Ribosomes are known as the protein makers of all cells. Proteins control and build life; therefore, ribosomes are essential to life. Despite their discovery in the 1950s, it took several decades before scientists truly elucidated the structure of ribosomes.
TL;DR (Too Long; Didn't Read)
Ribosomes, known as the protein factories of all cells, were first discovered by George E. Palade. However, the structure of ribosomes was determined decades later by Ada E. Yonath, Thomas A. Steitz and Venkatraman Ramakrishnan.
A Description of Ribosomes
Ribosomes get their name from the “ribo” of ribonucleic acid (RNA) and “soma,” which is Latin for “body.”
Scientists define ribosomes as a structure found in cells, one of several smaller cellular subsets called organelles. Ribosomes have two subunits, one large and one small. The nucleolus makes these subunits, which lock together. Ribosomal RNA and proteins (riboproteins) make up a ribosome.
Some ribosomes float among the cytoplasm of the cell, while others attach to the endoplasmic reticulum (ER). The endoplasmic reticulum studded with ribosomes is called rough endoplasmic reticulum (RER); the smooth endoplasmic reticulum (SER) has no ribosomes attached.
The Prevalence of Ribosomes
Depending on the organism, a cell can have several thousand or even millions of ribosomes. Ribosomes exist in both prokaryotic and eukaryotic cells. They can also be found in bacteria, mitochondria and chloroplasts. Ribosomes are more prevalent in cells that require constant protein synthesis, like brain or pancreatic cells.
Some ribosomes can be quite massive. In eukaryotes, they can have 80 proteins and be made of several million atoms. Their RNA portion takes up more of the mass than their protein portion.
Ribosomes Are Protein Factories
Ribosomes take codons, which are series of three nucleotides, from messenger RNA (mRNA). A codon serves as a template from the cell’s DNA to make a certain protein. Ribosomes then translate the codons and match them to an amino acid from transfer RNA (tRNA). This is known as translation. The ribosome has three tRNA binding sites: an aminoacyl binding site (A site) for attaching amino acids, a peptidyl site (P site) and an exit site (E site).
After this process, the translated amino acid builds upon a protein chain called a polypeptide, until the ribosomes complete their work of making a protein. Once the polypeptide is released into the cytoplasm, it goes on to become a functional protein. This process is why ribosomes are often defined as protein factories. The three stages of protein production are called initiation, elongation and translation.
These machinelike ribosomes work quickly, adjoining 200 amino acids per minute in some cases; prokaryotes can add 20 amino acids per second. Complex proteins take a few hours to assemble. Ribosomes make most of the approximately 10 billion proteins in the cells of mammals. Completed proteins may in turn undergo further changes or folding; this is called post-translational modification. In eukaryotes, the Golgi apparatus completes the protein before it is released. Once ribosomes finish their work, their subunits either get recycled or dismantled.
Who Discovered Ribosomes?
George E. Palade first discovered ribosomes in 1955. Palade’s ribosome description portrayed them as cytoplasmic particles that associated with the membrane of the endoplasmic reticulum. Palade and other researchers found the function of ribosomes, which was protein synthesis.
Francis Crick would go on to form the central dogma of biology, which summarized the process of building life as “DNA makes RNA makes protein.”
While the general shape was determined using electron microscopy images, it would take several more decades to determine the actual structure of ribosomes. This was due in large part to the comparatively immense size of ribosomes, which inhibited the analysis of their structure in a crystal form.
The Discovery of Ribosome Structure
While Palade discovered the ribosome, other scientists determined its structure. Three separate scientists discovered the structure of ribosomes: Ada E. Yonath, Venkatraman Ramakrishnan and Thomas A. Steitz. These three scientists were rewarded with the Nobel Prize in Chemistry in 2009.
The discovery of three-dimensional ribosome structure occurred in 2000. Yonath, born in 1939, opened the door for this revelation. Her initial work on this project began in the 1980s. She used microbes from hot springs to isolate their ribosomes, due to their robust nature in a harsh environment. She was able to crystallize ribosomes so they could be analyzed via X-ray crystallography. This generated a pattern of dots on a detector so that the positions of ribosomal atoms could be detected. Yonath eventually produced high-quality crystals using cryo-crystallography, meaning the ribosomal crystals were frozen to help keep them from breaking down.
Scientists then tried to elucidate the “phase angle” for the patterns of dots. As technology improved, refinements to the procedure led to detail at the single-atom level. Steitz, born in 1940, was able to discover which reaction steps involved which atoms, at the connections of amino acids. He found the phase information for the ribosome’s larger unit in 1998.
Ramakrishan, born in 1952, in turn worked to solve the phase of x-ray diffraction for a good molecular map. He found the phase information for the ribosome’s smaller subunit.
Today, further advancements in full ribosome crystallography have led to better resolution of ribosome complex structures. In 2010, scientists successfully crystalized the eukaryotic 80S ribosomes of Saccharomyces cerevisiae and were able to map its X-ray structure ("80S" is a type of categorization called a Svedberg value; more on this shortly). This in turn led to more information about protein synthesis and regulation.
Ribosomes of smaller organisms have so far proven to be the easiest to work with to determine ribosome structure. This is because the ribosomes themselves are smaller and less complex. More research is needed to help determine the structures of higher organisms’ ribosomes, such as those in humans. Scientists also hope to learn more about the ribosomal structure of pathogens, to aid in the fight against disease.
What Is a Ribozyme?
The term ribozyme refers to the larger of the two subunits of a ribosome. A ribozyme functions as an enzyme, hence its name. It serves as a catalyst in protein assembly.
Categorizing Ribosomes by Svedberg Values
Svedberg (S) values describe the rate of sedimentation in a centrifuge. Scientists often describe ribosomal units using Svedberg values. For example, prokaryotes possess 70S ribosomes that are comprised of one unit with 50S and one of 30S. These do not add up because the sedimentation rate has more to do with size and shape than molecular weight. Eukaryotic cells, on the other hand, contain 80S ribosomes.
The Importance of the Ribosome’s Structure
Ribosomes are essential to all life, for they make the proteins that ensure life and its building blocks. Some essential proteins for human life include hemoglobin in red blood cells, insulin and antibodies, among many others.
Once researchers unveiled the structure of ribosomes, it opened up new possibilities for exploration. One such avenue of exploration is for new antibiotic medicines. For example, new drugs might stop disease by targeting certain structural components of the ribosomes of bacteria.
Thanks to structure of ribosomes discovered by Yonath, Steitz and Ramakrishnan, researchers now know precise locations between amino acids and the locations where proteins leave ribosomes. Zeroing in on the location for where antibiotics attach to ribosomes opens up much higher precision in drug action. This is crucial in an era when formerly stalwart antibiotics have met with antibiotic-resistant strains of bacteria. The discovery of ribosome structure is therefore of great importance to medicine.
- Scitable by Nature Education: Ribosome
- Royal Society of Chemistry: Chemistry World: Biology’s Nobel Molecule Factory
- The Nobel Prize: The Nobel Prize in Chemistry 2009
- Florida State University: Molecular Expressions: Ribosomes
- RNA Society: Ribosome Biochemistry in Crystal Structure Determination
- British Society for Cell Biology: Ribosome