Carbohydrates Chapter 7 PDF

CHAPTER 7 Carbohydrates Steve Gschmeissner/Science Source  Recent studies on cell surface carbohydrates has demonstrated an unusually high concentration of sialic acid on the surface of some cancer cells. Sialic acid (IUPAC name N-acetylneuraminic acid) is a complex monosacch...

CHAPTER 7 Carbohydrates Steve Gschmeissner/Science Source  Recent studies on cell surface carbohydrates has demonstrated an unusually high concentration of sialic acid on the surface of some cancer cells. Sialic acid (IUPAC name N-acetylneuraminic acid) is a complex monosaccharide that is found on all our normal cells, as well. It is a signal that a cell is “self,” thereby protecting it from attack by our immune system. If certain cancer cells have an abundance of sialic acid on the surface, those cancer cells are essentially invisible to the cells of our immune system that should attack and destroy them. Steve Gschmeissner/Science Source Occurrence and Functions of Carbohydrates Carbohydrates are the most abundant class of bioorganic molecules on planet Earth. Although their abundance in the human body is relatively low, carbohydrates constitute about 75% by mass of dry plant materials. Occurrence and Functions of Carbohydrates Green (chlorophyll-containing) plants produce carbohydrates via photosynthesis. Two main uses for the carbohydrates in plants: 1. In the form of cellulose, carbohydrates serve as structural elements 2. In the form of starch, they provide energy reserves for the plants https://www.nagwa.com/en/explainers/573184167273/ Figure. Starch grains (green) in the parenchyma of a Clematis sp. plant. Starch is a polymer of glucose, a sugar produced by the plant during photosynthesis, and used as a source of energy. It is stored as grains/granules in structures called amyloplasts (yellow). Two main uses for the carbohydrates in plants: 1. In the form of cellulose, carbohydrates serve as structural elements 2. In the form of starch, they provide energy reserves for the plants https://doi.org/10.1016/j.bbrep.2017.02.004 Occurrence and Functions of Carbohydrates Dietary intake of plant materials is the major carbohydrate source for humans and animals. The average human diet should ideally be about two-thirds carbohydrate by mass. Carbohydrates have the following functions in humans: 1. Carbohydrate oxidation provides energy. (When “burned” by cells for energy, each gram (g) of carbohydrate releases approximately 4 kilocalories (kcal) of energy). 2. Carbohydrate storage, in the form of glycogen, provides a short-term energy reserve. 3. Carbohydrates supply carbon atoms for the synthesis of other biochemical substances (proteins, lipids, and nucleic acids). 4. Essential components in the mechanisms of genetic control of growth and development of living cells (ribose, deoxyribose) 5. Carbohydrates linked to lipids are structural components of cell membranes. 6. Carbohydrates linked to proteins function in a variety of cell–cell and cell– molecule recognition processes. Types of Carbohydrates Most simple carbohydrates have empirical formulas that fit the general formula CnH2nOn which can be written as Cn(H2O)n — the basis for the term carbohydrate (“hydrate of carbon”) A carbohydrate is a polyhydroxy aldehyde, a polyhydroxy ketone, or a compound that yields polyhydroxy aldehydes or polyhydroxy ketones upon hydrolysis. Types of Carbohydrates Carbohydrates are classified on the basis of molecular size as monosaccharides, disaccharides, oligosaccharides, and polysaccharides. 1. Monosaccharide - a carbohydrate that contains a single polyhydroxy aldehyde or polyhydroxy ketone unit. cannot be broken down into simpler units by hydrolysis reactions. water-soluble, white, crystalline solids 2. Disaccharide – is a carbohydrate that contains two monosaccharide units covalently bonded to each other Like monosaccharides, disaccharides are crystalline, water-soluble substances. Sucrose (table sugar) and lactose (milk sugar) are disaccharides. Hydrolysis of a disaccharide produces two monosaccharide units. Types of Carbohydrates 3. Oligosaccharide - a carbohydrate that contains 3-10 monosaccharide units covalently bonded to each other. “Free” oligosaccharides are seldom encountered in biochemical systems. They are usually found associated with proteins and lipids in complex molecules that have both structural and regulatory functions. Complete hydrolysis of an oligosaccharide produces several monosaccharide molecules; a trisaccharide produces 3 monosaccharide units, a hexasaccharide produces 6 monosaccharide units, and so on. 4. Polysaccharide - a polymeric carbohydrate that contains many monosaccharide units covalently bonded to each other. The number of monosaccharide units present in a polysaccharide varies from a few hundred units to over 50,000 units. Polysaccharides, like disaccharides and oligosaccharides, undergo hydrolysis under appropriate conditions to produce monosaccharides. Types of Carbohydrates Chirality: Handedness in Molecules The prefixes D- and L- found in the complete name of a monosaccharide are used to identify one of two possible isomeric forms called stereoisomers. By definition, each member of a pair of stereoisomers must have the same molecular formula and the same bonding pattern; D- and L- differ in the spatial arrangements of atoms in the molecule.  