Natural product

Source: Wikipedia, the free encyclopedia.

anticancer drug paclitaxel is a natural product derived from the yew tree.[1]

A natural product is a natural compound or substance produced by a living organism—that is, found in nature.[2][3] In the broadest sense, natural products include any substance produced by life.[4][5] Natural products can also be prepared by chemical synthesis (both semisynthesis and total synthesis) and have played a central role in the development of the field of organic chemistry by providing challenging synthetic targets. The term natural product has also been extended for commercial purposes to refer to cosmetics, dietary supplements, and foods produced from natural sources without added artificial ingredients.[6]

Within the field of organic chemistry, the definition of natural products is usually restricted to

cytotoxic and have been selected and optimized through evolution for use as "chemical warfare" agents against prey, predators, and competing organisms.[11]
Secondary or specialized metabolites are often unique to species, which is contrasted to primary metabolites which have broad use across kingdoms. Secondary metabolites are marked by chemical complexity which is why they are of such interest to chemists.

Natural sources may lead to basic research on potential bioactive components for commercial development as lead compounds in drug discovery.[12] Although natural products have inspired numerous drugs, drug development from natural sources has received declining attention in the 21st century by pharmaceutical companies, partly due to unreliable access and supply, intellectual property, cost, and profit concerns, seasonal or environmental variability of composition, and loss of sources due to rising extinction rates.[12]

Classes

The broadest definition of natural product is anything that is produced by life,

bodily fluids (e.g. milk, plant exudates), and other natural materials
(e.g. soil, coal).

Natural products may be classified according to their biological function, biosynthetic pathway, or source. Depending on the sources, the number of known natural product molecules ranges between 300,000[14][15] and 400,000.[16]

Function

Following Albrecht Kossel's original proposal in 1891,[17] natural products are often divided into two major classes, the primary and secondary metabolites.[18][19] Primary metabolites have an intrinsic function that is essential to the survival of the organism that produces them. Secondary metabolites in contrast have an extrinsic function that mainly affects other organisms. Secondary metabolites are not essential to survival but do increase the competitiveness of the organism within its environment. Because of their ability to modulate biochemical and signal transduction pathways, some secondary metabolites have useful medicinal properties.[20]

Natural products especially within the field of organic chemistry are often defined as primary and secondary metabolites. A more restrictive definition limiting natural products to secondary metabolites is commonly used within the fields of medicinal chemistry and pharmacognosy.[13]

Primary metabolites

Molecular building blocks of life

Primary metabolites as defined by Kossel are components of basic metabolic pathways that are required for life. They are associated with essential cellular functions such as nutrient assimilation, energy production, and growth/development. They have a wide species distribution that span many phyla and frequently more than one kingdom. Primary metabolites include the basic building blocks of life: carbohydrates, lipids, amino acids, and nucleic acids.[21]

Primary metabolites that are involved with energy production include

photosynthetic enzymes. Enzymes in turn are composed of amino acids and often non-peptidic cofactors that are essential for enzyme function.[22] The basic structure of cells and of organisms are also composed of primary metabolites. These include cell membranes (e.g. phospholipids), cell walls (e.g. peptidoglycan, chitin), and cytoskeletons (proteins).[23]

Primary metabolite enzymatic cofactors include members of the

pyridoxol, pyridoxal, and pyridoxamine) as pyridoxal 5′-phosphate is a cofactor for many enzymes especially transaminases involve in amino acid metabolism. Vitamin B12 (cobalamins) contain a corrin ring similar in structure to porphyrin and is an essential coenzyme for the catabolism of fatty acids as well for the biosynthesis of methionine.[24]
: Ch. 2 

genetic information, are composed of nucleic acid primary metabolites.[22]

diacyl glycerol etc.[25]

Secondary metabolites

Main article: Secondary metabolite
Representative examples of each of the major classes of secondary metabolites

Secondary in contrast to primary metabolites are dispensable and not absolutely required for survival. Furthermore, secondary metabolites typically have a narrow species distribution.[26]

