Nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles; because nucleophiles donate electrons, they are by definition Lewis bases. Nucleophilic describes the affinity of a nucleophile to the nuclei. Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance's nucleophilic character and is used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge; the terms nucleophile and electrophile were introduced by Christopher Kelk Ingold in 1933, replacing the terms anionoid and cationoid proposed earlier by A. J. Lapworth in 1925; the word nucleophile is derived from philos for love. In general, in a row across the periodic table, the more basic the ion the more reactive it is as a nucleophile.
Within a series of nucleophiles with the same attacking element, the order of nucleophilicity will follow basicity. Sulfur is in general a better nucleophile than oxygen. Many schemes attempting to quantify relative nucleophilic strength have been devised; the following empirical data have been obtained by measuring reaction rates for a large number of reactions involving a large number of nucleophiles and electrophiles. Nucleophiles displaying the so-called alpha effect are omitted in this type of treatment; the first such attempt is found in the Swain–Scott equation derived in 1953: log 10 = s n This free-energy relationship relates the pseudo first order reaction rate constant, k, of a reaction, normalized to the reaction rate, k0, of a standard reaction with water as the nucleophile, to a nucleophilic constant n for a given nucleophile and a substrate constant s that depends on the sensitivity of a substrate to nucleophilic attack. This treatment results in the following values for typical nucleophilic anions: acetate 2.7, chloride 3.0, azide 4.0, hydroxide 4.2, aniline 4.5, iodide 5.0, thiosulfate 6.4.
Typical substrate constants are 0.66 for ethyl tosylate, 0.77 for β-propiolactone, 1.00 for 2,3-epoxypropanol, 0.87 for benzyl chloride, 1.43 for benzoyl chloride. The equation predicts that, in a nucleophilic displacement on benzyl chloride, the azide anion reacts 3000 times faster than water; the Ritchie equation, derived in 1972, is another free-energy relationship: log 10 = N + where N+ is the nucleophile dependent parameter and k0 the reaction rate constant for water. In this equation, a substrate-dependent parameter like s in the Swain–Scott equation is absent; the equation states that two nucleophiles react with the same relative reactivity regardless of the nature of the electrophile, in violation of the reactivity–selectivity principle. For this reason, this equation is called the constant selectivity relationship. In the original publication the data were obtained by reactions of selected nucleophiles with selected electrophilic carbocations such as tropylium or diazonium cations: or ions based on Malachite green.
Many other reaction types have since been described. Typical Ritchie N+ values are: 0.5 for methanol, 5.9 for the cyanide anion, 7.5 for the methoxide anion, 8.5 for the azide anion, 10.7 for the thiophenol anion. The values for the relative cation reactivities are −0.4 for the malachite green cation, +2.6 for the benzenediazonium cation, +4.5 for the tropylium cation. In the Mayr-Patz equation: log = s The second order reaction rate constant k at 20 °C for a reaction is related to a nucleophilicity parameter N, an electrophilicity parameter E, a nucleophile-dependent slope parameter s; the constant s is defined as 1 with 2-methyl-1-pentene as the nucleophile. Many of the constants have been derived from reaction of so-called benzhydrylium ions as the electrophiles: and a diverse collection of π-nucleophiles:. Typical E values are +6.2 for R = chlorine, +5.90 for R = hydrogen, 0 for R = methoxy and -7.02 for R = dimethylamine. Typical N values with s in parenthesis are -4.47 for electrophilic aromatic substitution to toluene, -0.41 for electrophilic addition to 1-phenyl-2-propene, 0.96 for addition to 2-methyl-1-pentene, -0.13 for reaction with triphenylallylsilane, 3.61 for reaction with 2-methylfuran, +7.48 for reaction with isobutenyltributylstannane and +13.36 for reaction with the enamine 7.
