amino acids

Description

The ribosome is a large and complex molecular machine, found within all living cells, that serves as the primary site of biological protein synthesis. Ribosomes link amino acids together in the order specified by messenger RNA molecules. Ribosomes consist of two major subunits—the small ribosomal subunit reads the mRNA, while the large subunit joins amino acids to form a polypeptide chain. Each subunit is composed of one or more ribosomal RNA molecules and a variety of proteins. The sequence of DNA encoding for a protein may be copied many times into messenger RNA (mRNA) chains of a similar sequence. Ribosomes can bind to an mRNA chain and use it as a template for determining the correct sequence of amino acids in a particular protein. Amino acids are selected, collected and carried to the ribosome by transfer RNA (tRNA molecules), which enter one part of the ribosome and bind to the messenger RNA chain. The attached amino acids are then linked together by another part of the ribosome. Once the protein is produced, it can then 'fold' to produce a specific functional three-dimensional structure. A ribosome is made from complexes of RNAs and proteins and is therefore a ribonucleoprotein. Each ribosome is divided into two subunits: the smaller subunit binds to the mRNA pattern, while the larger subunit binds to the tRNA and the amino acids. When a ribosome finishes reading an mRNA molecule, these two subunits split apart. Ribosomes are ribozymes, because the catalytic peptidyl transferase activity that links amino acids together is performed by the ribosomal RNA. Ribosomes from bacteria, archaea and eukaryotes (the three domains of life on Earth) differ in their size, sequence, structure, and the ratio of protein to RNA. The differences in structure allow some antibiotics to kill bacteria by inhibiting their ribosomes, while leaving human ribosomes unaffected. In bacteria and archaea, more than one ribosome may move along a single mRNA chain at one time, each "reading" its sequence and producing a corresponding protein molecule. The ribosomes in the mitochondria of eukaryotic cells functionally resemble many features of those in bacteria, reflecting the likely evolutionary origin of mitochondria. Albert Claude, Christian de Duve, and George Emil Palade were jointly awarded the Nobel Prize in Physiology or Medicine, in 1974, for the discovery of the ribosomes. The Nobel Prize in Chemistry 2009 was awarded to Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath for determining the detailed structure and mechanism of the ribosome. Description Ribosomes consist of two subunits that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 3). Because they are formed from two subunits of non-equal size, they are slightly longer in the axis than in diameter. Prokaryotic ribosomes are around 20 nm (200 Å) in diameter and are composed of 65% ribosomal RNA and 35% ribosomal proteins. Eukaryotic ribosomes are between 25 and 30 nm (250--300 Å) in diameter and the ratio of rRNA to protein is close to 1. Bacterial subunits consist of one or two and eukaryotic of one or three very large RNA molecules (known as ribosomal RNA or rRNA) and multiple smaller protein molecules. Crystallographic work has shown that there are no ribosomal proteins close to the reaction site for polypeptide synthesis. This proves that the protein components of ribosomes do not directly participate in peptide bond formation catalysis, but rather suggests that these proteins act as a scaffold that may enhance the ability of rRNA to synthesize protein (See: Ribozyme). Ribosomes translate polypeptide chains (e.g., proteins) from the genetic instructions held within messenger RNA, using amino acids delivered by transfer RNA (tRNA). Free ribosomes are suspended in the cytosol (the semi-fluid portion of the cytoplasm); others are bound to the rough endoplasmic reticulum, giving it the appearan...