Secondary metabolites have a broad range of functions. These include

toxins etc.) that are used against competitors, prey, and predators.[27] For many other secondary metabolites, the function is unknown. One hypothesis is that they confer a competitive advantage to the organism that produces them.[28] An alternative view is that, in analogy to the immune system, these secondary metabolites have no specific function, but having the machinery in place to produce these diverse chemical structures is important and a few secondary metabolites are therefore produced and selected for.[29]

General structural classes of secondary metabolites include alkaloids, phenylpropanoids, polyketides, and terpenoids.[7]

Biosynthesis

Biosynthesis of primary and secondary metabolites.[24]: Ch. 2 

The biosynthetic pathways leading to the major classes of natural products are described below.[13][24]: Ch. 2 

Carbohydrates

Carbohydrates are an essential energy source for most life forms. In addition, polysaccharides formed from simpler carbohydrates are important structural components of many organisms such the cell walls of bacteria and plants.[citation needed]

Carbohydrate are the products of plant

pyruvate which in turn can be converted into carbohydrates in the liver.[citation needed
]

Fatty acids and polyketides

Through the process of

macrolide antibiotics.[citation needed
]

One molecule of acetyl-CoA (the "starter unit") and several molecules malonyl-CoA (the "extender units") are condensed by fatty acid synthase to produce fatty acids.[24]: Ch. 3  Fatty acid are essential components of lipid bilayers that form cell membranes as well as fat energy stores in animals.[citation needed]

Sources

Natural products may be extracted from the

bioassays. This effort to search for biologically active natural products is known as bioprospecting.[30][31]

Pharmacognosy provides the tools to detect, isolate and identify bioactive natural products that could be developed for medicinal use. When an "active principle" is isolated from a traditional medicine or other biological material, this is known as a "hit". Subsequent scientific and legal work is then performed to validate the hit (e.g. elucidation of mechanism of action, confirmation that there is no intellectual property conflict). This is followed by the hit to lead stage of drug discovery, where derivatives of the active compound are produced in an attempt to improve its potency and safety.[32][33] In this and related ways, modern medicines can be developed directly from natural sources.[citation needed]

Although traditional medicines and other biological material are considered an excellent source of novel compounds, the extraction and isolation of these compounds can be a slow, expensive and inefficient process. For large scale manufacture therefore, attempts may be made to produce the new compound by total synthesis or semisynthesis.

analogues with comparable potency and safety that are amenable to total/semisynthesis.[35]

Prokaryotic

Bacteria

Botulinum toxin types A and B (Botox, Dysport, Xeomin, MyoBloc), used both medicinally and cosmetically, are natural products from the bacterium Clostridium botulinum.[36]

The serendipitous discovery and subsequent clinical success of

rifamycins (from Amycolatopsis rifamycinica).[40] Antiparasitic and antiviral drugs have similarly been derived from bacterial metabolites.[41]

Although most of the drugs derived from bacteria are employed as anti-infectives, some have found use in other fields of medicine.

marine bacteria.[46]

Archaea

Because many

chemical compounds also, for example isoprenyl glycerol ethers 1 and 2 from Thermococcus S557 and Methanocaldococcus jannaschii, respectively.[48]

Eukaryotic

Fungi

The antibiotic penicillin is a natural product derived from the fungus Penicillium rubens.[49]

Several anti-infective medications have been derived from fungi including penicillin and the

panic attacks, and could potentially be used to treat anxiety.[citation needed
]

Plants

The opioid analgesic drug morphine is a natural product derived from the plant Papaver somniferum

Plants are a major source of complex and highly structurally diverse chemical compounds (

tubocurarine, muscarine, and nicotine.[24]
: Ch. 6 

Animals

The analgesic drug ω-conotoxin (ziconotide) is a natural product derived from the sea snail Conus magus.[56]

Animals also represent a source of bioactive natural products. In particular,

macromolecular target in the body (e.g. α-bungarotoxin from cobras).[57][58] As with plant feeding deterrents, this biological activity is attributed to natural selection, organisms capable of killing or paralyzing their prey and/or defending themselves against predators being more likely to survive and reproduce.[58]