The range of organic reactions include SN2 reactions: With E = -9.15 for the S-methyldibenzothiophenium ion, typical nucleophile values N are 15.63 for piperidine, 10.49 for methoxide, 5.20 for water. In short, nucleophilicities towards sp2 or sp3 centers follow the same pattern. In an effort to unify the above described equations the Mayr equation is rewritten as: log = s E s N ( N + E
A Schiff base is a compound with the general structure R2C=NR'. They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimines depending on their structure; the term is synonymous with azomethine which refers to secondary aldimines. A number of special naming systems exist for these compounds. For instance a Schiff base derived from an aniline, where R3 is a phenyl or a substituted phenyl, can be called an anil, while bis-compounds are referred to as salen-type compounds; the term Schiff base is applied to these compounds when they are being used as ligands to form coordination complexes with metal ions. Such complexes occur for instance in Corrin, but the majority of Schiff bases are artificial and are used to form many important catalysts, such as Jacobsen's catalyst. Schiff bases can be synthesized from an aliphatic or aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by a dehydration to generate an imine. In a typical reaction, 4,4'-diaminodiphenyl ether reacts with o-vanillin: Schiff bases have been investigated in relation to a wide range of contexts, including antimicrobial and anticancer activity.
They have been considered for the inhibition of amyloid-β aggregation. Schiff bases are common enzymatic intermediates where an amine, such as the terminal group of a lysine residue, reversibly reacts with an aldehyde or ketone of a cofactor or substrate; the common enzyme cofactor PLP forms a Schiff base with a lysine residue and is transaldiminated to the substrate. The cofactor retinal forms a Schiff base in rhodopsins, including human rhodopsin, key in the photoreception mechanism. Schiff bases are common ligands in coordination chemistry; the imine nitrogen exhibits pi-acceptor properties. The ligands are derived from alkyl diamines and aromatic aldehydes. Chiral Schiff bases were one of the first ligands used for asymmetric catalysis. In 1968 Ryōji Noyori developed a copper-Schiff base complex for the metal-carbenoid cyclopropanation of styrene. For this work he was awarded a share of the 2001 Nobel Prize in Chemistry. Schiff bases have been incorporated into MOFs. Conjugated Schiff bases absorb in the UV-visible region of the electromagnetic spectrum.
This absorption is the basis of the anisidine value, a measure of oxidative spoilage for fats and oils. Schiff bases can be used to mass-produce nanoclusters of transition metals inside halloysite; this abundant mineral has a structure of rolled nanosheets, which can support both the synthesis and the metal nanocluster products. These nanoclusters can be made of Ag, Ru, Rh, Pt or Co metals and can catalyze various chemical reactions. J. C. Hindson. Ulgut. H. Friend. C. Greenham. Norder. J. Dingemans. "All-aromatic liquid crystal triphenylamine-based polys as hole transport materials for opto-electronic applications". J. Mater. Chem. 20: 937–944. Doi:10.1039/B919159C. M. L. Petrus. Bein. J. Dingemans. "A Low Cost Azomethine-Based Hole Transporting Material for Perovskite Photovoltaics". J. Mater. Chem. A. 3: 12159–12162. Doi:10.1039/C5TA03046C.</ref> Organic field-effect transistor <ref>D. Isık. Santato. G. Skene. "Charge-Carrier Transport in Thin Films of π-Conjugated Thiopheno-Azomethines". Org. Electron. 13: 3022–3031.
Doi:10.1016/j.orgel.2012.08.018. L. Sicard. "On-Substrate Preparation of an Electroactive Conjugated Polyazomethine from Solution-Processable Monomers and its Application in Electrochromic Devices". Adv. Funct. Mater. 23: 3549–3559. Doi:10.1002/adfm.201203657
Chromosome 2 is one of the 23 pairs of chromosomes in humans. People have two copies of this chromosome. Chromosome 2 is the second-largest human chromosome, spanning more than 242 million base pairs and representing 8% of the total DNA in human cells. Chromosome 2 contains the HOXD homeobox gene cluster. All members of Hominidae except humans and Denisovans have 24 pairs of chromosomes. Humans have only 23 pairs of chromosomes. Human chromosome 2 is a result of an end-to-end fusion of two ancestral chromosomes; the evidence for this includes: The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the chimpanzee, has near-identical DNA sequences to human chromosome 2, but they are found in two separate chromosomes; the same is true of orangutan. The presence of a vestigial centromere. A chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere in the q21.3–q22.1 region. The presence of vestigial telomeres; these are found only at the ends of a chromosome, but in chromosome 2 there are additional telomere sequences in the q13 band, far from either end of the chromosome.
