6.4 Translation & Genetic Code

Translation is the process by which messenger RNA (mRNA) nucleotide sequences are decoded into specific amino acid chains. This protein synthesis machinery operates according to the universal genetic code.

The Genetic Code

Protein primary structure is specified by linear sequences of nucleotide triplets called codons. With 4 RNA bases (A, U, G, C), there are $4^3 = 64$ possible triplet codons.

• 61 Coding Codons: Specify the 20 standard amino acids.
• 1 START Codon: $5'-AUG-3'$, which codes for Methionine (Met) in eukaryotes and $N$-formylmethionine (fMet) in prokaryotes.
• 3 STOP Codons: $5'-UAA-3'$, $5'-UAG-3'$, and $5'-UGA-3'$. Mnemonic: U Are Annoying (UAA), U Are Gone (UAG), U Go Away (UGA). STOP codons do not encode amino acids; they bind protein release factors.

Key Features of the Genetic Code

1. Universal: Conserved across virtually all prokaryotes, eukaryotes, and viruses (with minor mitochondrial exceptions).
2. Non-Overlapping: Read sequentially 3 nucleotides at a time from the start codon without skipping or overlapping bases.
3. Degenerate (Redundant): Multiple codons code for the same amino acid (e.g., Leucine and Arginine are each coded by 6 distinct codons; only Tryptophan and Methionine are coded by single codons). Degeneracy buffers organisms against point mutations: 3rd-position mutations frequently cause synonymous (silent) mutations that do not alter the translated amino acid.

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The Wobble Hypothesis

To explain why cells require fewer than 61 distinct tRNA species (typically ~30–45 tRNAs cover all 61 amino-acid-coding codons), Francis Crick proposed the Wobble Hypothesis:

• Strict Watson-Crick Pairing: Enforced at codon positions 1 and 2 (pairing with anticodon positions 3 and 2).
• Wobble Flexibility: The 3rd base of the mRNA codon (which pairs with the 1st base at the 5' position of the tRNA anticodon) exhibits conformational flexibility, permitting non-standard hydrogen bonding.

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tRNA Architecture & Aminoacyl-tRNA Synthetase Charging

tRNA Secondary & Tertiary Structure

Transfer RNA (tRNA) molecules (~75–90 nt) fold into a cloverleaf secondary structure featuring four key domains:

1. Acceptor Stem: Contains an unpaired $5'-CCA-3'$ sequence at the 3' terminus where the specific amino acid is covalently attached.
2. Anticodon Loop: Contains the 3-nucleotide anticodon that base-pairs antiparallel to mRNA codons.
3. D Arm: Contains dihydrouridine residues; involved in synthetase recognition.
4. T$\psi$C Arm: Contains ribothymidine (T) and pseudouridine ($\psi$); involved in ribosome binding.

• Tertiary structure folds into an inverted L-shape.

Aminoacyl-tRNA Synthetase Charging Reaction

Charging is catalyzed by 20 specific Aminoacyl-tRNA Synthetases (aaRS) in a two-step energy-dependent process consuming 2 ATP equivalents:

1. Activation: $\text{Amino Acid} + \text{ATP} \rightarrow \text{Aminoacyl-AMP} + PP_i$. Inorganic pyrophosphatase hydrolyzes $PP_i \rightarrow 2 P_i$, driving activation forward.
2. Transfer: $\text{Aminoacyl-AMP} + \text{tRNA} ightarrow \text{Aminoacyl-tRNA} + \text{AMP}$. The amino acid carboxyl group forms a high-energy ester bond with the $3'-OH$ (or $2'-OH$) of the terminal adenosine of tRNA.

Synthetases possess an editing active site that hydrolyzes mischarged amino acids, maintaining an error rate $< 10^{-4}$.

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Translation Mechanism

Translation takes place on Ribosomes, which contain three tRNA-binding sites:

• A Site (Aminoacyl): Accepts incoming charged aminoacyl-tRNAs.
• P Site (Peptidyl): Holds the tRNA attached to the growing peptide chain.
• E Site (Exit): Holds uncharged tRNAs prior to release.

