cell, showing subcellular components including ribosomes" (3).
Organelles: (1) nucleolus(2) nucleus (3) ribosomes (little dots) (4) vesicle (5) rough endoplasmic reticulum(ER) (6) Golgi apparatus(7) Cytoskeleton(8) smooth ER (9) mitochondria (10) vacuole(11) cytoplasm(12) lysosome(13) centrioles within centrosome]
Ribosomes ("from ribonucleic acid and "Greek: soma ("meaning" body)") are complexes of
RNAand proteinthat are found in all cells. Ribosomes from bacteria, archaeaand eukaryotes, the three domains of life, have significantly different structure and RNA. Interestingly, the ribosomes in the mitochondrionof eukaryotic cells resemble those in bacteria, reflecting the evolutionary origin of this organelle. [cite journal |author=Benne R, Sloof P |title=Evolution of the mitochondrial protein synthetic machinery |journal=BioSystems |volume=21 |issue=1 |pages=51–68 |year=1987 |pmid=2446672 |doi=10.1016/0303-2647(87)90006-2]
The ribosome functions in the expression of the genetic code from nucleic acid into
protein, in a process called "translation". Ribosomes do this by catalyzing the assembly of individual amino acids into polypeptidechains; this involves binding a messenger RNAand then using this as a template to join together the correct sequence of amino acids. This reaction uses adapters called transfer RNAmolecules, which read the sequence of the messenger RNA and are attached to the amino acids.
Ribosomes are about 20nm (200
Ångström) in diameter and are composed of 65% ribosomal RNAand 35% ribosomal proteins (known as a Ribonucleoproteinor RNP). They translate messenger RNA (mRNA) to build polypeptidechains (e.g., proteins) using amino acids delivered by transfer RNA (tRNA). Their active sites are made of RNA, so ribosomes are now classified as " ribozymes." [cite journal |author=Rodnina MV, Beringer M, Wintermeyer W |title=How ribosomes make peptide bonds |journal=Trends Biochem. Sci. |volume=32 |issue=1 |pages=20–6 |year=2007 |pmid=17157507 |doi=10.1016/j.tibs.2006.11.007]
Ribosomes build proteins from the genetic instructions held within
messenger RNA. 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 appearance of roughness and thus its name, or to the nuclear envelope. As ribozymes are partly constituted from RNA, it is thought that they might be remnants of the RNA world. [cite journal |author=Cech T |title=Structural biology. The ribosome is a ribozyme |journal=Science |volume=289 |issue=5481 |pages=878–9 |year=2000 |pmid=10960319 | doi = 10.1126/science.289.5481.878 ] Catalysis of the peptide bondinvolves the C2 hydroxyl of RNA's P-site adenosine in a protein shuttle mechanism. The full function (i.e. translocation) of the ribosome is reliant on changes in protein conformations. Ribosomes are sometimes referred to as organelles, but the use of the term "organelle" is often used only in reference to sub-cellular components that include a phosholipid membrane, which ribosomes, being entirely particulate, do not. For this reason, ribosomes may sometimes be described as "non-membranous organelles".
Ribosomes are an extremely important structure in the cell. Ribosomes were first observed in the mid-1950s by
Romanian cell biologist George Paladeusing an electron microscopeas dense particles or granules [G.E. Palade. (1955) "A small particulate component of the cytoplasm." "J Biophys Biochem Cytol." Jan;1(1): pages 59-68. PMID 14381428] for which he would win the Nobel Prize. The term "ribosome" was proposed by scientist Richard B. Roberts in 1958:
The structure and function of the ribosomes and associated molecules, known as the "translational apparatus", has been of research interest since the mid-twentieth century and is a very active field of study today.
Ribosomes consist of two subunits (Figure 1) that fit together (Figure 2) and work as one to translate the
mRNAinto a polypeptide chain during protein synthesis (Figure 3). Bacterialsubunits consist of one or two and eukaryoticof one or three very large RNAmolecules (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 suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesize protein rather than directly participating in catalysis (See: Ribozyme).
In bacterial cells, ribosomes are synthesized in the cytoplasm through the transcription of multiple ribosome gene
operons. In eukaryotes and some bacterial cells, the process takes place both in the cell cytoplasm and in the nucleolus of eukaryotic cells. It involves the coordinated function of over 200 proteins in the synthesis and processing of the four rRNAs, as well as assembly of those rRNAs with the ribosomal proteins.
Ribosomes are classified as being either "free" or "membrane-bound."
Free ribosomes are free to move about anywhere in the
cytosol. Proteins that are formed from free ribosomes are used within the cell. Proteins containing disulfide bondsusing cysteine amino acidscannot be produced outside of the lumen of the endoplasmic reticulum.
When certain proteins are synthesized by a ribosome they can become "membrane-bound". The newly produced polypeptide chains are inserted directly into the
endoplasmic reticulumby the ribosome and are then transported to their destinations. Bound ribosomes usually produce proteins that are used within the cell membrane or are expelled from the cell via " exocytosis".