According to researcher Jacob W. Ijdo, "We conclude that the locus cloned in cosmids c8.1 and c29B is the relic of an ancient telomere-telomere fusion and marks the point at which two ancestral ape chromosomes fused to give rise to human chromosome 2." The following are some of the gene count estimates of human chromosome 2. Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome vary. Among various projects, the collaborative consensus coding sequence project takes an conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes; the following is a partial list of genes on human chromosome 2. For complete list, see the link in the infobox on the right. Partial list of the genes located on p-arm of human chromosome 2: Partial list of the genes located on q-arm of human chromosome 2: The following diseases and traits are related to genes located on chromosome 2: National Institutes of Health.
"Chromosome 2". Genetics Home Reference. Retrieved 2017-05-06. "Chromosome 2". Human Genome Project Information Archive 1990–2003. Retrieved 2017-05-06
A cofactor is a non-protein chemical compound or metallic ion, required for an enzyme's activity. Cofactors can be considered "helper molecules"; the rates at which these happen are characterized by enzyme kinetics. Cofactors can be subclassified as either inorganic ions or complex organic molecules called coenzymes, the latter of, derived from vitamins and other organic essential nutrients in small amounts. A coenzyme, or covalently bound is termed a prosthetic group. Cosubstrates are transiently bound to the protein and will be released at some point get back in; the prosthetic groups, on the other hand, are bound permanently to the protein. Both of them have the same function, to facilitate the reaction of enzymes and protein. Additionally, some sources limit the use of the term "cofactor" to inorganic substances. An inactive enzyme without the cofactor is called an apoenzyme, while the complete enzyme with cofactor is called a holoenzyme; some enzymes or enzyme complexes require several cofactors.
For example, the multienzyme complex pyruvate dehydrogenase at the junction of glycolysis and the citric acid cycle requires five organic cofactors and one metal ion: loosely bound thiamine pyrophosphate, covalently bound lipoamide and flavin adenine dinucleotide, cosubstrates nicotinamide adenine dinucleotide and coenzyme A, a metal ion. Organic cofactors are vitamins or made from vitamins. Many contain the nucleotide adenosine monophosphate as part of their structures, such as ATP, coenzyme A, FAD, NAD+; this common structure may reflect a common evolutionary origin as part of ribozymes in an ancient RNA world. It has been suggested that the AMP part of the molecule can be considered to be a kind of "handle" by which the enzyme can "grasp" the coenzyme to switch it between different catalytic centers. Cofactors can be divided into two major groups: organic Cofactors, such as flavin or heme, inorganic cofactors, such as the metal ions Mg2+, Cu+, Mn2+, or iron-sulfur clusters. Organic cofactors are sometimes further divided into prosthetic groups.
The term coenzyme refers to enzymes and, as such, to the functional properties of a protein. On the other hand, "prosthetic group" emphasizes the nature of the binding of a cofactor to a protein and, refers to a structural property. Different sources give different definitions of coenzymes and prosthetic groups; some consider bound organic molecules as prosthetic groups and not as coenzymes, while others define all non-protein organic molecules needed for enzyme activity as coenzymes, classify those that are bound as coenzyme prosthetic groups. These terms are used loosely. A 1980 letter in Trends in Biochemistry Sciences noted the confusion in the literature and the arbitrary distinction made between prosthetic groups and coenzymes group and proposed the following scheme. Here, cofactors were defined as an additional substance apart from protein and substrate, required for enzyme activity and a prosthetic group as a substance that undergoes its whole catalytic cycle attached to a single enzyme molecule.
