine binds to AUG codon.

• Large ribosomal subunit joins, forming a functional ribosome with tRNA in the P site.

2. Elongation

• Next tRNA enters the A site.

• Peptide bond forms between amino acids in the P and A sites.

• Ribosome shifts (translocates):

  • tRNA in P → E (exits)

  • tRNA in A → P (now holds the growing chain)

• Cycle repeats, elongating the polypeptide chain.

3. Termination

When a stop codon (UAA, UAG, UGA) enters the A site:

• No tRNA matches; instead, a release factor binds.

• Polypeptide is released from the ribosome.

• Ribosomal subunits detach.

Free ribosomes:

Float in cytoplasm; make proteins that function in cytosol, nucleus, mitochondria, or peroxisomes.

Bound ribosomes (Rough ER):

Make proteins destined for secretion, membranes, or organelles (Golgi, lysosomes).

Signal sequence:

Short amino acid sequence at the beginning of a polypeptide that directs ribosome to the ER.

SRP:

A protein-RNA complex that binds to the signal sequence and pauses translation

srp function

• The SRP guides the ribosome to the SRP receptor on the rough ER.

• Ribosome attaches to the ER membrane, translation resumes, and the growing polypeptide is fed into the ER lumen or membrane.

Protein synthesis at the Rough ER

Enters the ER lumen or membrane during translation.

Transport vesicle

buds off from ER → travels to Golgi apparatus.

Golgi apparatus

Modifies, sorts, and packages proteins.

Vesicle transport to final destination:

• Secretion (exocytosis): Protein released outside cell.

• Incorporation into plasma membrane

• Delivery to lysosome (enzymes)