The Atomic-Scale Structure of a Ribosome

John C. Kendrew determined the first atomic-scale (2A resolution) crystallographic structure of a protein, myoglobin (molecular mass 16,900 Da or 16.9 KDa) in 1959 and Max Perutz followed shortly afterward with atomic scale resolution of the tetrameric protein hemoglobin (64,500 Da). The first crystallographic structure for an enzyme, lysozyme (13,900 Da), was determined by David Phillips in 1965. The crystallographic analysis of the structure of the supermolecular photosynthetic reaction center of purple photosynthetic bacteria in 1985 led to a Nobel Prize in chemistry for Robert Huber, Johann Diesenhofer, and Hartmut Michel. The reaction center, a complete nanomachine embedded in the cell membrane of purple photosynthetic bacteria, consists of 4 protein subunits and 14 cofactors. 

The large (SOS) ribosomal subunit is where protein synthesis, catalyzed formation of peptide bonds, takes place. In light of the discoveries by Thomas Cech and Sidney Altman of ribozymes, RNA molecules that behave as enzymes, the question of catalysis of protein synthesis by RNA or proteins in the ribosome was a major one. The 3 OS ribosomal subunit is where transfer RNAs bind with ribosomal RNA codons. The 50S ribosomal subunit can be further separated into a 23S secondary subunit and a smaller 5S secondary subunit that are normally held together by protein molecules. The 23S subunit includes 3045 nucleotides and 31 proteins. There are six discrete RNA domains in the S23 unit particle. The 5S unit effectively adds a seventh domain. The proteins permeate the exterior of the RNA of S23, but are located over 18 A distant from the active site of catalytic protein bond formation. The structures of complexes of the S23 subunit with two inactive substrate molecules suggest mechanistic similarities to the enzyme chymotrypsin. The S23 structure indicates that an adenine base at the active site plays a role analogous to that of histidine- 

The mechanistic role of a rRNA base in protein synthesis 

57 in chymotrypsin. In summary, the ribosome behaves as a ribozyme in the synthesis of proteins. 

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