However, the author could not arrive at a single all-encompassing definition of a "coenzyme" and proposed that this term be dropped from use in the literature. Metal ions are common cofactors; the study of these cofactors falls under the area of bioinorganic chemistry. In nutrition, the list of essential trace elements reflects their role as cofactors. In humans this list includes iron, manganese, copper and molybdenum. Although chromium deficiency causes impaired glucose tolerance, no human enzyme that uses this metal as a cofactor has been identified. Iodine is an essential trace element, but this element is used as part of the structure of thyroid hormones rather than as an enzyme cofactor. Calcium is another special case, in that it is required as a component of the human diet, it is needed for the full activity of many enzymes, such as nitric oxide synthase, protein phosphatases, adenylate kinase, but calcium activates these enzymes in allosteric regulation binding to these enzymes in a complex with calmodulin.
Calcium is, therefore, a cell signaling molecule, not considered a cofactor of the enzymes it regulates. Other organisms require additional metals as enzyme cofactors, such as vanadium in the nitrogenase of the nitrogen-fixing bacteria of the genus Azotobacter, tungsten in the aldehyde ferredoxin oxidoreductase of the thermophilic archaean Pyrococcus furiosus, cadmium in the carbonic anhydrase from the marine diatom Thalassiosira weissflogii. In many cases, the cofactor includes both an organic component. One diverse set of examples is the heme proteins, which consist of a porphyrin ring coordinated to iron. Iron-sulfur clusters are complexes of iron and sulfur atoms held within proteins by cysteinyl residues, they play both structural and functional roles, including electron transfer, redox sensing, as structural modules. Organic cofactors are small organic molecules that can be either loosely or bound to the enzyme and directly participate in the reaction. In the latter case, when it is difficult to remove without denaturing the enzyme, it can be called a prosthetic group.
It is important to emphasize that there is no sharp division between loosely and bound cofactors. Indeed, many such as NAD+ can be bound in some enzymes, while it is loosely bound in others. Another example is thiamine pyrophosphate, bound in transketolase or pyruvate decarboxylase, while it is less tightly
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide is a cofactor found in all living cells. The compound is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains the other nicotinamide. Nicotinamide adenine dinucleotide exists in two forms: an oxidized and reduced form, abbreviated as NAD+ and NADH respectively. In metabolism, nicotinamide adenine dinucleotide is involved in redox reactions, carrying electrons from one reaction to another; the cofactor is, found in two forms in cells: NAD+ is an oxidizing agent – it accepts electrons from other molecules and becomes reduced. This reaction forms NADH, which can be used as a reducing agent to donate electrons; these electron transfer reactions are the main function of NAD. However, it is used in other cellular processes, most notably a substrate of enzymes that add or remove chemical groups from proteins, in posttranslational modifications; because of the importance of these functions, the enzymes involved in NAD metabolism are targets for drug discovery.
In organisms, NAD can be synthesized from simple building-blocks from the amino acids tryptophan or aspartic acid. In an alternative fashion, more complex components of the coenzymes are taken up from food as niacin. Similar compounds are released by reactions that break down the structure of NAD; these preformed components pass through a salvage pathway that recycles them back into the active form. Some NAD is converted into the coenzyme nicotinamide adenine dinucleotide phosphate; the chemistry of NADP is similar to that of NAD, but it has different role, being predominantly a cofactor in anabolic metabolism. NAD+ is written with a superscript plus sign because of the formal charge on one of its nitrogen atoms. NADH, on the other hand, is a doubly charged anion because of its two bridging phosphate groups. Nicotinamide adenine dinucleotide, like all dinucleotides, consists of two nucleosides joined by a pair of bridging phosphate groups; the nucleosides each contain a ribose ring, one with adenine attached to the first carbon atom and the other with nicotinamide at this position.
