Translation and Protein Processing

1. The Genetic Code

The genetic code is the set of rules by which nucleotide triplets (codons) in mRNA specify amino acids. There are 64 possible codons (4³): 61 code for the 20 standard amino acids, and 3 are stop codons (UAA, UAG, UGA) that signal termination. The code is degenerate (redundant): most amino acids are specified by more than one codon — e.g., leucine has six codons while tryptophan has only one. Degeneracy often occurs at the third ("wobble") position of the codon, where nonstandard base pairing (e.g., G–U) between the codon and anticodon is tolerated. The code is universal across nearly all organisms (with minor exceptions in mitochondria and some protists), unambiguous (each codon specifies only one amino acid), and non-overlapping (nucleotides are read in sequential, non-overlapping triplets). The start codon, AUG, codes for methionine and establishes the reading frame. A shift of one or two nucleotides produces a completely different polypeptide — a frameshift mutation.

2. tRNA and Aminoacyl-tRNA Synthetases

Transfer RNA (tRNA) is the adaptor molecule that translates the genetic code. Each tRNA is ~75–90 nucleotides long and folds into a characteristic cloverleaf secondary structure with four key arms: the acceptor stem (the 3′ CCA terminus where the amino acid is attached), the D arm, the TψC arm, and the anticodon arm (containing the triplet anticodon complementary to an mRNA codon). The anticodon is read 3′→5′ and pairs with the mRNA codon 5′→3′. tRNAs are charged with their cognate amino acids by aminoacyl-tRNA synthetases, a family of ~20 enzymes (one per amino acid) with exquisite specificity. The reaction occurs in two steps: (1) amino acid + ATP → aminoacyl-AMP + PPi (activation); (2) aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP (transfer). The resulting aminoacyl-tRNA (or "charged tRNA") carries a high-energy ester bond at the 3′ end, which provides the thermodynamic driving force for peptide bond formation. Synthetases possess proofreading (editing) domains that hydrolyze incorrectly paired amino acids, achieving an error rate below 1 in 10,000 — critical because the ribosome cannot distinguish correctly from incorrectly charged tRNAs.

3. Ribosomal Structure and Function

The ribosome is a massive ribonucleoprotein (~2.5 MDa in prokaryotes, ~4.3 MDa in eukaryotes) composed of two subunits. Prokaryotic (70S) ribosomes consist of a 50S large subunit (23S and 5S rRNA + ~34 proteins) and a 30S small subunit (16S rRNA + ~21 proteins). Eukaryotic (80S) ribosomes have a 60S large subunit (28S, 5.8S, 5S rRNA + ~49 proteins) and a 40S small subunit (18S rRNA + ~33 proteins). The ribosome has three tRNA-binding sites: the A site (aminoacyl-tRNA entry), the P site (peptidyl-tRNA), and the E site (exit for deacylated tRNA). Critically, the ribosome is a ribozyme — its peptidyl transferase activity resides in the 23S/28S rRNA, not in protein. This supports the RNA World hypothesis. The small subunit decodes mRNA (the decoding center), while the large subunit catalyzes peptide bond formation (the peptidyl transferase center). The mRNA passes between the two subunits, and the growing polypeptide chain exits through a tunnel in the large subunit (~100 Å long, ~10–20 Å wide).

4. Initiation, Elongation, and Termination

Translation proceeds in three phases, each requiring specific protein factors.

Initiation (Prokaryotes): The small ribosomal subunit (30S) binds the Shine-Dalgarno sequence (AGGAGG) in mRNA upstream of the start codon via complementary base pairing with 16S rRNA. Initiation factors (IF1, IF2, IF3) facilitate: IF3 prevents premature large subunit joining; IF1 blocks the A site; IF2 (a GTPase) delivers the initiator fMet-tRNAᶠᴹᵉᵗ to the P site. GTP hydrolysis triggers large subunit (50S) joining, forming the 70S initiation complex.

Initiation (Eukaryotes): The process is more complex. The pre-initiation complex (40S subunit + eIFs + Met-tRNAᶦᴹᵉᵗ) scans the mRNA from the 5′ cap (recognized by eIF4E) for the first AUG in a favorable Kozak consensus sequence (GCCRCCAUGG). Key difference: eukaryotic initiation uses methionine, not formyl-methionine. The closed-loop model — where poly(A)-binding protein (PABP) interacts with eIF4G, which binds eIF4E at the 5′ cap — circularizes mRNA for efficient re-initiation.