Because of these specific chemical-target interactions, venom constituents have proved important tools for studying

antihypertensive agents cilazapril and captopril.[58] Also, echistatin, a disintegrin from the venom of the saw-scaled viper Echis carinatus was a lead in the development of the antiplatelet drug tirofiban.[59]

In addition to the

bryostatins (from the bryozoan Bugula neritina).[64]

Medical uses

Natural products sometimes have pharmacological activity that can be of therapeutic benefit in treating diseases.[65][66][67] Moreover, synthetic analogs of natural products with improved potency and safety can be prepared and therefore natural products are often used as starting points for drug discovery. Natural product constituents have inspired numerous drug discovery efforts that eventually gained approval as new drugs[68][69]

Representative examples of drugs based on natural products

Modern natural product-derived drugs

A large number of currently prescribed drugs have been either directly derived from or inspired by natural products.[1][70]

Some of the oldest natural product based drugs are analgesics. The bark of the

acetylsalicylic acid better known as aspirin is a widely used pain reliever. Its mechanism of action is inhibition of the cyclooxygenase (COX) enzyme.[71] Another notable example is opium is extracted from the latex from Papaver somniferous (a flowering poppy plant). The most potent narcotic component of opium is the alkaloid morphine which acts as an opioid receptor agonist.[72] A more recent example is the N-type calcium channel blocker ziconotide analgesic which is based on a cyclic peptide cone snail toxin (ω-conotoxin MVIIA) from the species Conus magus.[73]

A significant number of

beta lactams work by inhibiting DD-transpeptidase enzyme that is required by bacteria to cross link peptidoglycan to form the cell wall.[74]

Several natural product drugs target

taxol, which is isolated from Taxus brevifolia (the pacific yew tree).[76]

A class of drugs widely used to lower cholesterol are the HMG-CoA reductase inhibitors, for example atorvastatin. These were developed from mevastatin, a polyketide produced by the fungus Penicillium citrinum.[77] Finally, a number natural product drugs are used to treat hypertension and congestive heart failure. These include the angiotensin-converting enzyme inhibitor captopril. Captopril is based on the peptidic bradykinin potentiating factor isolated from venom of the Brazilian arrowhead viper (Bothrops jararaca).[78]

Limiting and enabling factors

Numerous challenges limit the use of natural products for drug discovery, resulting in 21st century preference by pharmaceutical companies to dedicate discovery efforts toward

patent protection, vary in composition due to sourcing season or environment, and are susceptible to rising extinction rates.[12]

The biological resource for drug discovery from natural products remains abundant, with small percentages of microorganisms, plant species, and insects assessed for bioactivity.[12] In enormous numbers, bacteria and marine microorganisms remain unexamined.[79][80] As of 2008, the field of metagenomics was proposed to examine genes and their function in soil microbes,[80][81] but most pharmaceutical firms have not exploited this resource fully, choosing instead to develop "diversity-oriented synthesis" from libraries of known drugs or natural sources for lead compounds with higher potential for bioactivity.[12]

Isolation and purification

Ernst Boris Chain, Howard Florey,[a] and others, Fleming recognized the antibacterial activity and clinical potential of "pen G", but was unable to purify or stabilize it.[82] Developments in chromatographic separations and freeze drying helped move progress forward in the production of commercial quantities of penicillin and other natural products.[citation needed
]

All natural products begin as mixtures with other compounds from the natural source, often very complex mixtures, from which the product of interest must be isolated and purified. The isolation of a natural product refers, depending on context, either to the isolation of sufficient quantities of pure chemical matter for chemical structure elucidation, derivitzation/degradation chemistry, biological testing, and other research needs (generally milligrams to grams, but historically, often more),[citation needed] or to the isolation of "analytical quantities" of the substance of interest, where the focus is on identification and quantitation of the substance (e.g. in biological tissue or fluid), and where the quantity isolated depends on the analytical method applied (but is generally always sub-microgram in scale).[83][page needed] The ease with which the active agent can be isolated and purified depends on the structure, stability, and quantity of the natural product. The methods of isolation applied toward achieving these two distinct scales of product are likewise distinct, but generally involve extraction, precipitation, adsorptions, chromatography, and sometimes crystallizations. In both cases, the isolated substance is purified to chemical homogeneity, i.e. specific combined separation and analytical methods such as LC-MS methods are chosen to be "orthogonal"—achieving their separations based on distinct modes of interaction between substance and isolating matrix—with the goal being repeated detection of only a single species present in the putative pure sample. Early isolation is almost inevitably followed by structure determination, especially if an important pharmacologic activity is associated with the purified natural product.[citation needed]