Chirality: Handedness in Molecules A carbon atom that has four different groups bonded to it is called a chiral carbon atom. Any molecule containing a chiral carbon is a chiral molecule and will exist as a pair of enantiomers. Superimposable mirror images are images that coincide at all points when the images are laid upon each other. Nonsuperimposable mirror the simplest chiral carbohydrate; chiral, nonsuperimposable images are images where not all points coincide when the images are laid upon each other. Carbon 2 is a chiral carbon. Chirality: Handedness in Molecules Dextrorotatory and Levorotatory Compounds Enantiomers are said to be optically active because of the way they interact with plane- polarized light. An optically active compound is a compound that rotates the plane of polarized light. Chirality: Handedness in Molecules Dextrorotatory and Levorotatory Compounds A dextrorotatory compound is a chiral compound that rotates the plane of polarized light in a clockwise direction. A levorotatory compound is a chiral compound that rotates the plane of polarized light in a counterclockwise direction. If one member of an enantiomeric pair is dextrorotatory, then the other member must be levorotatory. Classification of Monosaccharides Classification based on the type of carbonyl group: 1. An aldose is a monosaccharide that contains an aldehyde functional group. Aldoses are polyhydroxy aldehydes. 2. A ketose is a monosaccharide that contains a ketone functional group. Ketoses are polyhydroxy ketones. Monosaccharides are often classified by both their number of carbon atoms and their functional group. An aldose with 3 carbons is an aldotriose A ketose with 6 carbons is a ketohexose aldopentose ketohexose aldohexose ketopentose  D aldoses containing three, four, C3H6O3 five, and six carbon atoms C4H8O4 C5H10O5 C6H12O6 C3H6O3 C4H8O4 C5H10O5 ketoses C6H12O6 Biochemically Important Monosaccharides D-Glyceraldehyde and Dihydroxyacetone chiral achiral The simplest of the monosaccharides, these two trioses are important intermediates in the process of glycolysis, a series of reactions whereby glucose is converted into two molecules of pyruvate. D-Glyceraldehyde is a chiral molecule, but dihydroxyacetone is not. Biochemically Important Monosaccharides D-Glucose D-Galactose Found in high amounts seldom encountered as a in ripe fruits free monosaccharide Blood sugar Synthesized from glucose in Also called dextrose the body for the production of lactose (a disaccharide of glucose and galactose called brain sugar because it is a component of glycoproteins found in brain and nerve tissue also present in the chemical markers that distinguish various types of blood— A, B, AB, and O Biochemically Important Monosaccharides D-Fructose biochemically the most important ketohexose. also known as levulose and fruit sugar sweetest-tasting of all sugars, D- fructose is found in many fruits and is present in honey in equal amounts with glucose used as a dietary sugar because less is needed for the same amount of sweetness Biochemically Important Monosaccharides D-Ribose 5-carbon sugar (pentose) Component of ribonucleic acids (RNAs) and energy-rich compounds such as adenosine triphosphate (ATP). Cyclic Forms of Monosaccharides However, experimental evidence indicates that for monosaccharides containing five or more carbon atoms, such open-chain structures are actually in equilibrium with two cyclic structures, and the cyclic structures are the dominant forms at equilibrium. The cyclic forms of monosaccharides result from the ability of their carbonyl group (C=O) to react intramolecularly with a hydroxyl group (-OH). Structurally, the resulting cyclic compounds are cyclic hemiacetals. https://dept.harpercollege.edu/chemistry/chm/100/dgodambe/thedisk/carbo/rxnfruct.gif D-Fructose  Biochemically Important Disaccharides Disaccharides consist of two monosaccharides joined through an “oxygen bridge.” (Recall glycoside formation; glycosidic bond). A glycosidic linkage is the bond between two monosaccharides resulting from the reaction between the hemiacetal carbon atom -OH group of one monosaccharide and an -OH group on the other monosaccharide. In biological systems, we commonly see only particular disaccharides, such as