Free and membrane-bound ribosomes differ only in their spatial distribution; they are identical in structure and function. Whether the ribosome exists in a free or membrane-bound state depends on the presence of a ER-targeting signal sequence on the protein being synthesized.
cite journal ">author=Schluenzen F, Tocilj A, Zarivach R, Harms J, Gluehmann M, Janell D, Bashan A, Bartels H, Agmon I, Franceschi F, Yonath A |title=Structure of functionally activated small ribosomal subunit at 3.3 angstroms resolution |journal=Cell |volume=102 |issue=5 |pages=615–23 |year=2000 |pmid=11007480 | doi = 10.1016/S0092-8674(00)00084-2 ] The ribosomal subunits of
bacterias and eukaryotes are quite similar. The Molecular Biology of the Cell, fourth eddition. Bruce Alberts, et al. Garland Science (2002) pg. 342 ISBN 0-8153-3218-1]
Bacteria have 70S ribosomes, each consisting of a small (
30S) and a large ( 50S) subunit. Their large subunit is composed of a 5S RNAsubunit (consisting of 120 nucleotides), a 23S RNA subunit (2900 nucleotides) and 34 proteins. The 30S subunit has a 1540 nucleotide RNA subunit (16S) bound to 21 proteins.
Eukaryotes have 80S ribosomes, each consisting of a small (
40S) and large ( 60S) subunit. Their large subunit is composed of a 5S RNA (120 nucleotides), a 28S RNA (4700 nucleotides), a 5.8S subunit (160 nucleotides) and ~49 proteins. The 40S subunit has a 1900 nucleotide (18S) RNA and ~33 proteins.
The ribosomes found in
chloroplasts and mitochondriaof eukaryotes also consist of large and small subunits bound together with proteins into one 70S particle. These organellesare believed to be descendants of bacteria(see Endosymbiotic theory) and as such their ribosomes are similar to those of bacteria. [ The Molecular Biology of the Cell, fourth edition. Bruce Alberts, et al. Garland Science (2002) pg. 808 ISBN 0-8153-3218-1]
The various ribosomes share a core structure which is quite similar despite the large differences in size. The extra
RNAin the larger ribosomes is in several long continuous insertions, such that they form loops out of the core structure without disrupting or changing it. The Molecular Biology of the Cell, fourth edition. Brusce Alberts, et al. Garland Science (2002) pg. 342 ISBN 0-8153-3218-1] All of the catalytic activity of the ribosome is carried out by the RNA, the proteins reside on the surface and seem to stabilize the structure.
The differences between the bacterial and eukaryotic ribosomes are exploited by pharmaceutical chemists to create
antibiotics that can destroy a bacterial infection without harming the cells of the infected person. Due to the differences in their structures, the bacterial 70S ribosomes are vulnerable to these antibiotics while the eukaryotic 80S ribosomes are not.cite journal |author=Recht MI, Douthwaite S, Puglisi JD |title=Basis for bacterial specificity of action of aminoglycoside antibiotics |journal=EMBO J |volume=18 |issue=11 |pages=3133–8 |year=1999 |pmid=10357824 | doi = 10.1093/emboj/18.11.3133 ] Even though mitochondriapossess ribosomes similar to the bacterial ones, mitochondria are not affected by these antibiotics because they are surrounded by a double membrane that does not easily admit these antibiotics into the organelle. [ O'Brien, T.W., The General Occurrence of 55S Ribosomes in Mammalian Liver Mitochondria. J. Biol. Chem., 245:3409 (1971).]
Haloarcula marismortui". Proteins are shown in blue and the two RNA strands in orange and yellow.cite journal |author=Ban N, Nissen P, Hansen J, Moore P, Steitz T |title=The complete atomic structure of the large ribosomal subunit at 2.4 A resolution |journal=Science |volume=289 |issue=5481 |pages=905–20 |year=2000 |pmid=10937989 | doi = 10.1126/science.289.5481.905 ] The small patch of green in the center of the subunit is the active site.] The general molecular structure of the ribosome has been known since the early 1970s. In the early 2000s the structure has been achieved at high resolutions, in the order of a few Ångströms.
