Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, transporting molecules from one location to another. Proteins differ from one another in their sequence of amino acids, dictated by the nucleotide sequence of their genes, which results in protein folding into a specific three-dimensional structure that determines its activity. A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than 20–30 residues, are considered to be proteins and are called peptides, or sometimes oligopeptides; the individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, encoded in the genetic code.
In general, the genetic code specifies 20 standard amino acids. Shortly after or during synthesis, the residues in a protein are chemically modified by post-translational modification, which alters the physical and chemical properties, stability and the function of the proteins. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors. Proteins can work together to achieve a particular function, they associate to form stable protein complexes. Once formed, proteins only exist for a certain period and are degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan covers a wide range, they can exist for years with an average lifespan of 1 -- 2 days in mammalian cells. Abnormal or misfolded proteins are degraded more either due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms and participate in every process within cells.
Many proteins are enzymes that are vital to metabolism. Proteins have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. Other proteins are important in cell signaling, immune responses, cell adhesion, the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized. Digestion breaks the proteins down for use in the metabolism. Proteins may be purified from other cellular components using a variety of techniques such as ultracentrifugation, precipitation and chromatography. Methods used to study protein structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance and mass spectrometry. Most proteins consist of linear polymers built from series of up to 20 different L-α- amino acids. All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, a variable side chain are bonded.
Only proline differs from this basic structure as it contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed conformation. The side chains of the standard amino acids, detailed in the list of standard amino acids, have a great variety of chemical structures and properties; the amino acids in a polypeptide chain are linked by peptide bonds. Once linked in the protein chain, an individual amino acid is called a residue, the linked series of carbon and oxygen atoms are known as the main chain or protein backbone; the peptide bond has two resonance forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are coplanar. The other two dihedral angles in the peptide bond determine the local shape assumed by the protein backbone; the end with a free amino group is known as the N-terminus or amino terminus, whereas the end of the protein with a free carboxyl group is known as the C-terminus or carboxy terminus.
The words protein and peptide are a little ambiguous and can overlap in meaning. Protein is used to refer to the complete biological molecule in a stable conformation, whereas peptide is reserved for a short amino acid oligomers lacking a stable three-dimensional structure. However, the boundary between the two is not well defined and lies near 20–30 residues. Polypeptide can refer to any single linear chain of amino acids regardless of length, but implies an absence of a defined conformation. Proteins can interact with many types of molecules, including with other proteins, with lipids, with carboyhydrates, with DNA, it has been estimated. Smaller bacteria, such as Mycoplasma or spirochetes contain fewer molecules, on the order of 50,000 to 1 million. By contrast, eukaryotic cells are larger and thus contain much more pro
Hyaluronic acid called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed throughout connective and neural tissues. It is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi apparatus, can be large: human synovial HA averages about 7 million Da per molecule, or about twenty thousand disaccharide monomers, while other sources mention 3–4 million Da. One of the chief components of the extracellular matrix, contributes to cell proliferation and migration, may be involved in the progression of some malignant tumors; the average 70 kg person has 15 grams of hyaluronan in the body, one-third of, turned over every day. Hyaluronic acid is a component of the group A streptococcal extracellular capsule, is believed to play a role in virulence; until the late 1970s, hyaluronic acid was described as a "goo" molecule, a ubiquitous carbohydrate polymer, part of the extracellular matrix. For example, hyaluronic acid is a major component of the synovial fluid, was found to increase the viscosity of the fluid.
Along with lubricin, it is one of the fluid's main lubricating components. Hyaluronic acid is an important component of articular cartilage, where it is present as a coat around each cell; when aggrecan monomers bind to hyaluronan in the presence of HAPLN1, large negatively charged aggregates form. These aggregates are responsible for the resilience of cartilage; the molecular weight of hyaluronan in cartilage decreases with age. A lubricating role of hyaluronan in muscular connective tissues to enhance the sliding between adjacent tissue layers has been suggested. A particular type of fibroblasts, embedded in dense fascial tissues, has been proposed as being cells specialized for the biosynthesis of the hyaluronan-rich matrix, their related activity could be involved in regulating the sliding ability between adjacent muscular connective tissues. Hyaluronic acid is a major component of skin, where it is involved in tissue repair; when skin is exposed to excessive UVB rays, it becomes inflamed and the cells in the dermis stop producing as much hyaluronan, increase the rate of its degradation.