maltose, lactose, or sucrose. These specific disaccharides are produced in cells because the reactions are catalyzed by enzymes. Biochemically Important Disaccharides Maltose often called malt sugar (malt=germinated barley that has been baked and ground; contains this disaccharide) produced whenever the polysaccharide starch breaks down made up of two D-glucose units, one of which must be α-D-glucose α(1→4) linkage Biochemically Important Disaccharides Maltose Biochemically Important Disaccharides Maltose Hydrolysis of D-maltose, whether in a laboratory flask or in a living organism, produces two molecules of D-glucose. An acidic environment (H+) or the enzyme maltase is needed for the hydrolysis to occur. Maltase, the enzyme that breaks the glucose–glucose α(1→4) linkage present in maltose, is found both in the human body and in yeast. Consequently, maltose is digested easily by humans and is readily fermented by yeast. Biochemically Important Disaccharides Cellobiose an intermediate in the hydrolysis of the polysaccharide cellulose Like maltose, cellobiose contains two D-glucose monosaccharide units Differs from maltose in that one of the d-glucose units—the one functioning as a hemiacetal—must have a β configuration instead of the α configuration (maltose) β(1→4) linkage Biochemically Important Disaccharides Cellobiose Like maltose, cellobiose is a reducing sugar, has three isomeric forms in aqueous solution, and upon hydrolysis produces two D-glucose molecules. Both the human body and yeast lack the enzyme cellobiase needed to break the glucose–glucose β(1→4) linkage of cellobiose. Thus, cellobiose cannot be digested by humans or fermented by yeast. Biochemically Important Disaccharides Lactose major sugar found in milk made up of a β-D-galactose unit and a D-glucose unit joined by a β(1→4) linkage glycosidic linkage. The glucose hemiacetal center is unaffected when galactose bonds to glucose in the formation of lactose, so lactose is a reducing sugar (the glucose ring can open to give an aldehyde). Biochemically Important Disaccharides Lactose Lactose can be hydrolyzed by acid or by the enzyme lactase, forming an equimolar mixture of galactose and glucose. In the human body, the galactose so produced is then converted to glucose by other enzymes. lactose intolerance, is a condition in which people lack the enzyme lactase; lowest among Scandinavians and other northern Europeans and highest among native North Americans, Southeast Asians, Africans, and Greeks. 60% of adults (the majority) are lactose intolerant. There are no milk-drinking adult animals; only baby animals drink milk. Biochemically Important Disaccharides Lactose When lactose molecules remain in the intestine undigested, they attract water to themselves, causing fullness, discomfort, cramping, nausea, and diarrhea. Bacterial fermentation of the lactose further along the intestinal tract produces acid (lactic acid) and gas, adding to the discomfort.  Biochemically Important Disaccharides Sucrose common table sugar is the most abundant of all disaccharides and occurs throughout the plant kingdom. α-D-glucose and β-F-fructose in an α,β(1→2) glycosidic linkage unlike maltose, cellobiose, and lactose, is a nonreducing sugar; the hemiacetal center (anomeric carbon atom) of each monosaccharide is involved in the glycosidic linkage. exists in only one form—there are no α and β isomers, and an open-chain form is not possible. Biochemically Important Disaccharides Sucrose When sucrose is cooked with acid-containing foods such as fruits or berries, partial hydrolysis takes place, forming some invert sugar. Jams and jellies prepared in this manner are actually sweeter than the pure sucrose Disaccharide Hydrolysis Oligosaccharides Oligosaccharides are carbohydrates that contain three to ten monosaccharide units. Two naturally occurring oligosaccharides found in onions, cabbage, broccoli, brussel sprouts, whole wheat, and all types of beans are the trisaccharide raffinose and the tetrasaccharide stachyose. Humans lack the digestive enzymes necessary to metabolize either raffinose or stachyose. Hence these oligosaccharides, when ingested in food, pass undigested into the large intestine, where bacteria act upon them. This bacterial action usually produces discomfort and flatulence (gas). Oligosaccharides Oligosaccharides Many plants, including the potato plant, produce toxins as a defense against insects and predators. Solanine is the potato plant’s toxin. Solanine amounts in potatoes increase when potatoes sprout and when they are exposed to sunlight (green coloration). Blood Types and Oligosaccharides Blood Types and Oligosaccharides Type O: universal donor Type AB: Universal acceptor Polysaccharides Glycan is an alternate name for a polysaccharide. Important parameters that distinguish various polysaccharides (or glycans) from each other are: 1. The identity of the monosaccharide repeating unit(s) in the polymer chain. A homopolysaccharide is a polysaccharide in which only one type of monosaccharide monomer is present. A heteropolysaccharide is a polysaccharide in which more than one (usually two) type of monosaccharide monomer is present. 