The nicotinamide moiety can be attached in two orientations to this anomeric carbon atom. Because of these two possible structures, the compound exists as two diastereomers, it is the β-nicotinamide diastereomer of NAD+, found in organisms. These nucleotides are joined together by a bridge of two phosphate groups through the 5' carbons. In metabolism, the compound donates electrons in redox reactions; such reactions involve the removal of two hydrogen atoms from the reactant, in the form of a hydride ion, a proton. The proton is released into solution, while the reductant RH2 is oxidized and NAD+ reduced to NADH by transfer of the hydride to the nicotinamide ring. RH2 + NAD+ → NADH + H+ + R; the midpoint potential of the NAD+/NADH redox pair is −0.32 volts, which makes NADH a strong reducing agent. The reaction is reversible, when NADH reduces another molecule and is re-oxidized to NAD+; this means the coenzyme can continuously cycle between the NAD+ and NADH forms without being consumed. In appearance, all forms of this coenzyme are white amorphous powders that are hygroscopic and water-soluble.
The solids are stable. Solutions of NAD+ are colorless and stable for about a week at 4 °C and neutral pH, but decompose in acids or alkalis. Upon decomposition, they form products. Both NAD+ and NADH absorb ultraviolet light because of the adenine. For example, peak absorption of NAD+ is at a wavelength of 259 nanometers, with an extinction coefficient of 16,900 M−1cm−1. NADH absorbs at higher wavelengths, with a second peak in UV absorption at 339 nm with an extinction coefficient of 6,220 M−1cm−1; this difference in the ultraviolet absorption spectra between the oxidized and reduced forms of the coenzymes at higher wavelengths makes it simple to measure the conversion of one to another in enzyme assays – by measuring the amount of UV absorption at 340 nm using a spectrophotometer. NAD+ and NADH differ in their fluorescence. NADH in solution has an emission peak at 460 nm and a fluorescence lifetime of 0.4 nanoseconds, while the oxidized form of the coenzyme does not fluoresce. The properties of the fluorescence signal changes when NADH binds to proteins, so these changes can be used to measure dissociation constants, which are useful in the study of enzyme kinetics.
These changes in fluorescence are used to measure changes in the redox state of living cells, through fluorescence microscopy. In rat liver, the total amount of NAD+ and NADH is 1 μmole per gram of wet weight, about 10 times the concentration of NADP+ and NADPH in the same cells; the actual concentration of NAD+ in cell cytosol is harder to measure, with recent estimates in animal cells ranging around 0.3 mM, 1.0 to 2.0 mM in yeast. However, more than 80% of NADH fluorescence in mitochondria is from bound form, so the concentration in solution is much lower. Data for other compartments in the cell are limited, although in the mitochondrion the concentration of NAD+ is similar to that in the cytosol; this NAD+ is carried into the mitochondrion by a specific membrane transport protein, since the coenzyme cannot diffuse across membranes. The balance between the oxidized and reduced forms of nicotinam
A base pair is a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA. Dictated by specific hydrogen bonding patterns, Watson–Crick base pairs allow the DNA helix to maintain a regular helical structure, subtly dependent on its nucleotide sequence; the complementary nature of this based-paired structure provides a redundant copy of the genetic information encoded within each strand of DNA. The regular structure and data redundancy provided by the DNA double helix make DNA well suited to the storage of genetic information, while base-pairing between DNA and incoming nucleotides provides the mechanism through which DNA polymerase replicates DNA and RNA polymerase transcribes DNA into RNA. Many DNA-binding proteins can recognize specific base-pairing patterns that identify particular regulatory regions of genes. Intramolecular base pairs can occur within single-stranded nucleic acids.