The first papers giving the structure of the ribosome at atomic resolution were published in rapid succession in late 2000. First, the 50S (large bacteria) subunit from the
archea, "Haloarcula marismortui" was published. Soon after the structure of the 30S subunit from " Thermus thermophilus" was published.cite journal |author=Schluenzen F, Tocilj A, Zarivach R, Harms J, Gluehmann M, Janell D, Bashan A, Bartels H, Agmon I, Franceschi F, Yonath A |title=Structure of functionally activated small ribosomal subunit at 3.3 angstroms resolution |journal=Cell |volume=102 |issue=5 |pages=615–23 |year=2000 |pmid=11007480 | doi = 10.1016/S0092-8674(00)00084-2 ] Shortly thereafter a more detailed structure was published. [Wimberly BT, Brodersen DE, Clemons WM Jr, Morgan-Warren RJ, Carter AP, Vonrhein C, Hartsch T, Ramakrishnan V. Structure of the 30S ribosomal subunit. "Nature." 2000 Sep 21;407(6802):327-39. PMID 11014182] Early the next year (May 2001) these coordinates were used to reconstruct the entire "T. thermophilus" 70S particle at 5.5 Ångström resolution. [Yusupov MM, Yusupova GZ, Baucom A, Lieberman K, Earnest TN, Cate JH, Noller HF. Crystal structure of the ribosome at 5.5 Å resolution. "Science." 2001 May 4;292(5518):883-96. Epub 2001 Mar 29. PMID 11283358]
Two papers were published in November 2005 with structures of the "
Escherichia coli" 70S ribosome. The structures of vacant ribosome were determined at 3.5 Ångström resolution using x-ray crystallography. [Schuwirth BS, Borovinskaya MA, Hau CW, Zhang W, Vila-Sanjurjo A, Holton JM, Cate JH. Structures of the bacterial ribosome at 3.5 Ångström resolution. "Science." 2005 Nov 4;310(5749):827-34. PMID 16272117] Then, two weeks later, a structure based on cryo-electron microsopy was published, [Mitra K, Schaffitzel C, Shaikh T, Tama F, Jenni S, Brooks CL 3rd, Ban N, Frank J. Structure of the "E. coli" protein-conducting channel bound to a translating ribosome. "Nature." 2005 Nov 17;438(7066):318-24. PMID 16292303] which depicts the ribosome at 11-15 Ångström resolution in the act of passing a newly synthesized protein strand into the protein-conducting channel.
First atomic structures of the ribosome complexed with
tRNAand mRNAmolecules were solved by using X-ray crystallography by two groups independently, at 2.8 Ångström [Selmer, M., Dunham, C.M., Murphy, F.V IV, Weixlbaumer, A., Petry S., Kelley, A.C., Weir, J.R. and Ramakrishnan, V. (2006). Structure of the 70S ribosome complexed with mRNA and tRNA. Science , 313, 1935-1942. PMID 16959973 ] and at 3.7 Ångström. [ Korostelev A, Trakhanov S, Laurberg M, Noller HF. Crystal structure of a 70S ribosome-tRNA complex reveals functional interactions and rearrangements. Cell. 2006 Sep 22;126(6):1065-77] These structures allow one to see the details of interactions of the " Thermus thermophilus" ribosome with mRNAand with tRNAs bound at classical ribosomal sites. Interactions of the ribosome with long mRNAs containing Shine-Dalgarno sequences were visualized soon after that at 4.5 to 5.5 Ångström resolution. [Yusupova G, Jenner L, Rees B, Moras D, Yusupov M. Structural basis for messenger RNA movement on the ribosome. Nature. 2006 Nov 16;444(7117):391-4]
Ribosomes are the workhorses of
protein biosynthesis, the process of translating mRNA into protein. The mRNA comprises a series of codons that dictate to the ribosome the sequence of the amino acids needed to make the protein. Using the mRNA as a template, the ribosome traverses each codon (3 nucleotides) of the mRNA, pairing it with the appropriate amino acid provided by a tRNA. Molecules of transfer RNA(tRNA) contain a complementary anticodonon one end and the appropriate amino acid on the other. The small ribosomal subunit, typically bound to a tRNA containing the amino acid methionine, binds to an AUG codon on the mRNA and recruits the large ribosomal subunit. The ribosome then contains three RNA binding sites, designated A, P, and E. The A site binds an aminoacyl-tRNA (a tRNA bound to an amino acid); the P site binds a peptidyl-tRNA (a tRNA bound to the peptide being synthesized); and the E site binds a free tRNA before it exits the ribosome. Protein synthesis begins at a start codonAUG near the 5' end of the mRNA. mRNA binds to the P site of the ribosome first. The ribosome is able to identify the start codon by use of the Shine-Dalgarno sequence of the mRNA in prokaryotes and Kazak box in eukaryotes.
In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (towards the 5' end of the mRNA). The ribosome uses
tRNAwhich matches the current codon (triplet) on the mRNA to append an amino acidto the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually in bacterial cells, several ribosomes are working parallel on a single mRNA, forming what is called a "polyribosome" or " polysome".
Wobble base pair
* [http://www.bio.cmu.edu/Courses/BiochemMols/ribosome/70S.htm 70S Ribosome Architecture Animation of a working ribosome] . Requires the Chime browser plugin from [http://www.mdl.com/products/framework/chime/ this site] (where registration is required).
* [http://news.com.com/Lab+computer+simulates+ribosome+in+motion/2100-11395_3-5907401.html Lab computer simulates ribosome in motion]
* [http://www.cytochemistry.net/Cell-biology/ribosome.htm Role of the Ribosome] , Gwen V. Childs, copied [http://cellbio.utmb.edu/cellbio/ribosome.htm here]
* [http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/index.html Molecule of the Month] [http://home.rcsb.org/ © RCSB Protein Data Bank] :
** [http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb10_1.html Ribosome]
** [http://www.pdb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb81_1.html Elongation Factors]
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