Ribosomes - A Complex Supramolecular Machine
Authors: Nimmy.M.S, Vinod Kumar, Era Vaidya Malhotra and Lal Bahadur Singh
George E. Palade, recipient of the Nobel Prize of the year 1974 discovered ribosomes and described them as small particles in the cytoplasm that preferentially associated with the endoplasmic reticulum membrane. Along with other scientists, Palade mentioned that ribosomes do perform protein synthesis in cells. After many years, 2009 Nobel Prize in Chemistry was awarded to Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath for the detailed mapping of the ribosome ie, both the structure and function. Ribosomes translate the passive DNA information into form and function or in other words the RNA message is decoded on ribosomes. Any cell typically may contain many ribosomes. In case of prokaryote E.coli, there are about 20,000 ribosomes which accounts 25% of the dry weight of the cell. There are 10 million ribosomes in mammalian cells. Ribosomes can be found freely floating inside the cell or it can be attached with ER (Rough ER). Free ribosomes synthesize proteins mostly for use within the cell and bound ribosomes synthesize proteins for export (secretion) from the cell.
First characterized as particles detected by ultracentrifugation of cell lysates, ribosomes are usually designated according to their rates of sedimentation:70S for bacterial ribosomes and 80S for the somewhat larger ribosomes of eukaryotic cells. In the late 1960s, Masayasu Nomura and colleagues demonstrated that both ribosomal subunits can be broken down into their RNA and protein components, and reconstituted in vitro. The basic form of the ribosome has been conserved in evolution. Ribosomes are large ribonucleoprotein particles that contain more RNA than protein and dissociate into large and small subunits.
The ribosome conformation is flexible during protein synthesis. The most abundant RNAs in cells are the ribosomal RNAs (rRNAs) which accounts 80% of the RNA. Ribosomal RNAs, form the basic structure of the ribosome and catalyse protein synthesis. The rRNAs appear to serve as a framework to which ribosomal proteins are bound. In prokaryotes, the association and dissociation of ribosomal subunits depends on interactions with 16S rRNA. Ribosomal RNA is exposed on the 30S surface. 16S rRNA plays an active role in protein synthesis. The 3’ terminus of the rRNA interacts directly with mRNA at initiation by pairing with the Shine-Dalgarno sequence in the ribosome binding site of mRNA. 23 S rRNA has peptidyl transferase activity. 23 S rRNA forms a highly structured pocket that, through a network of hydrogen bonds, precisely orients the two reactants (growing polypeptide chain and an aminoacyl-tRNA) and thereby greatly accelerates their covalent joining. The 55 proteins in the bacterial ribosome serve as either enzymes or structural components in protein synthesis. The ribosomes of eukaryotic cells (other than mitochondrial and chloroplast ribosomes) are larger than those of bacteria. The total content of both RNA and protein is greater; the major RNA molecules are longer (18S&28S) and there are more proteins. As no living creature can survive without ribosomes, they are the perfect targets for drugs. Many of today’s antibiotics attack the ribosomes of bacteria, but leave those of humans alone. Scientists study these structures in order to design new and more effective antibiotics.
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Scientist at NRCPB, New Delhi.
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