Molecular Biology
Ribosome Structure
Two RNA-protein subunits, three tRNA sites, one ribozyme active center
A ribosome is a two-subunit molecular machine that builds proteins by reading mRNA codons and stitching amino acids together. Bacteria use a 70S ribosome (50S + 30S subunits); eukaryotes use a larger 80S (60S + 40S). Ribosomal RNA is not just scaffolding — the active site that forms each peptide bond is made of RNA. The ribosome is a ribozyme, the enzymatic relic of an RNA-only world that preceded protein-dominated life.
- Bacterial70S = 50S + 30S, ~2.5 MDa
- Eukaryotic80S = 60S + 40S, ~4.3 MDa
- Active site23S rRNA (peptidyl transferase center)
- tRNA sitesA (aminoacyl), P (peptidyl), E (exit)
- Per cell (mammalian)~10⁶–10⁷
- Translation rate~20 aa/sec eukaryote, ~20 aa/sec bacteria
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Architecture in two halves
Every ribosome is built from two unequal subunits that meet only when there is work to do. The small subunit handles decoding — it grips the mRNA and verifies that each incoming tRNA's anticodon correctly pairs with the codon. The large subunit handles catalysis — it forges the peptide bond between amino acids and houses the tunnel through which the new protein emerges. Between them, the two subunits clamp three binding sites for tRNA: A, P, and E.
┌─────── 50S (60S) ──────┐
│ │
│ peptide exit tunnel │
│ │ │
│ P T C ← peptidyl transferase
│ ┌──┐ ┌──┐ ┌──┐ │ (23S rRNA, NOT protein)
nascent │ │ E│ │ P│ │ A│ │
polypeptide ─┘ │ │ │ │ │ │ │
└──┘ └──┘ └──┘ │
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
mRNA 5'━━━━━━━━━━━━━━━━━━ 3'
└─ codon ─┘
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
┌──────── 30S (40S) ─────┐
│ decoding center │
│ (16S/18S rRNA) │
└────────────────────────┘The bacterial ribosome was the first to be solved at atomic resolution, and the surprises were structural: most of the molecule's volume is RNA, and the closest protein residue to the peptidyl transferase center is more than 18 Å away. Peptide-bond catalysis is RNA-driven, full stop. Carl Woese had argued for an "RNA world" decades earlier; the ribosome structures by Ada Yonath, Tom Steitz, and Venki Ramakrishnan (~2000) made the case undeniable, earning them the 2009 Nobel Prize.
What's in each subunit
The Svedberg unit (S) is a sedimentation coefficient — how fast a particle falls through a centrifuge gradient. It depends on shape and density, not just mass, which is why 50S + 30S = 70S, not 80S. The numbers are old, structural, and often confusing for newcomers, but the labels are entrenched.
| Bacterial 70S | Mitochondrial 55S | Yeast 80S | Human 80S | Plant chloroplast 70S | Archaeal 70S | |
|---|---|---|---|---|---|---|
| Total mass | ~2.5 MDa | ~2.7 MDa | ~4.0 MDa | ~4.3 MDa | ~2.5 MDa | ~2.5 MDa |
| Large subunit | 50S (23S + 5S) | 39S (16S only) | 60S (25S, 5.8S, 5S) | 60S (28S, 5.8S, 5S) | 50S (23S + 4.5S + 5S) | 50S (23S + 5S) |
| Small subunit | 30S (16S) | 28S (12S) | 40S (18S) | 40S (18S) | 30S (16S) | 30S (16S) |
| Total proteins | ~54 | ~80 | ~79 | ~80 | ~58 | ~64 |
| Total rRNA | ~4500 nt | ~2400 nt | ~5500 nt | ~7200 nt | ~4500 nt | ~4500 nt |
| Antibiotic-targetable? | Yes (clinical) | Yes (toxic side-effects) | No | No | Yes (incidental) | No |
| Lineage | Bacteria | α-proteobacterial endosymbiont | Eukaryote | Eukaryote | Cyanobacterial endosymbiont | Archaea (some eukaryote-like proteins) |
Mitochondrial and chloroplast ribosomes look bacterial because they descend from bacteria — endosymbionts captured by an early eukaryotic cell. They've drifted in size and protein complement, but the rRNA core structure is recognizably 70S. Drugs that target bacterial ribosomes can therefore poison mitochondrial protein synthesis, which is why aminoglycosides cause hearing loss (cochlear hair cells are mitochondrially demanding) and chloramphenicol causes aplastic anemia.
