Chitin n, a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, the scales of fish and lissamphibians; the structure of chitin is comparable to another polysaccharide - cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal and biotechnological purposes; the English word "chitin" comes from the French word chitine, derived in 1821 from the Greek word χιτών, meaning covering. A similar word, "chiton", refers to a marine animal with a protective shell; the structure of chitin was determined by Albert Hofmann in 1929. Chitin is a modified polysaccharide; these units form covalent β--linkages. Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an acetyl amine group.
This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength. In its pure, unmodified form, chitin is translucent, pliable and quite tough. In most arthropods, however, it is modified, occurring as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and molluscs, chitin produces a much stronger composite; this composite material is much harder and stiffer than pure chitin, is tougher and less brittle than pure calcium carbonate. Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar to the stiff, light elytron of a beetle. In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various iridescent colors serving phenotypic signaling and communication for mating and foraging.
The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in biomimicry. Scarab beetles in the genus Cyphochilus utilize chitin to form thin scales that diffusely reflect white light; these scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of nanometres, which serve to scatter light. The multiple scattering of light is thought to play a role in the unusual whiteness of the scales. In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper. Chitosan is produced commercially by deacetylation of chitin. Nanofibrils have been made using chitosan. Chitin-producing organisms like protozoa, fungi and nematodes are pathogens in other species. Humans and other mammals have chitinase-like proteins that can degrade chitin. Chitin is sensed in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells.
Keratinocytes in skin can react to chitin or chitin fragments. According to in vitro studies, chitin is sensed by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, mannose receptors; the immune response can sometimes clear the chitin and its associated organism, but sometimes the immune response is pathological and becomes an allergy. Plants have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein; the first chitin receptor was cloned in 2006. When the receptors are activated by chitin, genes related to plant defense are expressed, jasmonate hormones are activated, which in turn activate systematic defenses. Commensal fungi have ways to interact with the host immune response that as of 2016 were not well understood; some pathogens produce chitin-binding proteins. Zymoseptoria tritici is an example of a fungal pathogen. Chitin was present in the exoskeletons of Cambrian arthropods such as trilobites; the oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a scorpion encased in amber.
Chitin is a good inducer of plant defense mechanisms for controlling diseases. It has been assessed as a fertilizer that can improve overall crop yields. Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods. Processes to size and strengthen paper employ chitosan. How chitin interacts with the immune system of plants and animals has been an active area of research, including the identity of key receptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, mechanisms by which immune systems respond. Chitin and chitosan have bee
Biochemistry, sometimes called biological chemistry, is the study of chemical processes within and relating to living organisms. Biochemical processes give rise to the complexity of life. A sub-discipline of both biology and chemistry, biochemistry can be divided in three fields. Over the last decades of the 20th century, biochemistry has through these three disciplines become successful at explaining living processes. All areas of the life sciences are being uncovered and developed by biochemical methodology and research. Biochemistry focuses on understanding how biological molecules give rise to the processes that occur within living cells and between cells, which in turn relates to the study and understanding of tissues and organism structure and function. Biochemistry is related to molecular biology, the study of the molecular mechanisms by which genetic information encoded in DNA is able to result in the processes of life. Much of biochemistry deals with the structures and interactions of biological macromolecules, such as proteins, nucleic acids and lipids, which provide the structure of cells and perform many of the functions associated with life.
The chemistry of the cell depends on the reactions of smaller molecules and ions. These can be inorganic, for example water and metal ions, or organic, for example the amino acids, which are used to synthesize proteins; the mechanisms by which cells harness energy from their environment via chemical reactions are known as metabolism. The findings of biochemistry are applied in medicine and agriculture. In medicine, biochemists investigate the cures of diseases. In nutrition, they study how to maintain health wellness and study the effects of nutritional deficiencies. In agriculture, biochemists investigate soil and fertilizers, try to discover ways to improve crop cultivation, crop storage and pest control. At its broadest definition, biochemistry can be seen as a study of the components and composition of living things and how they come together to become life, in this sense the history of biochemistry may therefore go back as far as the ancient Greeks. However, biochemistry as a specific scientific discipline has its beginning sometime in the 19th century, or a little earlier, depending on which aspect of biochemistry is being focused on.
