Lactase is an enzyme produced by many organisms. It is located in the brush border of the small intestine of other mammals. Lactase is essential to the complete digestion of whole milk. Lacking lactase, a person consuming dairy products may experience the symptoms of lactose intolerance. Lactase can be purchased as a food supplement, is added to milk to produce "lactose-free" milk products. Lactase, a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine. In humans, lactase is encoded by the LCT gene. Lactase is an enzyme. Without it they can't break down the natural lactose in milk, leaving them with diarrhea and bloating when drinking regular milk. Technology to produce lactose-free milk, ice cream and yogurt was developed by the USDA Agricultural Research Service in 1985.
This technology is used to add lactase to milk, thereby hydrolyzing the lactose found in milk, leaving it sweet but digestible by everyone. Without lactase, lactose intolerant people pass the lactose undigested to the colon where bacteria break it down creating carbon dioxide and that leads to bloating and flatulence. Lactase supplements are sometimes used to treat lactose intolerance. Lactase produced commercially can be extracted both from yeasts such as Kluyveromyces fragilis and Kluyveromyces lactis and from molds, such as Aspergillus niger and Aspergillus oryzae, its primary commercial use, in supplements such as Lacteeze and Lactaid, is to break down lactose in milk to make it suitable for people with lactose intolerance, the U. S. Food and Drug Administration has not formally evaluated the effectiveness of these products. Lactase is used to screen for blue white colonies in the multiple cloning sites of various plasmid vectors in Escherichia coli or other bacteria; the optimum temperature for human lactase is about 37 °C for its activity and the optimum pH is 6.
In metabolism, the β-glycosidic bond in D-lactose is hydrolyzed to form D-galactose and D-glucose, which can be absorbed through the intestinal walls and into the bloodstream. The overall reaction that lactase catalyzes is C12H22O11 + H2O → C6H12O6 + C6H12O6 + heat; the catalytic mechanism of D-lactose hydrolysis retains the substrate anomeric configuration in the products. While the details of the mechanism are uncertain, the stereochemical retention is achieved through a double displacement reaction. Studies of E. coli lactase have proposed that hydrolysis is initiated when a glutamate nucleophile on the enzyme attacks from the axial side of the galactosyl carbon in the β-glycosidic bond. The removal of the D-glucose leaving group may be facilitated by Mg-dependent acid catalysis; the enzyme is liberated from the α-galactosyl moiety upon equatorial nucleophilic attack by water, which produces D-galactose. Substrate modification studies have demonstrated that the 3′-OH and 2′-OH moieties on the galactopyranose ring are essential for enzymatic recognition and hydrolysis.
The 3′-hydroxy group is involved in initial binding to the substrate while the 2′- group is not necessary for recognition but needed in subsequent steps. This is demonstrated by the fact. Elimination of specific hydroxyl groups on the glucopyranose moiety does not eliminate catalysis. Lactase catalyzes the conversion of phlorizin to phloretin and glucose. Preprolactase, the primary translation product, has a single polypeptide primary structure consisting of 1927 amino acids, it can be divided into five domains: a 19-amino-acid cleaved signal sequence. The signal sequence is cleaved in the endoplasmic reticulum, the resulting 215-kDa pro-LPH is sent to the Golgi apparatus, where it is glycosylated and proteolytically processed to its mature form; the prodomain has been shown to act as an intramolecular chaperone in the ER, preventing trypsin cleavage and allowing LPH to adopt the necessary 3-D structure to be transported to the Golgi apparatus. Mature human lactase consists of a single 160-kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells.