Elongation: A three-step cycle repeated for each codon:

1. Decoding / tRNA selection: An aminoacyl-tRNA–EF-Tu–GTP complex enters the A site. Correct codon-anticodon pairing triggers GTP hydrolysis by EF-Tu, which dissociates, leaving the tRNA in the A site.
2. Peptide bond formation: The peptidyl transferase center (23S/28S rRNA) catalyzes nucleophilic attack by the A-site amino acid's α-amino group on the P-site peptidyl-tRNA's ester carbonyl, forming a peptide bond. The polypeptide is transferred to the A-site tRNA (the ribosome acts as an entropy trap — no ATP needed for catalysis).
3. Translocation: EF-G (eEF2 in eukaryotes) binds with GTP and drives movement of the ribosome by one codon. The peptidyl-tRNA moves from A→P site; the deacylated tRNA moves P→E site → exits. GTP hydrolysis by EF-G resets the ribosome for the next cycle.

Termination: When a stop codon (UAA, UAG, UGA) enters the A site, release factors (RFs) recognize it — no tRNA exists for stop codons. Prokaryotes: RF1 recognizes UAA/UAG; RF2 recognizes UAA/UGA. Eukaryotes: a single factor eRF1 recognizes all three. RF binding triggers peptidyl transferase to hydrolyze the ester bond between the polypeptide and the P-site tRNA (using a water molecule instead of an aminoacyl-tRNA nucleophile), releasing the protein. RF3/eRF3 (a GTPase) then mediates dissociation of the ribosomal subunits, mRNA, and deacylated tRNA.

5. Post-Translational Modification

Once synthesized, most polypeptides are chemically modified to become functional proteins. Key modifications relevant to the MCAT include:

• Proteolytic cleavage: Removal of the initiator methionine (or N-formylmethionine) by methionine aminopeptidase; cleavage of signal sequences; activation of zymogens (e.g., trypsinogen → trypsin; proinsulin → insulin + C-peptide).
• Phosphorylation: Addition of phosphate groups to Ser, Thr, or Tyr residues by kinases; reversible removal by phosphatases. This is a central regulatory mechanism in signaling cascades.
• Glycosylation: Addition of carbohydrate groups. N-linked glycosylation occurs on Asn residues (Asn-X-Ser/Thr motif) in the ER; O-linked on Ser/Thr in the Golgi. Important for protein folding, stability, and cell-surface recognition (e.g., ABO blood group antigens).
• Hydroxylation: Addition of –OH groups to Pro and Lys residues in collagen, requiring vitamin C as a cofactor — deficiency causes scurvy.
• Acetylation: Addition of acetyl groups to Lys residues (e.g., histone acetylation regulating chromatin structure) or at the N-terminus.
• Ubiquitination: Covalent attachment of ubiquitin (a 76-amino-acid protein) to Lys residues, targeting proteins for degradation by the 26S proteasome. Polyubiquitin chains signal destruction; monoubiquitination can regulate trafficking.
• Lipidation: Addition of lipid anchors (e.g., prenylation, myristoylation, palmitoylation) that tether proteins to membranes (e.g., Ras, Src).

6. Protein Folding and Targeting

Protein folding is the process by which a polypeptide chain assumes its native three-dimensional conformation, determined primarily by its amino acid sequence (Anfinsen's dogma). Folding is driven by the hydrophobic effect, hydrogen bonding, van der Waals interactions, and disulfide bond formation. Molecular chaperones assist folding and prevent aggregation: Hsp70 binds exposed hydrophobic patches on nascent chains; chaperonins (e.g., GroEL/GroES in prokaryotes, Hsp60 in mitochondria) provide an isolated folding chamber. Protein disulfide isomerase (PDI) catalyzes disulfide bond formation and reshuffling in the ER.

Protein targeting directs proteins to their correct destinations. Signal sequences — N-terminal hydrophobic stretches of ~15–30 amino acids — direct nascent proteins to the ER via the signal recognition particle (SRP). SRP binds the signal sequence, pauses translation, and docks at the SRP receptor on the ER membrane. Translation resumes through the translocon (Sec61 complex), delivering the protein into the ER lumen (co-translational translocation). In the ER, proteins fold, are modified, and are sorted via the secretory pathway: ER → Golgi → plasma membrane or secretion (default/constitutive pathway) or lysosomes. Lysosomal enzymes acquire a mannose-6-phosphate tag in the Golgi for receptor-mediated sorting. Proteins destined for mitochondria, chloroplasts, peroxisomes, or the nucleus have distinct targeting signals (e.g., nuclear localization signals — NLS — are internal, basic amino acid-rich sequences). Misfolded proteins are retained in the ER by chaperones; if irreparable, they undergo ER-associated degradation (ERAD) — retrotranslocation to the cytosol and proteasomal degradation.