Some argued that the beginning of biochemistry may have been the discovery of the first enzyme, diastase, in 1833 by Anselme Payen, while others considered Eduard Buchner's first demonstration of a complex biochemical process alcoholic fermentation in cell-free extracts in 1897 to be the birth of biochemistry. Some might point as its beginning to the influential 1842 work by Justus von Liebig, Animal chemistry, or, Organic chemistry in its applications to physiology and pathology, which presented a chemical theory of metabolism, or earlier to the 18th century studies on fermentation and respiration by Antoine Lavoisier. Many other pioneers in the field who helped to uncover the layers of complexity of biochemistry have been proclaimed founders of modern biochemistry, for example Emil Fischer for his work on the chemistry of proteins, F. Gowland Hopkins on enzymes and the dynamic nature of biochemistry; the term "biochemistry" itself is derived from a combination of chemistry. In 1877, Felix Hoppe-Seyler used the term as a synonym for physiological chemistry in the foreword to the first issue of Zeitschrift für Physiologische Chemie where he argued for the setting up of institutes dedicated to this field of study.
The German chemist Carl Neuberg however is cited to have coined the word in 1903, while some credited it to Franz Hofmeister. It was once believed that life and its materials had some essential property or substance distinct from any found in non-living matter, it was thought that only living beings could produce the molecules of life. In 1828, Friedrich Wöhler published a paper on the synthesis of urea, proving that organic compounds can be created artificially. Since biochemistry has advanced since the mid-20th century, with the development of new techniques such as chromatography, X-ray diffraction, dual polarisation interferometry, NMR spectroscopy, radioisotopic labeling, electron microscopy, molecular dynamics simulations; these techniques allowed for the discovery and detailed analysis of many molecules and metabolic pathways of the cell, such as glycolysis and the Krebs cycle, led to an understanding of biochemistry on a molecular level. Philip Randle is well known for his discovery in diabetes research is the glucose-fatty acid cycle in 1963.
He confirmed. High fat oxidation was responsible for the insulin resistance. Another significant historic event in biochemistry is the discovery of the gene, its role in the transfer of information in the cell; this part of biochemistry is called molecular biology. In the 1950s, James D. Watson, Francis Crick, Rosalind Franklin, Maurice Wilkins were instrumental in solving DNA structure and suggesting its relationship with genetic transfer of information. In 1958, George Beadle and Edward Tatum received the Nobel Prize for work in fungi showing that one gene produces one enzyme. In 1988, Colin Pitchfork was the first person convicted of murder with DNA evidence, which led to the growth of forensic science. More Andrew Z. Fire and Craig C. Mello received the 2006 Nobel Prize for discovering the role of RNA interference, in the silencing of gene expression. Around two dozen of the 92
Ambiguity is a type of meaning in which a phrase, statement or resolution is not explicitly defined, making several interpretations plausible. A common aspect of ambiguity is uncertainty, it is thus an attribute of any idea or statement whose intended meaning cannot be definitively resolved according to a rule or process with a finite number of steps. The concept of ambiguity is contrasted with vagueness. In ambiguity and distinct interpretations are permitted, whereas with information, vague, it is difficult to form any interpretation at the desired level of specificity. Context may play a role in resolving ambiguity. For example, the same piece of information may be ambiguous in one context and unambiguous in another. Lexical ambiguity is contrasted with semantic ambiguity; the former represents a choice between a finite number of known and meaningful context-dependent interpretations. The latter represents a choice between any number of possible interpretations, none of which may have a standard agreed-upon meaning.
This form of ambiguity is related to vagueness. Linguistic ambiguity can be a problem in law, because the interpretation of written documents and oral agreements is of paramount importance; the lexical ambiguity of a word or phrase pertains to its having more than one meaning in the language to which the word belongs. "Meaning" here refers to. For instance, the word "bank" has several distinct lexical definitions, including "financial institution" and "edge of a river". Or consider "apothecary". One could say "I bought herbs from the apothecary"; this could mean one spoke to the apothecary or went to the apothecary. The context in which an ambiguous word is used makes it evident which of the meanings is intended. If, for instance, someone says "I buried $100 in the bank", most people would not think someone used a shovel to dig in the mud. However, some linguistic contexts do not provide sufficient information to disambiguate a used word. Lexical ambiguity can be addressed by algorithmic methods that automatically associate the appropriate meaning with a word in context, a task referred to as word sense disambiguation.