It is oriented with the C-terminus in the cytosol. LPH contains two catalytic glutamic acid sites. In the human enzyme, the lactase activity has been connected to Glu-1749, while Glu-1273 is the site of phlorizin hydrolase function. Lactase is encoded by a single genetic locus on chromosome 2, it is expressed by mammalian small intestine enterocytes and in low levels in the colon during fetal development. Humans are born with high levels of lactase expression. In most of the world’s population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine, which causes the common symptoms of adult-type hypolactasia, or lactose intolerance; some population segments exhibit lactase persistence resulting from a mutation, postulated to have occurred 5,000–10,000 years ago, coinciding with the rise of cattle domestication. This mutation has allowed half of the world’s population to metabolize lactose without symptoms. Studies have linked the occurrence of lactase persistence to two differe
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
Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. The various types of sugar are derived from different sources. Simple sugars are called monosaccharides and include glucose and galactose. "Table sugar" or "granulated sugar" refers to a disaccharide of glucose and fructose. In the body, sucrose is hydrolysed into glucose. Sugars are found in the tissues of most plants, but sucrose is concentrated in sugarcane and sugar beet, making them ideal for efficient commercial extraction to make refined sugar. Sugarcane originated in tropical Indian subcontinent and Southeast Asia, is known of from before 6,000 BP, sugar beet was first described in writing by Olivier de Serres and originated in southwestern and Southeast Europe along the Atlantic coasts and the Mediterranean Sea, in North Africa, Macaronesia, to Western Asia. In 2016, the combined world production of those two crops was about two billion tonnes. Other disaccharides include lactose. Longer chains of sugar molecules are called polysaccharides.
Some other chemical substances, such as glycerol and sugar alcohols, may have a sweet taste, but are not classified as sugar. Sucrose is used in prepared foods, is sometimes added to commercially available beverages, may be used by people as a sweetener for foods and beverages; the average person consumes about 24 kilograms of sugar each year, or 33.1 kilograms in developed countries, equivalent to over 260 food calories per day. As sugar consumption grew in the latter part of the 20th century, researchers began to examine whether a diet high in sugar refined sugar, was damaging to human health. Excessive consumption of sugar has been implicated in the onset of obesity, cardiovascular disease and tooth decay. Numerous studies have tried to clarify those implications, but with varying results because of the difficulty of finding populations for use as controls that consume little or no sugar. In 2015, the World Health Organization recommended that adults and children reduce their intake of free sugars to less than 10%, encouraged a reduction to below 5%, of their total energy intake.
The etymology reflects the spread of the commodity. From Sanskrit शर्करा, meaning "ground or candied sugar," "grit, gravel", came Persian shakar, whence Arabic سكر, whence Medieval Latin succarum, whence 12th-century French sucre, whence the English word sugar. Italian zucchero, Spanish azúcar, Portuguese açúcar came directly from Arabic, the Spanish and Portuguese words retaining the Arabic definite article; the earliest Greek word attested is σάκχαρις. The English word jaggery, a coarse brown sugar made from date palm sap or sugarcane juice, has a similar etymological origin: Portuguese jágara from the Malayalam ചക്കരാ, itself from the Sanskrit शर्करा. Sugar has been produced in the Indian subcontinent since ancient times and its cultivation spread from there into modern-day Afghanistan through the Khyber Pass, it was not plentiful or cheap in early times, in most parts of the world, honey was more used for sweetening. People chewed raw sugarcane to extract its sweetness. Sugarcane was a native of Southeast Asia.
Different species seem to have originated from different locations with Saccharum barberi originating in India and S. edule and S. officinarum coming from New Guinea. One of the earliest historical references to sugarcane is in Chinese manuscripts dating to 8th century BCE, which state that the use of sugarcane originated in India. In the tradition of Indian medicine, the sugarcane is known by the name Ikṣu and the sugarcane juice is known as Phāṇita, its varieties and characterics are defined in nighaṇṭus such as the Bhāvaprakāśa. Sugar remained unimportant until the Indians discovered methods of turning sugarcane juice into granulated crystals that were easier to store and to transport. Crystallized sugar was discovered by the time of the Imperial Guptas, around the 5th century CE. In the local Indian language, these crystals were called khanda, the source of the word candy. Indian sailors, who carried clarified butter and sugar as supplies, introduced knowledge of sugar along the various trade routes they travelled.