Structure determination refers to methods applied to determine the

nuclear magnetic resonance methods, often multidimensional, and, when feasible, small molecule crystallography.[citation needed] For instance, the chemical structure of penicillin was determined by Dorothy Crowfoot Hodgkin in 1945, work for which she later received a Nobel Prize in Chemistry (1964).[84]

Synthesis

Many natural products have very complex structures. The perceived complexity of a natural product is a qualitative matter, consisting of consideration of its molecular mass, the particular arrangements of substructures (

stereochemical elements, the physical properties of the molecule and its intermediates (which bear on the ease of its handling and purification), all of these viewed in the context of the novelty of the structure and whether preceding related synthetic efforts have been successful (see below for details).[citation needed
]

Some natural products, especially those less complex, are easily and cost-effectively prepared via complete chemical synthesis from readily available, simpler chemical ingredients, a process referred to as total synthesis (especially when the process involves no steps mediated by biological agents). Not all natural products are amenable to total synthesis, cost-effective or otherwise. In particular, those most complex often are not. Many are accessible, but the required routes are simply too expensive to allow synthesis on any practical or industrial scale. However, to be available for further study, all natural products must yield to isolation and purification. This may suffice if isolation provides appropriate quantities of the natural product for the intended purpose (e.g. as a drug to alleviate disease). Drugs such as penicillin, morphine, and paclitaxel proved to be affordably acquired at needed commercial scales solely via isolation procedures (without any significant synthetic chemistry contributing).[citation needed] However, in other cases, needed agents are not available without synthetic chemistry manipulations.[citation needed]

Semisynthesis

Main article: Semisynthesis

The process of isolating a natural product from its source can be costly in terms of committed time and material expense, and it may challenge the availability of the relied upon natural resource (or have ecological consequences for the resource). For instance, it has been estimated that the bark of an entire yew tree (Taxus brevifolia) would have to be harvested to extract enough paclitaxel for just a single dose of therapy.[85] Furthermore, the number of structural analogues obtainable for structure–activity analysis (SAR) simply via harvest (if more than one structural analogue is even present) is limited by the biology at work in the organism, and so outside of the experimentalist's control.[citation needed]

In such cases where the ultimate target is harder to come by, or limits SAR, it is sometimes possible to source a middle-to-late stage biosynthetic precursor or analogue from which the ultimate target can be prepared. This is termed semisynthesis or partial synthesis. With this approach, the related biosynthetic intermediate is harvested and then converted to the final product by conventional procedures of chemical synthesis.[citation needed]

This strategy can have two advantages. Firstly, the intermediate may be more easily extracted, and in higher yield, than the ultimate desired product. An example of this is paclitaxel, which can be manufactured by extracting

penicillins are an illustration of the benefit of this approach.[citation needed
]

Total synthesis

Main article: Total synthesis
methyl or 5'-adenosyl group, or a cyanide or hydroxide anion. The "proof" by synthesis of vitamin B12 was accomplished in 1972 by the groups of Robert Burns Woodward[87] and Albert Eschenmoser.[88]

In general, the total synthesis of natural products is a non-commercial research activity, aimed at deeper understanding of the synthesis of particular natural product frameworks, and the development of fundamental new synthetic methods. Even so, it is of tremendous commercial and societal importance. By providing challenging synthetic targets, for example, it has played a central role in the development of the field of organic chemistry.[89][90] Prior to the development of analytical chemistry methods in the twentieth century, the structures of natural products were affirmed by total synthesis (so-called "structure proof by synthesis").[91] Early efforts in natural products synthesis targeted complex substances such as cobalamin (vitamin B12), an essential cofactor in cellular metabolism.[87][88]