Hyaluronan degradation products accumulate in the skin after UV exposure. While it is abundant in extracellular matrices, hyaluronan contributes to tissue hydrodynamics and proliferation of cells, participates in a number of cell surface receptor interactions, notably those including its primary receptors, CD44 and RHAMM. Upregulation of CD44 itself is accepted as a marker of cell activation in lymphocytes. Hyaluronan's contribution to tumor growth may be due to its interaction with CD44. Receptor CD44 participates in cell adhesion interactions required by tumor cells. Although hyaluronan binds to receptor CD44, there is evidence hyaluronan degradation products transduce their inflammatory signal through toll-like receptor 2, TLR4, or both TLR2 and TLR4 in macrophages and dendritic cells. TLR and hyaluronan play a role in innate immunity. There are limitations including the in vivo loss of this compound limiting the duration of effect. In some cancers, hyaluronic acid levels correlate well with poor prognosis.
Hyaluronic acid is, thus used as a tumor marker for prostate and breast cancer. It may be used to monitor the progression of the disease; as shown in Figure 1, the various types of molecules that interact with hyaluronan can contribute to many of the stages of cancer metastasis, i.e. further the spread of cancer. Hyaluronic acid synthases play roles in all stages of cancer metastasis. By producing anti-adhesive HA, HAS can allow tumor cells to release from the primary tumor mass, if HA associates with receptors such as CD44, the activation of Rho GTPases can promote epithelial–mesenchymal transition of the cancer cells. During the processes of intravasation or extravasation, the interaction of HAS produced HA with receptors such as CD44 or RHAMM promote the cell changes that allow for the cancer cells to infiltrate the vascular or lymphatic systems. While traveling in these systems, HA produced by HAS protects the cancer cell from physical damage. In the formation of a metastatic lesion, HAS produces HA to allow the cancer cell to interact with native cells at the secondary site and to produce a tumor for itself.
HA fragments promote angiogenesis, hyaluronidases produce these fragments. Hypoxia increases production of HA and activity of hyaluronidases; the hyaluronic acid receptors, CD44 and RHAMM, are most studied in terms of their roles in cancer metastasis. Increased clinical CD44 expression has been positively correlated to metastasis in a number of tumor types. In terms of mechanics, CD44 affects adhesion of cancer cells to each other and to endothelial cells, rearranges the cytoskeleton through the Rho GTPases, increases the activity of ECM degrading enzymes. Increased RHAMM expression has been clinically correlated with cancer metastasis. In terms of mechanics, RHAMM promotes cancer cell motility through a number of pathways including focal adhesion kinase, MAP kinase, pp60, the downstream targets of Rho kinase. RHAMM can cooperate with CD44 to promote angiogenesis toward the metastatic lesion. Hyaluronic acid is a main component of the extracellular matrix, has a key role in tissue regeneration, inflammation response, angiogenesis, which are phases of skin wound repair.
As of 2016, reviews assessing its effect to promote wo
Amino acids are organic compounds containing amine and carboxyl functional groups, along with a side chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen and nitrogen, although other elements are found in the side chains of certain amino acids. About 500 occurring amino acids are known and can be classified in many ways, they can be classified according to the core structural functional groups' locations as alpha-, beta-, gamma- or delta- amino acids. In the form of proteins, amino acid residues form the second-largest component of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first carbon atom have particular importance, they are known as α-amino acids. They include the 22 proteinogenic amino acids, which combine into peptide chains to form the building-blocks of a vast array of proteins.