Stepwise Translation Stages

[Initiation: eIFs + 40S scan Kozak -> 60S joins at P site]
  ---> [Elongation: A site Decoding -> 28S rRNA Peptidyl Transferase -> eEF2 Translocation]
  ---> [Termination: STOP codon -> eRF1 Water Hydrolysis -> Ribosome Disassembly]

1. Initiation

• Eukaryotes: Eukaryotic initiation factor 4E (eIF4E) binds the 5' $m^7G$ cap. The 43S pre-initiation complex (comprising the 40S subunit, eIF2-GTP, and $\text{Met-tRNA}_i$) scans the 5' UTR until it locates the Kozak consensus sequence ($5'-(A/G)CCATGG-3'$). Anticodon pairing with $AUG$ triggers eIF2 GTP hydrolysis and joining of the 60S subunit, forming an 80S initiation complex with $\text{Met-tRNA}_i$ in the P site.
• Prokaryotes: The 16S rRNA of the 30S subunit pairs directly with the Shine-Dalgarno sequence ($5'-AGGAGG-3'$), ~8 nt upstream of $AUG$.

2. Elongation (Three-Step Cycle)

1. Decoding: Elongation factor eEF1A-GTP (EF-Tu in bacteria) delivers the matching aminoacyl-tRNA to the vacant A site. Codon-anticodon pairing triggers GTP hydrolysis, locking the tRNA into the A site.
2. Peptide Bond Formation (Transpeptidation): Peptidyl Transferase—a ribozyme catalytic activity of the 28S rRNA in the 60S subunit (23S rRNA in bacteria)—catalyzes nucleophilic attack of the A-site $\alpha$-amino group on the ester carbonyl carbon of the P-site peptidyl-tRNA. The polypeptide chain is transferred to the A-site tRNA.
3. Translocation: Elongation factor eEF2-GTP (EF-G in bacteria) hydrolyzes GTP to advance the ribosome 1 codon ($3\text{ nt}$) $5' \rightarrow 3'$. Uncharged tRNA shifts to the E site and exits; peptidyl-tRNA shifts from A to P site.

3. Termination

When a STOP codon ($UAA$, $UAG$, or $UGA$) enters the A site, it is recognized by eRF1 (eukaryotic Release Factor 1) in complex with eRF3-GTP. eRF1 binds the A site and induces peptidyl transferase to transfer the polypeptide chain to a water molecule ($H_2O$) instead of an amino acid. Ester bond hydrolysis releases the completed protein, and ribosome recycling factors disassemble the 80S ribosome.

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Post-Translational Modifications (PTMs)

Nascent polypeptides exiting the ribosomal exit tunnel undergo processing to achieve functional activity:

• Protein Folding: Molecular Chaperones (Hsp70, Hsp60/GroEL-GroES) bind exposed hydrophobic patches, preventing misfolding and toxic protein aggregation.
• Phosphorylation: Addition of phosphate ($PO_4^{3-}$) to Ser, Thr, or Tyr residues by Protein Kinases (reversed by Phosphatases). Introduces negative charges, altering enzyme activity or signaling pathways.
• Glycosylation: Attachment of carbohydrates (N-linked to Asn in the ER; O-linked to Ser/Thr in the Golgi). Crucial for protein stability and cell surface receptor recognition.
• Ubiquitination: E1-E2-E3 enzymatic cascade attaches a 76-amino acid ubiquitin tag to Lysine residues. Polyubiquitination via Lys48 linkages targets proteins to the 26S Proteasome for degradation.
• Proteolytic Cleavage (Zymogen Activation): Irreversible cleavage of peptide bonds converts inactive pro-proteins into active forms. Examples: proinsulin is cleaved in Golgi vesicles to yield mature active insulin and C-peptide; pancreatic zymogens (trypsinogen $\rightarrow$ trypsin) are activated in the duodenum.