Symmetry

Examination of

steric hindrance at the core of the molecule, as most natural products dimerize and trimerize in a head-to-head fashion rather than head-to-tail.[92]

Research and teaching

Research and teaching activities related to natural products fall into a number of diverse academic areas, including organic chemistry, medicinal chemistry, pharmacognosy,

chemoinformatics.[94]

Chemistry

Natural products chemistry is a distinct area of chemical research which was important in the development and

Evans aldol reaction), as well as the discovery of completely new chemical reactions (e.g., the Woodward cis-hydroxylation, Sharpless epoxidation, and Suzuki–Miyaura cross-coupling reactions).[100]

History

Antoine Lavoisier (1743–1794)
Friedrich Wöhler (1800–1882)
Hermann Emil Fischer
(1852–1919)

Foundations of organic and natural product chemistry

The concept of natural products dates back to the early 19th century, when the foundations of organic chemistry were laid. Organic chemistry was regarded at that time as the chemistry of substances that plants and animals are composed of. It was a relatively complex form of chemistry and stood in stark contrast to inorganic chemistry, the principles of which had been established in 1789 by the Frenchman Antoine Lavoisier in his work Traité Élémentaire de Chimie.[101]

Isolation

Lavoisier showed at the end of the 18th century that organic substances consisted of a limited number of elements: primarily carbon and hydrogen and supplemented by oxygen and nitrogen. He quickly focused on the isolation of these substances, often because they had an interesting pharmacological activity. Plants were the main source of such compounds, especially alkaloids and

Eugène Chevreul isolated cholesterol, a crystalline substance, from animal tissue that belongs to the class of steroids,[103] and in 1819 strychnine, an alkaloid was isolated.[104]

Synthesis

A second important step was the synthesis of organic compounds. Whereas the synthesis of inorganic substances had been known for a long time, the synthesis of organic substances was a difficult hurdle. In 1827 the Swedish chemist

atomic theory. The idea of vitalism especially fitted in with beliefs in medicine; the most traditional healing practices believed that disease was the result of some imbalance in the vital energies that distinguishes life from nonlife. A first attempt to break the vitalism idea in science was made in 1828, when the German chemist Friedrich Wöhler succeeded in synthesizing urea, a natural product found in urine, by heating ammonium cyanate, an inorganic substance:[105]

This reaction showed that there was no need for a life force in order to prepare organic substances. This idea, however, was initially met with a high degree of skepticism, and only 20 years later, with the synthesis of acetic acid from carbon by

Adolph Wilhelm Hermann Kolbe, was the idea accepted. Organic chemistry has since developed into an independent area of research dedicated to the study of carbon-containing compounds, since that element in common was detected in a variety of nature-derived substances. An important factor in the characterization of organic materials was on the basis of their physical properties (such as melting point, boiling point, solubility, crystallinity, or color).[citation needed
]

Structural theories

A third step was the structure elucidation of organic substances: although the elemental composition of pure organic substances (irrespective of whether they were of natural or synthetic origin) could be determined fairly accurately, the molecular structure was still a problem. The urge to do structural elucidation resulted from a dispute between Friedrich Wöhler and

better source needed
]

Expanding the concept

The concept of natural product, which initially based on organic compounds that could be isolated from plants, was extended to include animal material in the middle of the 19th century by the German

Hermann Emil Fischer in 1884, turned his attention to the study of carbohydrates and purines, work for which he was awarded the Nobel Prize in 1902. He also succeeded to make synthetically in the laboratory in a variety of carbohydrates, including glucose and mannose. After the discovery of penicillin by Alexander Fleming in 1928, fungi and other micro-organisms were added to the arsenal of sources of natural products.[102]

Milestones

By the 1930s, several large classes of natural products were known. Important milestones included:[according to whom?]

See also

Journals

References

Footnotes

  1. Nobel Prize in Medicine
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Citations

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Further reading

External links

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