The use of multi-defined words requires the author or speaker to clarify their context, sometimes elaborate on their specific intended meaning. The goal of clear concise communication is that the receiver have no misunderstanding about what was meant to be conveyed. An exception to this could include a politician whose "weasel words" and obfuscation are necessary to gain support from multiple constituents with mutually exclusive conflicting desires from their candidate of choice. Ambiguity is a powerful tool of political science. More problematic are words whose senses express related concepts. "Good", for example, can mean "useful" or "functional", "exemplary", "pleasing", "moral", "righteous", etc. I have a good daughter"; the various ways to apply prefixes and suffixes can create ambiguity. Semantic ambiguity happens when a sentence contains an ambiguous word or phrase—a word or phrase that has more than one meaning. In "We saw her duck", the word "duck" can refer either to the person's bird, or to a motion she made.
Syntactic ambiguity arises when a sentence can have two different meanings because of the structure of the sentence—its syntax. This is due to a modifying expression, such as a prepositional phrase, the application of, unclear. "He ate the cookies on the couch", for example, could mean that he ate those cookies that were on the couch, or it could mean that he was sitting on the couch when he ate the cookies. "To get in, you will need an entrance fee of $10 or your voucher and your drivers' license." This could mean that you need EITHER ten dollars OR BOTH your license. Or it could mean that you need you need EITHER ten dollars OR a voucher. Only rewriting the sentence, or placing appropriate punctuation can resolve a syntactic ambiguity. For the notion of, theoretic results about, syntactic ambiguity in artificial, formal languages, see Ambiguous grammar. Spoken language can contain many more types of ambiguities which are called phonological ambiguities, where there is more than one way to compose a set of sounds into words.
For example, "ice cream" and "I scream". Such ambiguity is resolved according to the context. A mishearing of such, based on incorrectly resolved ambiguity, is called a mondegreen. Metonymy involves the use of the name of a subcomponent part as an abbreviation, or jargon, for the name of the whole object. In modern vocabulary, critical semiotics, metonymy encompasses any ambiguous word substitution, based on contextual contiguity, or a function or process that an object performs, such as "sweet ride" to refer to a nice car. Metonym miscommunication is considered a primary mechanism of linguistic humor. Philosophers
A dimer is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. The term homodimer is used when the two molecules are heterodimer when they are not; the reverse of dimerisation is called dissociation. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs. Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers. Borane occurs as the dimer diborane, due to the high Lewis acidity of the boron center. Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but do form the excimers Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation. Molecular dimers are formed by the reaction of two identical compounds e.g.: 2A → A-A.
In this example, monomer "A" is said to dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene: 2 C2 → 2C=C2Carbenes are reactive and form bonds. Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" to give identical monomers: C10H12 → 2 C5H6Many nonmetallic elements occur as dimers: hydrogen, oxygen, the halogens, i.e. fluorine, chlorine and iodine. Noble gases can form dimers linked for example dihelium or diargon. Mercury occurs as a mercury cation, formally a dimeric ion. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include Li2, Na2, K2, Rb2 and Cs2. Many small organic molecules, most notably formaldehyde form dimers; the dimer of formaldehyde is dioxetane. In the context of polymers, "dimer" refers to the degree of polymerization 2, regardless of the stoichiometry or condensation reactions.
This is applicable to disaccharides. For example, cellobiose is a dimer of glucose though the formation reaction produces water: 2C6H12O6 → C12H22O11 + H2OHere, the dimer has a stoichiometry different from the pair of monomers. Amino acids can form dimers, which are called dipeptides. An example is glycylglycine. Other examples are carnosine. Pyrimidine dimers are formed by a photochemical reaction from pyrimidine DNA bases; this cross-linking causes DNA mutations, causing skin cancers. Monomer Trimer Polymer Protein dimer "IUPAC "Gold Book" definition". Retrieved 2009-04-30
Lysozyme known as muramidase or N-acetylmuramide glycanhydrolase, is an antimicrobial enzyme produced by animals that forms part of the innate immune system. Lysozyme is a glycoside hydrolase that catalyzes the hydrolysis of 1,4-beta-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in peptidoglycan, the major component of gram-positive bacterial cell wall; this hydrolysis in turn compromises the integrity of bacterial cell walls causing lysis of the bacteria. Lysozyme is abundant in secretions including tears, human milk, mucus, it is present in cytoplasmic granules of the macrophages and the polymorphonuclear neutrophils. Large amounts of lysozyme can be found in egg white. C-type lysozymes are related to alpha-lactalbumin in sequence and structure, making them part of the same family. In humans, the lysozyme enzyme is encoded by the LYZ gene. Lysozyme is thermally stable, with a melting point reaching up to 72 ℃ at pH 5.0. However, in human milk it loses activity quickly at that temperature.