These are all L-stereoisomers, although a few D-amino acids occur in bacterial envelopes, as a neuromodulator, in some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codons in the genetic code and are known as "standard" amino acids; the other two are selenocysteine, pyrrolysine. Pyrrolysine and selenocysteine are encoded via variant codons. N-formylmethionine is considered as a form of methionine rather than as a separate proteinogenic amino acid. Codon–tRNA combinations not found in nature can be used to "expand" the genetic code and form novel proteins known as alloproteins incorporating non-proteinogenic amino acids. Many important proteinogenic and non-proteinogenic amino acids have biological functions. For example, in the human brain and gamma-amino-butyric acid are the main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major component of the connective tissue collagen, is synthesised from proline. Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport. Nine proteinogenic amino acids are called "essential" for humans because they cannot be produced from other compounds by the human body and so must be taken in as food. Others may be conditionally essential for medical conditions. Essential amino acids may differ between species; because of their biological significance, amino acids are important in nutrition and are used in nutritional supplements, fertilizers and food technology. Industrial uses include the production of drugs, biodegradable plastics, chiral catalysts; the first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus, subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, remained undiscovered until 1884. Glycine and leucine were discovered in 1820; the last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. Usage of the term "amino acid" in the English language is from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". In the structure shown at the top of the page, R represents a side chain specific to each amino acid; the carbon atom next to the carboxyl group is called the α–carbon. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids; these include amino acids such as proline which contain secondary amines, which used to be referred to as "imino acids". The alpha amino acids are the most common form found in nature, but only when occurring in the L-isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon. Therefore, all alpha amino acids but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amin
Vesicle (biology and chemistry)
In cell biology, a vesicle is a large structure within a cell, or extracellular, consisting of liquid enclosed by a lipid bilayer. Vesicles form during the processes of secretion and transport of materials within the plasma membrane. Alternatively, they may be prepared artificially. If there is only one phospholipid bilayer, they are called unilamellar liposome vesicles; the membrane enclosing the vesicle is a lamellar phase, similar to that of the plasma membrane and vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can fuse with other organelles within the cell. Vesicles perform a variety of functions; because it is separated from the cytosol, the inside of the vesicle can be made to be different from the cytosolic environment. For this reason, vesicles are a basic tool used by the cell for organizing cellular substances. Vesicles are involved in metabolism, buoyancy control, temporary storage of food and enzymes, they can act as chemical reaction chambers.
The 2013 Nobel Prize in Physiology or Medicine was shared by James Rothman, Randy Schekman and Thomas Südhof for their roles in elucidating the makeup and function of cell vesicles in yeasts and in humans, including information on each vesicle's parts and how they are assembled. Vesicle dysfunction is thought to contribute to Alzheimer's disease, some hard-to-treat cases of epilepsy, some cancers and immunological disorders and certain neurovascular conditions. Vacuoles are cellular organelles which contain water. Plant cells have a large central vacuole in the center of the cell, used for osmotic control and nutrient storage. Contractile vacuoles are found in certain protists those in Phylum Ciliophora; these vacuoles take water from the cytoplasm and excrete it from the cell to avoid bursting due to osmotic pressure. Lysosomes are involved in cellular digestion. Food can be taken from outside the cell into food vacuoles by a process called endocytosis; these food vacuoles fuse with lysosomes which break down the components so that they can be used in the cell.
This form of cellular eating is called phagocytosis. Lysosomes are used to destroy defective or damaged organelles in a process called autophagy, they fuse with the membrane of the damaged organelle. Transport vesicles can move molecules between locations inside the cell, e.g. proteins from the rough endoplasmic reticulum to the Golgi apparatus. Membrane-bound and secreted proteins are made on ribosomes found in the rough endoplasmic reticulum. Most of these proteins mature in the Golgi apparatus before going to their final destination which may be to lysosomes, peroxisomes, or outside of the cell; these proteins travel within the cell inside of transport vesicles. Secretory vesicles contain materials. Cells have many reasons to excrete materials. One reason is to dispose of wastes. Another reason is tied to the function of the cell. Within a larger organism, some cells are specialized to produce certain chemicals; these chemicals are released when needed. Synaptic vesicles are located at presynaptic terminals in neurons and store neurotransmitters.