This is important in RNA molecules, where Watson–Crick base pairs permit the formation of short double-stranded helices, a wide variety of non-Watson–Crick interactions allow RNAs to fold into a vast range of specific three-dimensional structures. In addition, base-pairing between transfer RNA and messenger RNA forms the basis for the molecular recognition events that result in the nucleotide sequence of mRNA becoming translated into the amino acid sequence of proteins via the genetic code; the size of an individual gene or an organism's entire genome is measured in base pairs because DNA is double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands; the haploid human genome is estimated to be about 3.2 billion bases long and to contain 20,000–25,000 distinct protein-coding genes. A kilobase is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA; the total amount of related DNA base pairs on Earth is estimated at 5.0×1037 and weighs 50 billion tonnes.
In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC. Hydrogen bonding is the chemical interaction. Appropriate geometrical correspondence of hydrogen bond donors and acceptors allows only the "right" pairs to form stably. DNA with high GC-content is more stable than DNA with low GC-content. But, contrary to popular belief, the hydrogen bonds do not stabilize the DNA significantly; the larger nucleobases and guanine, are members of a class of double-ringed chemical structures called purines. Purines are complementary only with pyrimidines: pyrimidine-pyrimidine pairings are energetically unfavorable because the molecules are too far apart for hydrogen bonding to be established. Purine-pyrimidine base-pairing of AT or GC or UA results in proper duplex structure; the only other purine-pyrimidine pairings would be AC and GT and UG. The GU pairing, with two hydrogen bonds, does occur often in RNA. Paired DNA and RNA molecules are comparatively stable at room temperature, but the two nucleotide strands will separate above a melting point, determined by the length of the molecules, the extent of mispairing, the GC content.
Higher GC content results in higher melting temperatures. On the converse, regions of a genome that need to separate — for example, the promoter regions for often-transcribed genes — are comparatively GC-poor. GC content and melting temperature must be taken into account when designing primers for PCR reactions; the following DNA sequences illustrate pair double-stranded patterns. By convention, the top strand is written from the 5' end to the 3' end. A base-paired DNA sequence: ATCGATTGAGCTCTAGCG TAGCTAACTCGAGATCGCThe corresponding RNA sequence, in which uracil is substituted for thymine in the RNA strand: AUCGAUUGAGCUCUAGCG UAGCUAACUCGAGAUCGC Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors in DNA replication and DNA transcription; this is due to their isosteric chemistry. One common mutagenic base analog is 5-bromouracil, which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators, fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are known or suspected carcinogens. Examples include ethidium acridine. An unnatural base pair is a designed subunit of DNA, created in a laboratory and does not occur in nature. DNA sequences have been described which use newly created nucleobases to form a third base pair, in addition to the two ba
Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions; the molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. All metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps; the study of enzymes is called enzymology and a new field of pseudoenzyme analysis has grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, reflected in their amino acid sequences and unusual'pseudocatalytic' properties. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Most enzymes are proteins; the latter are called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures. Like all catalysts, enzymes increase the reaction rate by lowering its activation energy; some enzymes can make their conversion of substrate to product occur many millions of times faster.
An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds. Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, many enzymes are denatured when exposed to excessive heat, losing their structure and catalytic properties; some enzymes are used commercially, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.
By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms, he wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."In 1877, German physiologist Wilhelm Kühne first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. The word enzyme was used to refer to nonliving substances such as pepsin, the word ferment was used to refer to chemical activity produced by living organisms.
Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts when there were no living yeast cells in the mixture, he named the enzyme that brought about the fermentation of sucrose "zymase". In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate or to the type of reaction; the biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others argued that proteins were carriers for the true enzymes and that proteins per se were incapable of catalysis. In 1926, James B. Sumner crystallized it; the conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin and chymotrypsin.
These three scientists were awarded the 1946 Nobel Prize in Chemistry. The discovery that enzymes could be crystallized allowed their structures to be solved by x-ray crystallography; this was first done for lysozyme, an enzyme found in tears and egg whites that digests the coating of some bacteria. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. An enzyme's name is derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. Examples are alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes; the International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers. The first number broadly classifies the enzyme based on its mechanism; the top-level classification is: EC 1, Oxidoreductases: catalyze oxidation/reducti