Its isoelectric point is 11.35. In a large range of pH lysozyme can survive; the enzyme functions by attacking and breaking glycosidic bonds in peptidoglycans. The enzyme can break glycosidic bonds in chitin, although not as as true chitinases. Lysozymes active site binds the peptidoglycan molecule in the prominent cleft between its two domains, it attacks peptidoglycans, its natural substrate, between N-acetylmuramic acid and the fourth carbon atom of N-acetylglucosamine. Shorter saccharides like tetrasaccharide have shown to be viable substrates but via an intermediate with a longer chain. Chitin has been shown to be a viable lysozyme substrate. Artificial substrates have been developed and used in lysozyme; the Phillips Mechanism proposed that the enzyme's catalytic power came from both steric strain on the bound substrate and electrostatic stabilization of an oxo-carbenium intermediate. From X-ray crystallographic data, Phillips proposed the active site of the enzyme, where a hexasaccharide binds.
The lysozyme distorts the fourth sugar in the hexasaccharide into a half-chair conformation. In this stressed state, the glycosidic bond is more broken. An ionic intermediate containing an oxo-carbenium is created as a result of the glycosidic bond breaking, thus distortion causing the substrate molecule to adopt a strained conformation similar to that of the transition state will lower the energy barrier of the reaction. The proposed oxo-carbonium intermediate was speculated to be electrostatically stabilized by aspartate and glutamate residues in the active site by Arieh Warshel in 1978; the electrostatic stabilization argument was based on comparison to bulk water, the reorientation of water dipoles can cancel out the stabilizing energy of charge interaction. In Warshel's model, the enzyme acts as a super-sovlent, which fixes the orientation of ion pairs and provides super-solvation, lower the energy when to ions are close to each other; the rate-determining step in this mechanism is related to formation of the oxo-carbenium intermediate.
There were some contradictory results to indicate the exact RDS. By tracing the formation of product, it was discovered that the RDS can change over different temperatures, a reason for those contradictory results. At a higher temperature the RDS is formation of glycosyl enzyme intermediate and at a lower temperature the break down of that intermediate. In an early debate in 1969, Dahlquist proposed a covalent mechanism for lysozyme based on kinetic isotope effect, but for a long time the ionic mechanism was more accepted. In 2001, a revised mechanism was proposed by Vocadlo via a covalent but not ionic intermediate. Evidence from ESI-MS analysis indicated a covalent intermediate. A 2-fluoro substituted substrate was used to lower the reaction rate and accumulate an intermediate for characterization; the amino acid side-chains glutamic acid 35 and aspartate 52 have been found to be critical to the activity of this enzyme. Glu35 acts as a proton donor to the glycosidic bond, cleaving the C-O bond in the substrate, whereas Asp52 acts as a nucleophile to generate a glycosyl enzyme intermediate.
The Glu35 reacts with water to form hydroxyl ion, a stronger nucleophile than water, which attacks the glycosyl enzyme intermediate, to give the product of hydrolysis and leaving the enzyme unchanged. This covalent mechanism was named after Koshland. More quantum mechanics/ molecular mechanics molecular dynamics simulations have been using the crystal of HEWL and predict the existence of a covalent intermediate. Evidence for the ESI-MS and X-ray structures indicate the existence of covalent intermediate, but rely on using a less active mutant or non-native substrate. Thus, QM/MM molecular dynamics provides the unique ability to directly investigate the mechanism of wild-type HEWL and native substrate; the calculations revealed that the covalent intermediate from the Koshland mechanism is ~30 kcal/mol more stable than the ionic intermediate from the Phillips mechanism. These calculation demonstrate that the ionic intermediate is energetically unfavorable and the covalent intermediates observed from experiments using less active mutant or non-native substrates provide useful insight into the mechanism of wild-type HEWL.
Imidazole derivatives can form a charge-transfer complex with some residues to achieve a competitive inhibition of lysozyme. In Gram-negative bacteria, the lipopolysaccharide acts as a non-competitive inhib