Acarbohydrate (/ˌkɑːrboʊˈhaɪdreɪt/) is a sugar (saccharide) or a sugar derivative.[1] For the simplest carbohydrates, the carbon-to-hydrogen-to-oxygen atomic ratio is 1:2:1, i.e. they are often represented by theempirical formulaC(H2O)n. Together with amino acids, fats, and nucleic acids, the carbohydrates are one of the major families of biomolecules.[2]
Carbohydrates perform numerous roles in living organisms.[3] Polysaccharides serve as anenergy store (e.g.,starch andglycogen) and as structural components (e.g., cellulose in plants andchitin in arthropods and fungi). The 5-carbon monosaccharideribose is an important component ofcoenzymes (e.g.,ATP,FAD andNAD) and the backbone of the genetic molecule known asRNA. The relateddeoxyribose is a component of DNA. Saccharides and their derivatives play key roles in theimmune system,fertilization, preventingpathogenesis,blood clotting, anddevelopment.[4]
Carbohydrates are central tonutrition and are found in a wide variety of natural and processed foods. Starch is a polysaccharide and is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cerealflour, such asbread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted fromsugarcane orsugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally inhoney, manyfruits, and some vegetables. Table sugar, milk, or honey is often added to drinks and many prepared foods such as jam, biscuits and cakes.
The term "carbohydrate" has many synonyms and the definition can depend on context. Terms associated with carbohydrate include "sugar", "saccharide", "glucan",[5] and "glucide".[6] Infood science and the term "carbohydrate" often means any food that is rich in thestarch (such as cereals, bread and pasta) or simple carbohydrates, or fairly simple sugars such as sucrose (found in candy,jams, and desserts). Carbohydrates can also refer todietary fiber, like cellulose.[7][8]
The starting point for discussion of carbohydrates are the saccharides. Monosaccharides are the simplest carbohydrates in that they cannot behydrolyzed to smaller carbohydrates. Monosaccharides usually have the formula Cm (H2O)n.Disaccharides (e.g.sucrose) are common as arepolysaccharides/oligosaccharides (e.g.,starch,cellulose). Saccharides are polyhydroxy aldehydes, ketones as well as derived polymers having linkages of theacetal type. They may be classified according to theirdegree of polymerization. Manypolyols are also classified as carbohydrates. In many carbohydrates the OH groups are appended to or replaced byN-acetyl (e.g.,chitin),sulfate (e.g.,glycosaminoglycans),carboxylic acid and deoxy modifications (e.g.,fucose andsialic acid).[6]
Sugars may be linked to other types of biological molecules to formglycoconjugates. The enzymatic process ofglycosylation creates sugars/saccharides linked to themselves and to other molecules by the glycosidic bond, thereby producing glycans.Glycoproteins,proteoglycans andglycolipids are the most abundant glycoconjugates found in mammalian cells. They are found predominantly on the outer cell membrane and in secreted fluids. Glycoconjugates have been shown to be important in cell-cell interactions due to the presence on the cell surface of variousglycan binding receptors in addition to the glycoconjugates themselves.[10][11] In addition to their function inprotein folding and cellular attachment, theN-linked glycans of a protein can modulate the protein's function, in some cases acting as an on-off switch.[12]
The history of carbohydrates, to some extent, is thehistory of sugar cane, which was first grown inNew Guinea. The mass cultivation occurred in India where techniques were developed for the isolatoin of crystalline sugar.[13] Cane sugar and its cultivation reached Europe around the 13th Century and then expanded to the New World, where industrialization occurred.
The chemistry and biochemistry of carbohydrates can be traced to 1811. On that year Constantin Kirchhoff discovered that grape sugar (glucose) forms when starch is boiled with acid. Thestarch industry started the following year. Henri Braconnot discovered in 1819 that sugar is formed through the action ofsulfuric acid on cellulose.William Prout, after chemical analyses of sugar and starch byJoseph Louis Gay-Lussac and Thénard, gave this group of substances the group name "saccharine." The term "carbohydrate" was first proposed by German chemistCarl Schmidt (chemist) in 1844. In 1856,glycogen, a form of carbohydrate storage in animal livers, was discovered by French physiologistClaude Bernard.[14]Emil Fischer received the 1902Nobel Prize in Chemistry for his work on sugars andpurines. For the discovery of glucose metabolism,Otto Meyerhof received the 1922Nobel Prize in Physiology or Medicine.Hans von Euler-Chelpin, together withArthur Harden, received the 1929 Nobel Prize in Chemistry "for their research on sugar fermentation and the role of enzymes in this process." In 1947, bothBernardo Houssay for his discovery of the role of thepituitary gland in carbohydrate metabolism andCarl andGerty Cori for their discovery of the conversion ofglycogen received the Nobel Prize in Physiology or Medicine. For the discovery of sugarnucleotides in carbohydrate biosynthesis,Luis Leloir received the 1970 Nobel Prize in Chemistry.