When a signal comes down an axon, the synaptic vesicles fuse with the cell membrane releasing the neurotransmitter so that it can be detected by receptor molecules on the next nerve cell. In animals endocrine tissues release hormones into the bloodstream; these hormones are stored within secretory vesicles. A good example is the endocrine tissue found in the islets of Langerhans in the pancreas; this tissue contains many cell types. Secretory vesicles hold the enzymes that are used to make the cell walls of plants, fungi and Archaea cells as well as the extracellular matrix of animal cells. Bacteria, Archaea and parasites release membrane vesicles containing varied but specialized toxic compounds and biochemical signal molecules, which are transported to target cells to initiate processes in favour of the microbe, which include invasion of host cells and killing of competing microbes in the same niche. Extracellular vesicles are produced by all domains of life including complex eukaryotes, both Gram-negative and Gram-positive bacteria and fungi.
Exosomes: membraneous vesicles of endocytic origin enriched in CD63 and CD81. Microvesicle, that are shed directly from the plasma membrane. Membrane particles, or large membranous vesicles CD133+, CD63− Apoptotic blebs or blebbing vesicles: released by dying cells; these are separated by density by differential centrifugation. Ectosomes were named in 2008. In humans, endogenous extracellular vesicles play a role in coagulation, intercellular signaling and waste management, they are implicated in the pathophysiological processes involved in multiple diseases, including cancer. Extracellular vesicles have raised interest as a potential source of biomarker discovery because of their role in intercellular communication, release into accessible body fluids and the resemblance of their molecular content to that of the releasing cells; the extracellular vesicles of stem cells known as the secretome of stem cells, are being researched and applied for therapeutic purposes, predominantly degenerative, auto-immune and/or inflammatory diseases.
A lysosome is a membrane-bound organelle found in many animal cells and most plant cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane proteins, its lumenal proteins; the lumen's pH is optimal for the enzymes involved in hydrolysis, analogous to the activity of the stomach. Besides degradation of polymers, the lysosome is involved in various cell processes, including secretion, plasma membrane repair, cell signaling, energy metabolism; the lysosomes act as the waste disposal system of the cell by digesting unwanted materials in the cytoplasm, both from outside the cell and obsolete components inside the cell. Material from outside the cell is taken-up through endocytosis, while material from the inside of the cell is digested through autophagy, their sizes can be different—the largest ones can be more than 10 times the size of the smallest ones. They were discovered and named by Belgian biologist Christian de Duve, who received the Nobel Prize in Physiology or Medicine in 1974.
Lysosomes are known to contain more than 60 different enzymes, have more than 50 membrane proteins. Enzymes of the lysosomes are synthesised in the rough endoplasmic reticulum; the enzymes are imported from the Golgi apparatus in small vesicles, which fuse with larger acidic vesicles. Enzymes destined for a lysosome are tagged with the molecule mannose 6-phosphate, so that they are properly sorted into acidified vesicles. Synthesis of lysosomal enzymes is controlled by nuclear genes. Mutations in the genes for these enzymes are responsible for more than 30 different human genetic disorders, which are collectively known as lysosomal storage diseases; these diseases result from an accumulation of specific substrates, due to the inability to break them down. These genetic defects are related to several neurodegenerative disorders, cardiovascular diseases, ageing-related diseases. Lysosomes should not be confused with micelles. Christian de Duve, the chairman of the Laboratory of Physiological Chemistry at the Catholic University of Louvain in Belgium, had been studying the mechanism of action of a pancreatic hormone insulin in liver cells.
By 1949, he and his team had focused on the enzyme called glucose 6-phosphatase, the first crucial enzyme in sugar metabolism and the target of insulin. They suspected that this enzyme played a key role in regulating blood sugar levels; however after a series of experiments, they failed to purify and isolate the enzyme from the cellular extracts. Therefore, they tried a more arduous procedure of cell fractionation, by which cellular components are separated based on their sizes using centrifugation, they succeeded in detecting the enzyme activity from the microsomal fraction. This was the crucial step in the serendipitous discovery of lysosomes. To estimate this enzyme activity, they used that of standardised enzyme acid phosphatase, found that the activity was only 10% of the expected value. One day, the enzyme activity of purified cell fractions, refrigerated for five days was measured; the enzyme activity was increased to normal of that of the fresh sample. The result was the same no matter how many times they repeated the estimation, led to the conclusion that a membrane-like barrier limited the accessibility of the enzyme to its substrate, that the enzymes were able to diffuse after a few days.