2. The length of the polymer chain. from less than a hundred monomer units to over 50,000 monomer units. Polysaccharides 3. As with disaccharides, the type of glycosidic linkage between monomer units. 4. The degree of branching of the polymer chain. Storage Polysaccharides A storage polysaccharide is a polysaccharide that is a storage form for monosaccharides and is used as an energy source in cells. Starch Storage Polysaccharides A storage polysaccharide is a polysaccharide that is a storage form for monosaccharides and is used as an energy source in cells. Starch Storage Polysaccharides Glycogen Glycogen has a structure similar to that of amylopectin; all glycosidic linkages are of the a type, and both (1→4) and (1→6) linkages are present. Glycogen and amylopectin differ in the number of glucose units between branches and in the total number of glucose units present in a molecule. Glycogen is about three times more highly branched than amylopectin, and it is much larger, with up to 1,000,000 glucose units present. Glycogen is an ideal storage form for glucose. The large size of these macromolecules prevents them from diffusing out of cells. Structural Polysaccharides Cellulose the structural component of plant cell walls the most abundant naturally occurring polysaccharide; high concentrations can be found in the “woody” portions of plants—stems, stalks, and trunks Like amylose, cellulose is an unbranched glucose polymer. Unlike amylose which has α(1→4) linkages, cellulose has β(1→4), resulting to a difference in their shape. Cotton is almost pure cellulose (95%), and wood is about 50% cellulose. cellulose  amylose cellulose The linear (straight-chain) cellulose molecules, when aligned side by side, become water-insoluble fibers because of inter-chain hydrogen bonding involving the numerous hydroxyl groups present. Structural Polysaccharides Chitin the second most abundant naturally occurring polysaccharide. gives rigidity to the exoskeletons of crabs, lobsters, shrimp, insects, and other arthropods, in the cell walls of fungi Polymer of the amino sugar N-acetyl-D-glucosamine (NAG), a monosaccharide found in blood type oligosaccharide Dietary Considerations and Carbohydrates Foods high in carbohydrate content constitute over 50% of the diet of most people of the world A simple carbohydrate is a dietary monosaccharide or dietary disaccharide; are usually sweet to the taste and are commonly referred to as sugars. Two types: A natural sugar is a sugar naturally present in whole foods. Milk and fresh fruit are two important sources of natural sugars. A refined sugar is a sugar that has been separated from its plant source. Sugar beets and sugar cane are major sources of refined sugars. Refined sugars are often said to provide empty calories because they provide energy but few other nutrients. Natural sugars, on the other hand, are accompanied by nutrients. A tablespoon of sucrose (table sugar, refined) provides 50 calories of energy just as a small orange does. The small orange (natural sugar), however, also supplies vitamin C, potassium, calcium, and fiber; table sugar provides no other nutrients. A complex carbohydrate is a dietary polysaccharide. The main complex carbohydrates are starch and cellulose, substances not generally sweet to the taste. Glycolipids and Glycoproteins: Cell Recognition It is now known that mono-, di-, and oligosaccharides attached through glycosidic linkages to lipid molecules (glycolipids) and protein molecules (glycoproteins) have a wide range of biochemical functions, including allowing cells to interact with invading bacteria and viruses and enabling cells of differing function to recognize each other. A glycolipid is a lipid molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it. A glycoprotein is a protein molecule that has one or more carbohydrate (or carbohydrate derivative) units covalently bonded to it. Glycolipids and Glycoproteins: Cell Recognition In glycolipids and glycoproteins associated with cell membrane structure, the lipid or protein part of the glycolipid or glycoprotein is incorporated into the cell membrane structure and the carbohydrate (oligosaccharide) part functions as a marker on the outer cell membrane surface. Cell recognition generally involves the interaction between the carbohydrate marker of one cell and a protein or lipid that is part of the cell membrane of another cell. https://doi.org/10.3390/ijms23031451