A, P, and E sites — the assembly line
Three tRNAs can occupy the ribosome at any given moment, threaded through both subunits. They are read 5' → 3' along the mRNA, and they shuffle through three positions on each elongation cycle.
- A site (aminoacyl): where a new aminoacyl-tRNA arrives, delivered by elongation factor EF-Tu (eEF1A in eukaryotes). The codon is decoded here. Wrong tRNAs are kinetically rejected; the right one triggers GTP hydrolysis on EF-Tu, which releases its grip and lets the tRNA fully accommodate.
- P site (peptidyl): holds the tRNA that carries the entire growing peptide chain. The peptide bond is formed when the A-site amino acid attacks the carbonyl of the P-site peptidyl-tRNA — an aminolysis catalyzed by the 23S rRNA.
- E site (exit): where the now-empty tRNA pauses on its way out. After translocation (driven by EF-G/eEF2 GTP hydrolysis), the P-site tRNA shifts to E, the A-site tRNA shifts to P, and a new codon enters the A site.
Each cycle adds one amino acid. The whole loop takes about 50 milliseconds in bacteria — about 20 amino acids per second. Eukaryotic ribosomes run at roughly the same rate. A 1000-residue protein is therefore made in under a minute; a 30,000-residue titin filament takes ~25 minutes, which is among the slowest in the proteome.
The exit tunnel
From the peptidyl transferase center, a tunnel ~80 Å long and 10–20 Å wide threads through the large subunit. The nascent polypeptide grows into and through it; ~30–40 residues fit inside at once. Proteins emerge largely unfolded, but α-helices can form, and certain stalling sequences (SecM in E. coli, AAP in eukaryotes) actively pause the ribosome by interacting with the tunnel walls — buying time for protein-targeting machinery to engage the nascent chain.
Real numbers
- Exponential-phase E. coli: ~70,000 ribosomes, ~25% of the cell's dry mass.
- Mammalian fibroblast: ~10⁶ ribosomes; hepatocyte/pancreatic acinar: >10⁷.
- Elongation: ~20 aa/sec in bacteria and eukaryotes; rate-limiting step is GTP hydrolysis at decoding, not bond chemistry.
- Misincorporation: ~1 in 10⁴ per codon. Without EF-Tu proofreading: ~1 in 100.
- Ribosome is ~25 nm wide — visible by negative-stain EM (Palade, 1955).
- Yeast nucleoli synthesize ~2000 new ribosomes/min in fast growth; biogenesis consumes ~60% of cellular RNA polymerase output.
Variants and drugs
- Mitochondrial 55S ribosomes: bacterial-derived, drug-sensitive; encode 13 hydrophobic ETC subunits from mitochondrial DNA.
- Specialized ribosomes: protein and rRNA modifications vary across tissues, biasing which mRNAs get translated ("ribosome filter" hypothesis).
- rRNA modifications: ~200 nucleotide modifications (pseudouridines, 2'-O-methylations), guided by snoRNAs.
- Aminoglycosides (streptomycin, gentamicin): bind 16S rRNA decoding center, cause mistranslation. Bactericidal; ototoxic.
- Macrolides (erythromycin, azithromycin): plug the 50S exit tunnel. Tetracyclines: block A-site tRNA binding. Linezolid: blocks peptidyl transferase initiation, active against MRSA/VRE. Pleuromutilins (lefamulin): bind P/A sites.
- Chloramphenicol: blocks peptidyl transferase; reserved due to mitochondrial toxicity.
- Diphtheria toxin ADP-ribosylates eEF2, halting all eukaryotic translation. Ricin cleaves a single adenine from 28S rRNA, inactivating the 60S subunit.
Common misconceptions
- "Ribosomes are organelles." They're ribonucleoprotein particles, not membrane-bound.
- "The active site is protein." 23S rRNA catalyzes; proteins are peripheral.
- "50S + 30S = 80S." Svedberg units are sedimentation coefficients, not masses — they don't add. 50S + 30S → 70S.
- "Eukaryote ribosomes are eukaryote-only." Mitochondrial and chloroplast ribosomes are bacterial 70S. Human cells run both kinds.