Traveling Buddhist monks took sugar crystallization methods to China. During the reign of Harsha in North India, Indian envoys in Tang China taught methods of cultivating sugarcane after Emperor Taizong of Tang made known his interest in sugar. China established its first sugarcane plantations in the seventh century. Chinese documents confirm at least two missions to India, initiated in 647 CE, to obtain technology for sugar refining. In the Indian subcontinent, the Middle East and China, sugar became a staple of cooking and desserts. Nearchus, admiral of Alexander of Macedonia, knew of sugar during the year 325 B. C. because of his participation in the campaign of India led by Alexander. The Greek physician Pedanius Dioscorides in the 1st century CE described sugar in his medical treatise De Materia Medica, Pliny the Elder, a 1st-century CE Roman, described sugar in his Natural History: "Sugar is made in Arabia as well, but Indian sugar is better, it is a kind of honey found in cane, white as gum, it crunches between the teeth.
It comes in lumps the size of a hazelnut. Sugar is used only for medical purposes." Crusaders brought sugar back to Europe after their campaigns in the Hol
Alpha-Mannosidase is an enzyme involved in the cleavage of the alpha form of mannose. Its systematic name is alpha-D-mannoside mannohydrolase. Humans express the following three alpha-mannosidase isozymes: It can be utilized in experiments that determine the effects of the presence or absence of mannose on specific molecules, such as recombinant proteins that are used in vaccine development. A deficiency can lead to alpha-mannosidosis. GeneReviews/NCBI/NIH/UW entry on Alpha-Mannosidosis OMIM entries on Alpha-Mannosidosis alpha-Mannosidase at the US National Library of Medicine Medical Subject Headings
Sucrose is common sugar. It is a molecule composed of two monosaccharides: glucose and fructose. Sucrose is produced in plants, from which table sugar is refined, it has the molecular formula C12H22O11. For human consumption, sucrose is extracted, refined, from either sugar cane or sugar beet. Sugar mills are located where sugarcane is grown to crush the cane and produce raw sugar, shipped around the world for refining into pure sucrose; some sugar mills process the raw sugar into pure sucrose. Sugar beet factories are located in colder climates where the beet is grown and process the beets directly into refined sugar; the sugar refining process involves washing the raw sugar crystals before dissolving them into a sugar syrup, filtered and passed over carbon to remove any residual colour. The by-now clear sugar syrup is concentrated by boiling under a vacuum and crystallized as the final purification process to produce crystals of pure sucrose; these crystals are clear and have a sweet taste. En masse, the crystals appear white.
Sugar is an added ingredient in food production and food recipes. About 185 million tonnes of sugar were produced worldwide in 2017; the word sucrose was coined in 1857 by the English chemist William Miller from the French sucre and the generic chemical suffix for sugars -ose. The abbreviated term Suc is used for sucrose in scientific literature; the name saccharose was coined in 1860 by the French chemist Marcellin Berthelot. Saccharose is an obsolete name for sugars in general sucrose. In sucrose, the components glucose and fructose are linked via an ether bond between C1 on the glucosyl subunit and C2 on the fructosyl unit; the bond is called a glycosidic linkage. Glucose exists predominantly as two isomeric "pyranoses", but only one of these forms links to the fructose. Fructose itself exists as a mixture of "furanoses", each of which having α and β isomers, but only one particular isomer links to the glucosyl unit. What is notable about sucrose is that, unlike most disaccharides, the glycosidic bond is formed between the reducing ends of both glucose and fructose, not between the reducing end of one and the nonreducing end of the other.
This linkage inhibits further bonding to other saccharide units. Since it contains no anomeric hydroxyl groups, it is classified as a non-reducing sugar. Sucrose crystallizes in the monoclinic space group P21 with room-temperature lattice parameters a = 1.08631 nm, b = 0.87044 nm, c = 0.77624 nm, β = 102.938°. The purity of sucrose is measured by polarimetry, through the rotation of plane-polarized light by a solution of sugar; the specific rotation at 20 °C using yellow "sodium-D" light is +66.47°. Commercial samples of sugar are assayed using this parameter. Sucrose does not deteriorate at ambient conditions. Sucrose does not melt at high temperatures. Instead, it decomposes at 186 °C to form caramel. Like other carbohydrates, it combusts to carbon water. Mixing sucrose with the oxidizer potassium nitrate produces the fuel known as rocket candy, used to propel amateur rocket motors. C12H22O11 + 6 KNO3 → 9 CO + 3 N2 + 11 H2O + 3 K2CO3This reaction is somewhat simplified though; some of the carbon does get oxidized to carbon dioxide, other reactions, such as the water-gas shift reaction take place.