How it works

The translation cycle can be visualized as a coordinated molecular machine:

1. Charging: Aminoacyl-tRNA synthetases charge tRNAs with their cognate amino acids, consuming ATP. This is the only step where the genetic code is actually "read" — by the synthetase, not the ribosome. The editing function ensures accuracy: a valine mischarged onto tRNAᴵˡᵉ is hydrolyzed.

1. Initiation: In eukaryotes, the 43S pre-initiation complex (40S + eIF2–GTP–Met-tRNAᶦᴹᵉᵗ + eIF1, eIF1A, eIF3, eIF5) binds mRNA at the 5′ cap. It scans 5′→3′, melting secondary structure with ATP-dependent helicases (eIF4A). When an AUG in a strong Kozak context is recognized, eIF2 hydrolyzes GTP, eIFs dissociate, and the 60S subunit joins.

1. Elongation cycle (repeat ~400 times for an average 50-kDa protein): The ribosome (~2–20 amino acids per second in eukaryotes) iterates: EF-Tu delivers tRNA to A site → GTP hydrolysis → accommodation → peptide bond formation (spontaneous in the peptidyl transferase center) → EF-G–GTP drives translocation → deacylated tRNA exits E site.

1. Termination: A stop codon triggers RF binding → the polypeptide is hydrolyzed and released → ribosomal subunits dissociate. Polysomes (multiple ribosomes translating a single mRNA simultaneously) amplify protein output.

1. Maturation: The nascent polypeptide exits the ribosome tunnel and is met by chaperones, modification enzymes, and targeting factors. Folding, modification, and sorting occur co- and post-translationally.

How it works

The translation cycle can be visualized as a coordinated molecular machine:

1. Charging: Aminoacyl-tRNA synthetases charge tRNAs with their cognate amino acids, consuming ATP. This is the only step where the genetic code is actually "read" — by the synthetase, not the ribosome. The editing function ensures accuracy: a valine mischarged onto tRNAᴵˡᵉ is hydrolyzed.

1. Initiation: In eukaryotes, the 43S pre-initiation complex (40S + eIF2–GTP–Met-tRNAᶦᴹᵉᵗ + eIF1, eIF1A, eIF3, eIF5) binds mRNA at the 5′ cap. It scans 5′→3′, melting secondary structure with ATP-dependent helicases (eIF4A). When an AUG in a strong Kozak context is recognized, eIF2 hydrolyzes GTP, eIFs dissociate, and the 60S subunit joins.

1. Elongation cycle (repeat ~400 times for an average 50-kDa protein): The ribosome (~2–20 amino acids per second in eukaryotes) iterates: EF-Tu delivers tRNA to A site → GTP hydrolysis → accommodation → peptide bond formation (spontaneous in the peptidyl transferase center) → EF-G–GTP drives translocation → deacylated tRNA exits E site.

1. Termination: A stop codon triggers RF binding → the polypeptide is hydrolyzed and released → ribosomal subunits dissociate. Polysomes (multiple ribosomes translating a single mRNA simultaneously) amplify protein output.

1. Maturation: The nascent polypeptide exits the ribosome tunnel and is met by chaperones, modification enzymes, and targeting factors. Folding, modification, and sorting occur co- and post-translationally.

Comparisons

• Biochemistry: The genetic code, codon-anticodon pairing (wobble), and the central dogma are foundational. The energy cost of translation — 4 high-energy phosphate bonds per amino acid (1 ATP for charging + 2 GTP for elongation + 1 GTP for initiation/termination averaged) — ties to bioenergetics.
• Genetics/Molecular Biology: Frameshift mutations (insertions or deletions not divisible by 3) and nonsense mutations (premature stop codon) illustrate how the reading frame determines the final polypeptide. Silent mutations exploit codon degeneracy.
• Pharmacology: Many antibiotics target translation, exploiting prokaryotic-eukaryotic ribosomal differences. Streptomycin binds 30S and causes misreading; tetracycline blocks A site; chloramphenicol inhibits peptidyl transferase (50S); erythromycin blocks the exit tunnel (50S); puromycin mimics aminoacyl-tRNA and causes premature termination (in both prokaryotes and eukaryotes). Diphtheria toxin ADP-ribosylates eEF2, blocking eukaryotic translocation.
• Cell Biology: The secretory pathway (ER → Golgi → vesicles → plasma membrane) and signal hypothesis (Blobel, Nobel Prize 1999) are core cell biology concepts. Diseases of protein targeting (I-cell disease, cystic fibrosis ΔF508) are high-yield clinical correlates.
• Pathology: Prion diseases (Creutzfeldt-Jakob, kuru) demonstrate that protein misfolding can be infectious — a misfolded protein (PrPˢᶜ) induces conformational change in normal PrPᶜ. Amyloid diseases (Alzheimer's, Parkinson's, type II diabetes) involve aggregation of misfolded proteins.

