Carbohydrates.pdf

Carbohydrates  Proteins and nucleic acids could satisfy three of the five fundamental characteristics of life: information, replication and evolution Carbohydrates play an important role in a fourth characteristic— energy.  The term carbohydrate, or sugar, encompasses the monomers cal...

Carbohydrates  Proteins and nucleic acids could satisfy three of the five fundamental characteristics of life: information, replication and evolution Carbohydrates play an important role in a fourth characteristic— energy.  The term carbohydrate, or sugar, encompasses the monomers called monosaccharides (literally, “one sugar”) Small polymers called oligosaccharides (“few-sugars”). Large polymers called polysaccharides (“many-sugars”).  The name “carbohydrate” is logical because the molecular formula of many of these molecules is (CH2O)n, where the n indicates the number of “carbon-hydrate” groups (“hydrate” refers to water). Carbohydrates do not consist of carbon atoms bonded to water molecules. Instead, they are made up of a carbonyl group (C=O), several hydroxyl groups (-OH), along with multiple carbon–hydrogen bonds (C-H).  Sugars are fundamental to life. They provide chemical energy in cells and furnish some of the molecular building blocks required for the synthesis of larger, more complex compounds. Monosaccharides were important during chemical evolution, early in Earth’s history, too. For example, the sugar called ribose is required for the formation of the nucleotides that make up nucleic acids. What distinguishes one Monosaccharide from another? Monosaccharides, or simple sugars, are the monomers of carbohydrates. The carbonyl group that serves as one of monosaccharides’ distinguishing features can be found either at the end of the molecule, forming an aldehyde sugar (an aldose), or within the carbon chain, forming a ketone sugar (a ketose).  The presence of a carbonyl group along with multiple polar hydroxyl groups means that even the simplest sugars have many reactive and hydrophilic functional groups. Sugars are polar molecules that form hydrogen bonds with water and are easily dissolved in aqueous solutions.  The number of carbon atoms present also varies among monosaccharides. Three-carbon sugars are called trioses. Ribose, which acts as a building block for nucleotides, has five carbons and is called a pentose. The glucose is a six carbon sugar, or a hexose. Monosaccharides can vary in the spatial arrangement of their atoms.  Because the structures of glucose and galactose differ, their functions differ. Even seemingly simple changes in structure—like the location of a single hydroxyl group—can have enormous consequences for function.  It’s actually rare for sugars consisting of five or more carbons to exist in linear form. In aqueous solution they spontaneously form ring structures when the carbonyl group bonds to a carbon with a hydroxyl group.  When the ring structure is formed in sugars, the position of the newly formed C-1 hydroxyl group will be fixed in one of two possible orientations: below or above the plane of the ring. So there are two possible forms of glucose: α-glucose and β-glucose. The two forms exist in equilibrium, but β-glucose is more common because it is slightly more stable than α- glucose. Can Monosaccharides Form by Chemical Evolution? Laboratory simulations have shown that most monosaccharides are readily synthesized under conditions that mimic those predicted for early Earth. For example, when formaldehyde (CH2O) molecules are heated in solution, they react with one another to form almost all the possible types of pentoses and hexoses. These reactions may have occurred in the hot water released from undersea volcanoes and hydrothermal vents.  The Structure of Polysaccharides Simple sugars covalently link to form chains of varying lengths called complex carbohydrates. These chains range in size from short oligosaccharides to long polysaccharides. When just two sugars link together, the resulting molecule is known as a disaccharide. Monosaccharides polymerize when a condensation reaction occurs between two hydroxyl groups, resulting in a covalent bond called a glycosidic linkage. The inverse reaction, hydrolysis, cleaves these linkages.  Peptide bonds and phosphodiester linkages form between the same locations in their monomers, giving proteins and nucleic acids a standard backbone structure, but this is not the case for carbohydrates. Because glycosidic linkages form between hydroxyl groups, and because every monosaccharide contains at least two hydroxyls, the location and geometry of glycosidic linkages can vary widely among oligosaccharides and polysaccharides.  Similar to proteins and nucleic acids, the structure and function of larger carbohydrates depends on the types of monomers involved and how they are linked together. The variation in how polysaccharides are formed allows organisms to use them in radically different ways. For example, polysaccharides may be used to store chemical energy in the cells of plants or provide structural support in the exoskeletons of insects.  Maltose, or malt sugar, is a disaccharide formed by two glucose molecules and is abundant in the starter liquid used to brew beer. Lactose, an important sugar in milk, is a disaccharide of glucose and galactose.  