A more accurate theoretical equation is: C12H22O11 + 6.288 KNO3 → 3.796 CO2 + 5.205 CO + 7.794 H2O + 3.065 H2 + 3.143 N2 + 2.998 K2CO3 + 0.274 KOH Sucrose burns with chloric acid, formed by the reaction of hydrochloric acid and potassium chlorate: 8 HClO3 + C12H22O11 → 11 H2O + 12 CO2 + 8 HClSucrose can be dehydrated with sulfuric acid to form a black, carbon-rich solid, as indicated in the following idealized equation: H2SO4 + C12H22O11 → 12 C + 11 H2O + Heat. The formula for sucrose's decomposition can be represented as a two-step reaction: the first simplified reaction is dehydration of sucrose to pure carbon and water, carbon oxidises to CO2 with O2 from air. C12H22O11 + heat → 12 C + 11 H2O 12 C + 12 O2 → 12 CO2 Hydrolysis breaks the glycosidic bond converting sucrose into glucose and fructose. Hydrolysis is, however, so slow. If the enzyme sucrase is added, the reaction will proceed rapidly. Hydrolysis can be accelerated with acids, such as cream of tartar or lemon juice, both weak acids.
Gastric acidity converts sucrose to glucose and fructose during digestion, the bond between them being an acetal bond which can be broken by an acid. Given heats of combustion of 1349.6 kcal/mol for sucrose, 673.0 for glucose, 675.6 for fructose, hydrolysis releases about 1.0 kcal per mole of sucrose, or about 3 small calories per gram of product. The biosynthesis of sucrose proceeds via the precursors UDP-glucose and fructose 6-phosphate, catalyzed by the enzyme sucrose-6-phosphate synthase; the energy for the reaction is gained by the cleavage of uridine diphosphate. Sucrose is formed by plants and cyanobacteria but not by other organisms. Sucrose is found in many food plants along with the monosaccharide fructose. In many fruits, such as pineapple and apricot, sucrose is the main sugar. In others, such as grapes and pears, fructose is the main sugar. Although sucrose is invariably isolated from natural sources, its chemical synthesis was first achieved in 1953 by Raymond Lemieux. In nature, sucrose is present in many plants, in particular their roots and nectars, because it serves as a way to store energy from photosynthesis.
Many mammals, birds and bacteria accumulate and feed on the sucrose in plants and for some it is their main food sou
A brush border is the microvilli-covered surface of simple cuboidal epithelium and simple columnar epithelium cells found in certain locations of the body. Microvilli are 100 nanometers in diameter and their length varies from 100 to 2,000 nanometers in length; because individual microvilli are so small and are packed in the brush border, individual microvilli can only be resolved using electron microscopes. This fuzzy appearance gave rise to the term brush border, as early anatomists noted that this structure appeared much like the bristles of a paintbrush. Brush border cells are found in the following main locations: The small intestine tract: This is where absorption takes place; the brush borders of the intestinal lining are the site of terminal carbohydrate digestions. The microvilli that constitute the brush border have enzymes for this final part of digestion anchored into their apical plasma membrane as integral membrane proteins; these enzymes are found near to the transporters that will allow absorption of the digested nutrients.