Common confusions

• AUG reads as Methionine, not "Start." There is no special "start" tRNA — the same Met-tRNAᶦᴹᵉᵗ used for initiation also inserts methionine at internal AUG codons. The distinction is the initiation factor context (eIF2 brings Met-tRNAᶦᴹᵉᵗ during initiation; EF1α brings Met-tRNAᴹᵉᵗ during elongation), not the tRNA itself. Prokaryotes use formyl-methionine (fMet) only for initiation.
• The wobble position bridges codon degeneracy. When the MCAT asks how a single tRNA can recognize multiple codons, the answer is wobble base pairing at the third codon position (e.g., inosine at the anticodon wobble position can pair with U, C, or A). Don't confuse this with degeneracy of the code itself — wobble is the mechanism that enables some degeneracy.
• The ribosome is a ribozyme — the protein components are NOT the catalytic engine. Students sometimes attribute peptidyl transferase activity to ribosomal proteins. The catalytic core is the 23S rRNA (or 28S in eukaryotes). Antibiotics like chloramphenicol and puromycin target this RNA-based active site.
• Stop codons are recognized by protein release factors, not tRNA. There are no "stop tRNAs." The MCAT will test this directly: "Which tRNA recognizes UAG?" The correct answer is "none." Release factors (RFs) are proteins that mimic tRNA structure to occupy the A site and trigger termination.
• GTP hydrolysis provides energy, but peptide bond formation itself does not require direct NTP input. The energy for peptide bond formation comes from the high-energy ester bond in aminoacyl-tRNA (created during charging, at the cost of ATP). GTP is consumed for fidelity and translocation, not catalysis. Questions asking "how many high-energy bonds are used per amino acid" must count: 1 ATP → AMP + PPi (tRNA charging) + 2 GTP (EF-Tu + EF-G).
• Post-translational modifications are covalent, not just conformational. The MCAT distinguishes between a polypeptide chain and a functional protein: the latter requires proper folding AND often covalent modifications. A question may ask whether phosphorylation, glycosylation, or proteolytic cleavage changes primary structure — yes, all three do (cleavage irreversibly shortens it; phosphorylation and glycosylation add chemical groups).

Quick review

• Genetic code: 64 codons, 61 for amino acids, 3 stops (UAA, UAG, UGA). Degenerate, universal, unambiguous, non-overlapping. Start codon = AUG (Met).
• tRNA: cloverleaf structure; 3′ CCA carries amino acid; anticodon reads codon 3′→5′. Charged by aminoacyl-tRNA synthetase (2-step reaction: ATP → aminoacyl-AMP → aminoacyl-tRNA). Synthetases have proofreading.
• Ribosome: prokaryotic 70S (50S + 30S), eukaryotic 80S (60S + 40S). Three sites: A (entry), P (peptidyl), E (exit). Peptidyl transferase = ribozyme (23S/28S rRNA).
• Initiation: prokaryotes: Shine-Dalgarno + fMet-tRNAᶠᴹᵉᵗ; eukaryotes: 5′ cap scanning + Kozak sequence + Met-tRNAᶦᴹᵉᵗ. Closed-loop (PABP–eIF4G–eIF4E).
• Elongation: EF-Tu delivers tRNA to A site → GTP hydrolysis → peptide bond (no ATP needed) → EF-G translocates (A→P, P→E).
• Termination: stop codon → release factor (RF1/RF2 prokaryotes; eRF1 eukaryotes) → water hydrolyzes peptide → subunits dissociate.
• Post-translational modifications: phosphorylation (kinases), glycosylation (N-linked in ER, O-linked in Golgi), proteolytic cleavage (zymogen activation), ubiquitination (26S proteasome targeting), hydroxylation (collagen, vitamin C).
• Protein targeting: signal sequence → SRP → SRP receptor → translocon (Sec61) → ER. Secretory pathway: ER → Golgi → PM/secretion. Mannose-6-phosphate tag for lysosomal enzymes. NLS for nuclear import.
• Antibiotics: streptomycin (30S, misreading), tetracycline (30S, blocks A site), chloramphenicol (50S, peptidyl transferase), erythromycin (50S, tunnel block), puromycin (mimics tRNA, premature termination).
• Folding: chaperones (Hsp70, chaperonins); misfolded → ERAD → proteasome. Prion disease = misfolded protein induces misfolding.