- "Antibiotics kill all ribosomes." They prefer bacterial 70S over human 80S, but selectivity is never absolute — hence the side effects of even the best ribosome-targeting drugs.
- "Free and bound ribosomes are different." Identical machines. What differs is whether the nascent peptide has an ER-targeting signal peptide.
Frequently asked questions
Why is the ribosome called a ribozyme?
Because the active site — the peptidyl transferase center where the peptide bond is forged — is made entirely of RNA, not protein. The 2000 high-resolution crystal structure of the 50S subunit by Steitz, Moore, and others showed that the closest protein side chain is more than 18 Å from the catalytic site. Ribosomal proteins stabilize the RNA architecture but do no catalysis. This finding was decisive evidence that an "RNA world" preceded the protein-dominated cell — the ribosome is a fossil from that era. Cech, Altman, and the structural biologists shared Nobels for the broader ribozyme discoveries; Yonath, Steitz, and Ramakrishnan got the 2009 Nobel for the ribosome structures themselves.
What's the difference between 70S and 80S ribosomes?
Bacteria and archaea use a 70S ribosome (2.5 MDa) made of a 50S large subunit and a 30S small subunit. The 50S contains 23S and 5S rRNAs plus 33 proteins; the 30S contains 16S rRNA plus 21 proteins. Eukaryotes use a larger 80S ribosome (4.3 MDa) — 60S large (28S, 5.8S, 5S rRNAs, 47 proteins) plus 40S small (18S rRNA, 33 proteins). Mitochondria and chloroplasts run their own ribosomes that closely resemble the bacterial 70S, fossil evidence of the endosymbiotic origin of those organelles. The S in 70S/80S is a Svedberg unit — a sedimentation coefficient, not a mass; values aren't additive (50 + 30 = 70 only by accident).
What are the A, P, and E sites?
Three tRNA binding sites on the ribosome, threaded through both subunits. A (aminoacyl) — incoming aminoacyl-tRNAs deliver their amino acid here, paired to the codon on mRNA. P (peptidyl) — the tRNA carrying the growing polypeptide chain sits here. E (exit) — empty deacylated tRNA pauses briefly before leaving. Each elongation cycle is a one-step shift: A-site tRNA and P-site tRNA exchange roles after the peptide bond forms; the spent tRNA moves P → E → out. The translocation is driven by GTP hydrolysis on the elongation factor EF-G (eEF2 in eukaryotes).
How many ribosomes are in a cell?
A fast-growing E. coli holds about 70,000 ribosomes — roughly a quarter of the dry mass of the cell. A typical mammalian cell has 10⁶–10⁷ ribosomes; an actively secreting cell like a hepatocyte tops 10⁷. Ribosomes are made in nucleoli (rDNA tandem repeats — ~200 copies in humans, ~150 in yeast). At maximum growth a yeast cell synthesizes about 2000 new ribosomes per minute. Ribosome production consumes about 60% of cellular transcription effort and over 30% of energy budget in a fast-dividing cell.
Why do antibiotics target ribosomes?
Bacterial 70S and human 80S ribosomes diverged a billion years ago. They share the same overall architecture — they have to, because the underlying chemistry of peptide-bond formation is conserved — but their rRNA sequences and surface contours differ enough that small molecules can selectively poison one and not the other. Aminoglycosides (streptomycin, gentamicin) bind 16S rRNA in the bacterial decoding center and cause mistranslation. Macrolides (erythromycin, azithromycin) plug the 50S exit tunnel. Tetracyclines block the A site. Linezolid blocks the peptidyl transferase center. Chloramphenicol does too — but its narrow margin against mitochondrial ribosomes (which are bacterial) is why it can cause aplastic anemia.
Are ribosomes free or bound?
Both, in eukaryotes. Free ribosomes float in the cytosol and produce cytosolic and nuclear proteins. Bound ribosomes attach to the rough endoplasmic reticulum membrane via the SRP/SRP-receptor pathway and feed nascent secretory and membrane proteins through the Sec61 translocon as they translate. The ribosome is the same in both cases — what differs is whether the protein being made carries a signal peptide that triggers ER targeting. Bacteria have no rough ER, so all ribosomes are free; secreted proteins are passed to membrane translocons after translation finishes.