The kidney: Here the brush border is useful in distinguishing the proximal tubule from the distal tubule. The large intestine has microvilli on the surface of its colonocytes; the brush border morphology increases a cell's surface area, a trait, useful in absorptive cells. Cells that absorb substances need a large surface area in contact with the substance to be efficient. In intestinal cells, the microvilli are referred to as striated border and are protoplasmic extensions contrary to villi which are submucosal folds, while in the kidneys, microvilli are referred to as brush border
Chitinases are hydrolytic enzymes that break down glycosidic bonds in chitin. As chitin is a component of the cell walls of fungi and exoskeletal elements of some animals, chitinases are found in organisms that either need to reshape their own chitin or dissolve and digest the chitin of fungi or animals. Chitinivorous organisms include many bacteria, which may be detritivorous, they attack living arthropods, zooplankton or fungi or they may degrade the remains of these organisms. Fungi, such as Coccidioides immitis possess degradative chitinases related to their role as detritivores and to their potential as arthropod pathogens. Chitinases are present in plants. Expression is mediated by the NPR1 gene and the salicylic acid pathway, both involved in resistance to fungal and insect attack. Other plant chitinases may be required for creating fungal symbioses. Although mammals do not produce chitin, they have two functional chitinases and acidic mammalian chitinase, as well as chitinase-like proteins that have high sequence similarity but lack chitinase activity.
Endochitinases randomly split chitin at internal sites of the chitin microfibril, forming soluble, low molecular mass multimer products. The multimer products includes di-acetylchitobiose and chitotetraose, with the dimer being the predominant product. Exochitinases have been divided into two sub categories: Chitobiosidases act on the non-reducing end of the chitin microfibril, releasing the dimer, di-acetylchitobiose, one by one from the chitin chain. Therefore, there is no release of oligosaccharides in this reaction. Β-1,4- N-acetylglucosaminidases split the multimer products, such as di-acetylchitobiose and chitotetraose, into monomers of N-acetylglucoseamine. Chitinases were classified based on the amino acid sequences, as that would be more helpful in understanding the evolutionary relationships of these enzymes to each other. Therefore, the chitinases were grouped into three families: 18, 19, 20. Both families 18 and 19 consists of endochitinases from a variety of different organisms, including viruses, fungi and plants.
However, family 19 comprises plant chitinases. Family 20 includes a similar enzyme, N-acetylhexosaminidase, and as the gene sequences of the chitinases were known, they were further classified into six classes based on their sequences. Characteristics that determined the classes of chitinases were the N-terminal sequence, localization of the enzyme, isoelectric pH, signal peptide, inducers. Class I chitinases had a cysteine-rich N-terminal, leucine- or valine-rich signal peptide, vacuolar localization, and Class I chitinases were further subdivided based on their acidic or basic nature into Class Ia and Class Ib, respectively. Class 1 chitinases were found to comprise only plant chitinases and endochitinases. Class II chitinases did not have the cysteine-rich N-terminal but had a similar sequence to Class I chitinases. Class II chitinases were found in plants and bacteria and consisted of exochitinases. Class III chitinases did not have similar sequences to chitinases in Class I or Class II. Class IV chitinases had similar characteristics, including the immunological properties, as Class I chitinases.
However, Class IV chitinases were smaller in size compared to Class I chitinases. Class V and Class VI chitinases are not well characterized. However, one example of a Class V chitinase showed two chitin binding domains in tandem, based on the gene sequence, the cysteine-rich N-terminal seemed to have been lost during evolution due to less selection pressure that caused the catalytic domain to lose its function. Like cellulose, chitin is an abundant biopolymer, resistant to degradation, it is not digested by animals, though certain fish are able to digest chitin. It is assumed that chitin digestion by animals requires bacterial symbionts and lengthy fermentations, similar to cellulase digestion by ruminants. Chitinases have been isolated from the stomachs of certain mammals, including humans. Chitinase activity can be detected in human blood and cartilage; as in plant chitinases this may be related to pathogen resistance. Chitinases produced in the human body may be related in response to allergies, asthma has been linked to enhanced chitinase expression levels.
Human chitinases may explain the link between some of the most common allergies and worm infections, as part of one version of the hygiene hypothesis. The link between chitinases and salicylic acid in plants is well established—but there is a hypothetical link between salicylic acid and allergies in humans. Regulation varies from species to species, within an organism, chitinases with different physiological functions would be under different regulation mechanisms. For example, chitinases that are involved in maintenance, such as remodeling the cell wall, are constitutively expressed. However, chitinases that have specialized function