Their primary function is to translate the genetic information encoded in messenger RNA. These organelles are in eukaryotes, prokaryotes, and nearly all cells, except for certain highly differentiated cells such as mature red blood cells. Ribosomes operate like a fully-powered protein manufacturing factory where polypeptides are produced at an extremely rapid pace. In eukaryotes, peptide chains expand by several amino acids per second. While 20 or more amino acids are added in peptide every second by prokaryotes.
Their size in prokaryotes is smaller. The sedimentation coefficient is 70S approximately. Two different subunits are the main component whose molecular weight and sedimentation coefficient is 1600 KDa and 900 KDa (50S and 30S) respectively. Things are similar in eukaryotes (80S, 60S, and 40S; 2800 KDa and 1400 KDa). In chloroplasts and mitochondria, researchers have also found ribosomes that resemble those in prokaryotes. They speculate that our ancestors engulfed certain primordial bacteria that survived and coevolved within them. A little of proteins is made in mitochondria or chloroplasts. However, most enzymes and structural components are actually encoded by cell nucleus, as the genetic material of symbiotic bacteria has been transferred to their host during evolution.
Free Ribosome vs. Attached Ribosome
If we disregard organelles, there are two types in eukaryotic cytoplasm. Their structures are completely identical, but the products they manufacture are destined for different locations. The ones attached to endoplasmic reticulum (ER) or nuclear envelope are called bound or attached ribosomes. Peptide produced here enter the ER lumen for modification. They are then transported to Golgi apparatus for further processing. Some are secreted outside the cell, such as antibodies, protein hormones, and extracellular matrix components like collagen and proteoglycans. Others are embedded in plasma membrane or endomembrane system as transmembrane proteins. Additionally, some soluble proteins are transported to organelles, such as hydrolytic enzymes in lysosomes.
Free ribosomes float in cytosol without attaching to any biological membrane. The complete morphology only appears when they are working. Once synthesis is over, they spontaneously disassemble into subunits until the next task begins. The products synthesized here are released into cytoplasm directly. For example, microtubules, microfilaments, and intermediate filaments that constitute cytoskeleton are created by them.
Regardless of their type, they often aggregate on a single mRNA to form a polyribosome. They are working along mRNA like machines on an assembly line.
Ribosome is a Ribozyme
The idea that enzymes are proteins was widely accepted before the 1980s, so the proteins in ribosomes were taken for granted as the key to catalysis. However, it was discovered that strains with inhibited protein production often had mutations in genes encoding RNA. It’s surprising that the remaining RNA backbone could still works even after proteins were removed by proteases and ionic detergents. When RNase was used by researchers to dissolve RNA backbone, its biological activity disappeared. Therefore, ribosomes are one of the first discovered ribozymes. About 60% components are RNA that occupies the core regions and folded into a complex three-dimensional structure. The rest are proteins that cover surface or fill in gaps to maintain the RNA structure. When an enzyme effector binds to protein, its conformation changes slightly to regulate catalytic activity. There are three important sites in the core region. tRNA carrying amino acids enters from A site, and stays at P site. It is ejected from E site finally.
Large subunit, small subunit and initiator tRNA that all float in cytosol assemble into a complete ribosome near the start codon on mRNA. The cytosol contains various tRNAs whose one end connected to a specific amino acid and the other end recognizes codons on mRNA. Amino acids carried by tRNAs arrive at ribosome for polypeptide expanding that is catalyzed by ribozyme, as the entire apparatus moves from 5′ to 3′. When it encounters a stop codon, the synthesis halts and polypeptide is released. The subunits separate once again after translation is completed.