The termglycobiology[15] was coined in 1988 byRaymond Dwek to recognize the coming together of the traditional disciplines of carbohydrate chemistry andbiochemistry.[16] This coming together was as a result of a much greater understanding of the cellular andmolecular biology ofglycans. "Glycoscience" is a field that explores the structures and functions of glycans.[17]
Carbohydrate consumed in food yields 3.87 kilocalories of energy pergram for simple sugars,[18] and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods.[19] Relatively high levels of carbohydrate are associated with processed foods or refined foods made from plants, including sweets, cookies and candy, table sugar, honey, soft drinks, breads and crackers, jams and fruit products, pastas and breakfast cereals. Refined carbohydrates from processed foods such as white bread or rice, soft drinks, and desserts are readily digestible, and many are known to have a high glycemic index, which reflects a rapid assimilation of glucose. By contrast, the digestion of whole, unprocessed, fiber-rich foods such as beans, peas, and whole grains produces a slower and steadier release of glucose and energy into the body.[20] Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion oflactose.
Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of energy. Many organisms also have the ability to metabolize othermonosaccharides anddisaccharides but glucose is often metabolized first. InEscherichia coli, for example, thelac operon will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present, thelac operon is repressed, resulting in the glucose being used first (see:Diauxie).Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolizecellulose or other polysaccharides such aschitin andarabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists.Ruminants andtermites, for example, use microorganisms to process cellulose, fermenting it to caloric short-chain fatty acids. Even though humans lack the enzymes to digest fiber, dietary fiber represents an important dietary element for humans. Fibers promote healthy digestion, help regulate postprandial glucose and insulin levels, reduce cholesterol levels, and promote satiety.[21]
TheInstitute of Medicine recommends that American and Canadian adults get between 45 and 65% ofdietary energy from whole-grain carbohydrates.[22] TheFood and Agriculture Organization andWorld Health Organization jointly recommend that national dietary guidelines set a goal of 55–75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).[23] A 2017Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.[24]
The termcomplex carbohydrate was first used in theU.S. Senate Select Committee on Nutrition and Human Needs publicationDietary Goals for the United States (1977) where it was intended to distinguish sugars from other carbohydrates (which were perceived to be nutritionally superior).[27] However, the report put "fruit, vegetables and whole-grains" in the complex carbohydrate column, despite the fact that these may contain sugars as well as polysaccharides. The standard usage, however, is to classify carbohydrates chemically:simple if they are sugars (monosaccharides anddisaccharides) andcomplex if they arepolysaccharides (oroligosaccharides).[7][28] Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in thesmall intestine and "unavailable carbohydrates", which pass to thelarge intestine, where they are subject tofermentation by thegastrointestinal microbiota.[7]
Theglycemic index (GI) andglycemic load concepts characterize the potential for carbohydrates in food to raiseblood glucose compared to a reference food (generally pure glucose).[29] Expressed numerically as GI, carbohydrate-containing foods can be grouped as high-GI (score more than 70), moderate-GI (56–69), or low-GI (less than 55) relative to pure glucose (GI=100).[29] Consumption of carbohydrate-rich, high-GI foods causes an abrupt increase in blood glucose concentration that declines rapidly following the meal, whereas low-GI foods with lower carbohydrate content produces a lower blood glucose concentration that returns gradually after the meal.[29]
Glycemic load is a measure relating the quality of carbohydrates in a food (low- vs. high-carbohydrate content – the GI) by the amount of carbohydrates in a single serving of that food.[29]
Health effects of dietary carbohydrate restriction
Carbohydrate-restricted diets can be as effective as low-fat diets in helping achieve weight loss over the short term when overall calorie intake is reduced.[33] AnEndocrine Society scientific statement said that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet."[33] In the long term, low-carbohydrate diets do not appear to confer a "metabolic advantage," and effective weight loss or maintenance depends on the level ofcalorie restriction,[33] not the ratio ofmacronutrients in a diet.[34] The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing bloodinsulin levels, but a more balanced diet that restricts refined carbohydrates can also reduce serum glucose and insulin levels and may also suppress lipogenesis and promote fat oxidation.[35] However, as far as energy expenditure itself is concerned, the claim that low-carbohydrate diets have a "metabolic advantage" is not supported byclinical evidence.[33][36] Further, it is not clear how low-carbohydrate dieting affectscardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers ofcardiovascular disease risk.[37][38]
Carbohydrate-restricted diets are no more effective than a conventionalhealthy diet in preventing the onset oftype 2 diabetes, but for people with type 2 diabetes, they are a viable option for losing weight or helping withglycemic control.[39][40][41] There is limited evidence to support routine use of low-carbohydrate dieting in managingtype 1 diabetes.[42] TheAmerican Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients.[41]
Most dietary carbohydrates contain glucose, either as their only building block (as in the polysaccharides starch and glycogen), or together with another monosaccharide (as in the hetero-polysaccharides sucrose and lactose).[44] Unbound glucose is one of the main ingredients of honey. Glucose is extremely abundant and has been isolated from a variety of natural sources across the world, including male cones of the coniferous tree Wollemia nobilis in Rome,[45] the roots of Ilex asprella plants in China,[46] and straws from rice in California.[47]
Sugar content of selected common plant foods (in grams per 100 g)[48]
^A The carbohydrate value is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fiber".