They described this membrane-like barrier as a "saclike structure surrounded by a membrane and containing acid phosphatase."It became clear that this enzyme from the cell fraction came from membranous fractions, which were cell organelles, in 1955 De Duve named them "lysosomes" to reflect their digestive properties. The same year, Alex B. Novikoff from the University of Vermont visited de Duve's laboratory, obtained the first electron micrographs of the new organelle. Using a staining method for acid phosphatase, de Duve and Novikoff confirmed the location of the hydrolytic enzymes of lysosomes using light and electron microscopic studies. De Duve won the Nobel Prize in Medicine in 1974 for this discovery. De Duve had termed the organelles the "suicide bags" or "suicide sacs" of the cells, for their hypothesized role in apoptosis. However, it has since been concluded. Lysosomes contain a variety of enzymes, enabling the cell to break down various biomolecules it engulfs, including peptides, nucleic acids and lipids.
The enzymes responsible for this hydrolysis require an acidic environment for optimal activity. In addition to being able to break down polymers, lysosomes are capable of fusing with other organelles & digesting large structures or cellular debris, they are able to break-down virus particles or bacteria in phagocytosis of macrophages. The size of lysosomes varies from 0.1 μm to 1.2 μm. With a pH ranging from ~4.5–5.0, the interior of the lysosomes is acidic compared to the basic cytosol. The lysosomal membrane protects the cytosol, therefore the rest of the cell, from the degradative enzymes within the lysosome; the cell is additionally protected from any lysosomal acid hydrolases that drain into the cytosol, as these enzymes are pH-sensitive and do not function well or at all in the alkaline environment of the cytosol. This ensures that cytosolic molecules and organelles are not destroyed in case there is leakage of the hydrolytic enzymes from the lysosome; the lysosome maintains its pH differen
Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air, its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium; the most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life. The name derives from Latin calx "lime", obtained from heating limestone; some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy, who named the element. Calcium compounds are used in many industries: in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, in the manufacture of soaps.
On the other hand, the metal in pure form has few applications due to its high reactivity. Calcium is the fifth-most abundant element in the human body; as electrolytes, calcium ions play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes as well as proper bone formation. Calcium is a ductile silvery metal whose properties are similar to the heavier elements in its group, strontium and radium. A calcium atom has twenty electrons, arranged in the electron configuration 4s2. Like the other elements placed in group 2 of the periodic table, calcium has two valence electrons in the outermost s-orbital, which are easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a noble gas, in this case argon. Hence, calcium is always divalent in its compounds, which are ionic.
Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to disproportionation to the divalent salts and calcium metal, because the enthalpy of formation of MX2 is much higher than those of the hypothetical MX. This occurs because of the much greater lattice energy afforded by the more charged Ca2+ cation compared to the hypothetical Ca+ cation. Calcium, strontium and radium are always considered to be alkaline earth metals. Beryllium and magnesium are different from the other members of the group in their physical and chemical behaviour: they behave more like aluminium and zinc and have some of the weaker metallic character of the post-transition metals, why the traditional definition of the term "alkaline earth metal" excludes them; this classification is obsolete in English-language sources, but is still used in other countries such as Japan. As a result, comparisons with strontium and barium are more germane to calcium chemistry than comparisons with magnesium.
Calcium metal melts at 842 °C and boils at 1494 °C. It crystallises in the face-centered cubic arrangement like strontium, its density of 1.55 g/cm3 is the lowest in its group. Calcium can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it is a better conductor by mass than both due to its low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts with atmospheric oxygen, its use as such in space has been considered; the chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more than magnesium and less than strontium to produce calcium hydroxide and hydrogen gas, it reacts with the oxygen and nitrogen in the air to form a mixture of calcium oxide and calcium nitride. When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it forms a hydration coating in moist air, but below 30% relative humidity it may be stored indefinitely at room temperature.
Besides the simple oxide CaO, the peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, there is some evidence for a yellow superoxide Ca2. Calcium hydroxide, Ca2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. Calcium carbonate and calcium sulfate are abundant minerals. Like strontium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, calcium metal dissolves directly in liquid ammonia to give a dark blue solution. Due to the large size of the Ca2+ ion, high coordination numbers are common, up to 24 in some intermetallic compounds such as CaZn13. Calcium is complexed by oxygen chelates such as EDTA and polyphosphates, which are useful in an
The ribosome is a complex molecular machine, found within all living cells, that serves as the site of biological protein synthesis. Ribosomes link amino acids together in the order specified by messenger RNA molecules. Ribosomes consist of two major components: the small ribosomal subunits, which read the RNA, the large subunits, which join amino acids to form a polypeptide chain; each subunit consists of a variety of ribosomal proteins. The ribosomes and associated molecules are known as the translational apparatus; the sequence of DNA, which encodes the sequence of the amino acids in a protein, is copied into a messenger RNA chain. It may be copied many times into RNA chains. Ribosomes can bind to a messenger RNA chain and use its sequence for determining the correct sequence of amino acids for generating a given protein. Amino acids are selected and carried to the ribosome by transfer RNA molecules, which enter one part of the ribosome and bind to the messenger RNA chain, it is during this binding that the correct translation of nucleic acid sequence to amino acid sequence occurs.
For each coding triplet in the messenger RNA there is a distinct transfer RNA that matches and which carries the correct amino acid for that coding triplet. The attached amino acids are linked together by another part of the ribosome. Once the protein is produced, it can fold to produce a specific functional three-dimensional structure although during synthesis some proteins start folding into their correct form. A ribosome is therefore a ribonucleoprotein; each ribosome is divided into two subunits: a smaller subunit which binds to a larger subunit and the mRNA pattern, a larger subunit which binds to the tRNA, the amino acids, the smaller subunit. 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 are associated with the intracellular membranes that make up the rough endoplasmic reticulum. Ribosomes from bacteria and eukaryotes in the three-domain system, resemble each other to a remarkable degree, evidence of a common origin.
They differ in their size, sequence 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 mitochondrial ribosomes of eukaryotic cells, are produced from mitochondrial genes, functionally resemble many features of those in bacteria, reflecting the evolutionary origin of mitochondria. Ribosomes were first observed in the mid-1950s by Romanian-American cell biologist George Emil Palade, using an electron microscope, as dense particles or granules; the term "ribosome" was proposed by scientist Richard B. Roberts in the end of 1950s: During the course of the symposium a semantic difficulty became apparent. To some of the participants, "microsomes" mean the ribonucleoprotein particles of the microsome fraction contaminated by other protein and lipid material.
The phrase "microsomal particles" does not seem adequate, "ribonucleoprotein particles of the microsome fraction" is much too awkward. During the meeting, the word "ribosome" was suggested, which has a satisfactory name and a pleasant sound; the present confusion would be eliminated if "ribosome" were adopted to designate ribonucleoprotein particles in sizes ranging from 35 to 100S. Albert Claude, Christian de Duve, George Emil Palade were jointly awarded the Nobel Prize in Physiology or Medicine, in 1974, for the discovery of the ribosome; 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; the ribosome is a complex cellular machine. It is made up of specialized RNA known as ribosomal RNA as well as dozens of distinct proteins; the ribosomal proteins and rRNAs are arranged into two distinct ribosomal pieces of different size, known as the large and small subunit of the ribosome.
Ribosomes consist of two subunits that fit together and work as one to translate the mRNA into a polypeptide chain during protein synthesis. Because they are formed from two subunits of non-equal size, they are longer in the axis than in diameter. Prokaryotic ribosomes are around 20 nm in diameter and are composed of 65% rRNA and 35% ribosomal proteins. Eukaryotic ribosomes are between 25 and 30 nm in diameter with an rRNA-to-protein ratio, close to 1. 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 do not directly participate in peptide bond formation catalysis, but rather that these proteins act as a scaffold that may enhance the ability of rRNA to synthesize protein. The ribosomal subunits of prokaryotes and eukaryotes are quite similar; the unit of measurement used to describe the ribosomal subunits and the rRNA fragments is the Svedberg unit, a measure of the rate of sedimentation in centrifugation rather than size.
This accounts for why fragment names do not add up: for example, prokaryotic 70S ribosomes are made of 50S and 30S subunits. Prokaryotes have 70