The most important carbohydrate isglucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species:plants synthesize carbohydrates from carbon dioxide and water byphotosynthesis storing the absorbed energy internally, often in the form ofstarch orlipids. Plant components are consumed by animals andfungi, and used as fuel forcellular respiration. Oxidation of one gram of carbohydrate yields approximately 16 kJ (4 kcal) ofenergy, while the oxidation of one gram of lipids yields about 38 kJ (9 kcal). The human body stores between 300 and 500 g of carbohydrates depending on body weight, with the skeletal muscle contributing to a large portion of the storage.[49] Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form ofATP.[50] Organisms capable of anaerobic andaerobic respiration metabolize glucose andoxygen (aerobic) to release energy, withcarbon dioxide andwater as byproducts.
In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes calledglycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism. A 2 ATP investment is required in the early steps of glycolysis to phosphorylate Glucose toGlucose 6-Phosphate (G6P) andFructose 6-Phosphate (F6P) toFructose 1,6-biphosphate (FBP), thereby pushing the reaction forward irreversibly.[49] In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.
MS andHPLC are commonly applied to glycan cleaved either enzymatically or chemically from the target.[54] In case of glycolipids, they can be analyzed directly without separation of the lipid component.
N-glycans from glycoproteins are analyzed routinely by high-performance-liquid-chromatography (reversed phase, normal phase and ion exchange HPLC) after tagging the reducing end of the sugars with a fluorescent compound (reductive labeling).[55]A large variety of different labels were introduced in the recent years, where 2-aminobenzamide (AB), anthranilic acid (AA), 2-aminopyridin (PA), 2-aminoacridone (AMAC) and 3-(acetylamino)-6-aminoacridine (AA-Ac) are just a few of them.[56] Different labels have to be used for different ESI modes and MS systems used.[57]
Fractionated glycans fromhigh-performance liquid chromatography (HPLC) instruments can be further analyzed byMALDI-TOF-MS(MS) to get further information about structure and purity. Sometimes glycan pools are analyzed directly bymass spectrometry without prefractionation, although a discrimination between isobaric glycan structures is more challenging or even not always possible. Anyway, directMALDI-TOF-MS analysis can lead to a fast and straightforward illustration of the glycan pool.[58]
High performance liquid chromatography online coupled to mass spectrometry is useful. By choosing porous graphitic carbon as a stationary phase for liquid chromatography, even non derivatized glycans can be analyzed. Detection is here done by mass spectrometry, but in instead ofMALDI-MS, electrospray ionisation (ESI) is more frequently used.[59][60][61]
Although MRM has been used extensively in metabolomics and proteomics, its high sensitivity and linear response over a wide dynamic range make it especially suited for glycan biomarker research and discovery. MRM is performed on a triple quadrupole (QqQ) instrument, which is set to detect a predetermined precursor ion in the first quadrupole, a fragmented in the collision quadrupole, and a predetermined fragment ion in the third quadrupole. It is a non-scanning technique, wherein each transition is detected individually and the detection of multiple transitions occurs concurrently in duty cycles. This technique is being used to characterize the immune glycome.[12][62]
Chemical synthesis and manipulation of carbohydrates
Carbohydrate synthesis is a sub-field oforganic chemistry concerned specifically with the generation of natural and unnatural carbohydrate structures. Carbohydrate chemistry is a large and economically important branch of organic chemistry. This can include the synthesis ofmonosaccharide residues or structures containing more than one monosaccharide, known asoligosaccharides. Selective formation ofglycosidic linkages and selective reactions ofhydroxyl groups are very important, and the usage ofprotecting groups is extensive.
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