Maltose and lactose illustrate two of the most common glycosidic linkages, called the α-1,4- glycosidic linkage and the β-1,4-glycosidic linkage. The numbers refer to the carbons on either side of the linkage, indicating that both linkages are between the C-1 and C-4 carbons. Their geometry, however, is different: α and β refer to the contrasting orientations of the C-1 hydroxyls—on opposite sides of the plane of the glucose rings (i.e., “below” versus “above” the plane).  A functional consequence of the structural differences between maltose and lactose is that the enzymes used to hydrolyze maltose will not cleave lactose. Instead, lactose is digested by lactase—an enzyme that many humans stop secreting after childhood. Without lactase, adults may become lactose intolerant and suffer intestinal discomfort if they consume dairy products. Starch: A Storage Polysaccharide in Plants In plant cells, some monosaccharides are polymerized and stored for later use in the form of starch. Starch consists entirely of α-glucose joined by glycosidic linkages. Most of these linkages are between C-1 and C-4 carbons, and the angle of these bonds causes the chain of glucose residues to coil into a helix.  Starch is made up of two types of polymers. One is an unbranched molecule called amylose, which contains only α-1,4-glycosidic linkages. The other is a branched molecule called amylopectin. Branching occurs when a glycosidic linkage forms between a C-1 carbon and a C-6 carbon (an α-1,6 linkage). In amylopectin, branching occurs at about one out of every 30 glucose residues. Glycogen: A Highly Branched Storage Polysaccharide in Animals Glycogen performs the same storage role in animals that starch performs in plants. In humans, for example, glycogen is stored in the cells of liver and muscle tissues. Glycogen is a helical polymer of α-glucose and is nearly identical to the branched form of starch. However, instead of an α-1,6-glycosidic linkage occurring in about 1 out of every 30 residues in amylopectin, a branch occurs in about 1 out of every 10 glucose subunits.  Cellulose: A Structural Polysaccharide in Plants All cells are enclosed by a membrane, and the cells of most organisms are also surrounded by a protective layer of material called a cell wall. In plants, bacteria, fungi and many other groups, the cell wall is composed primarily of one or more polysaccharides.  In plants, the major component of the cell wall is cellulose. Cellulose is a polymer made from β- glucose monomers joined by β-1,4- glycosidic linkages. The geometry of the linkage is such that each glucose residue in the chain is flipped in relation to the adjacent residue. The flipped orientation is important because (1) it generates a linear molecule, rather than the helix seen in starch. (2) it permits multiple hydrogen bonds to form between adjacent, parallel strands of cellulose. As a result, cellulose forms long, parallel strands that are joined by hydrogen bonds. The interacting cellulose fibers are strong and give the cell structural support.  Chitin: A Structural Polysaccharide in Fungi and Animals Chitin is a polysaccharide that stiffens the cell walls of fungi. It is the most important component of the external skeletons of insects. Chitin is similar to cellulose, but instead of consisting of glucose residues, the monosaccharide involved is one called N-acetylglucosamine (NAG). These NAG monomers are joined by β-1,4-glycosidic linkages. Peptidoglycan: A Structural Polysaccharide in Bacteria Most bacteria, like all plants and fungi, have cell walls. The primary structural component of bacterial cell walls consists of a polysaccharide called peptidoglycan. It has a long backbone formed by NAG and N-acetylmuramic acid (NAM) that alternate with each other and are linked by β-1,4- glycosidic linkages.  In addition, a short chain of amino acids is attached at the C-3 carbon of NAM. When molecules of peptidoglycan align, peptide bonds link the amino acid chains on adjacent strands. These links serve the same purpose as the hydrogen bonds between the parallel strands of cellulose and chitin in the cell walls of other organisms.  Polysaccharides and Chemical Evolution No plausible mechanism exists for the polymerization of monosaccharides under conditions that prevailed early in Earth’s history. To date, no polysaccharide has been discovered that can catalyze polymerization reactions. The monomers in polysaccharides are not capable of complementary base pairing. Even though polysaccharides probably did not play a significant role in the earliest forms of life, they became enormously important once cellular life evolved.  What Do Carbohydrates Do? One of the basic functions that carbohydrates perform in organisms is to serve as a substrate for synthesizing more-complex molecules. For example, recall that RNA contains the five-carbon sugar ribose (C5H10O5) and DNA contains the modified sugar deoxyribose (C5H10O4). Carbohydrates have diverse functions in cells: In addition to serving as precursors to larger molecules, they (1) provide fibrous structural materials (2) indicate cell identity (3) store chemical energy.  Carbohydrates Can Provide Structural Support Cellulose and chitin, along with the modified polysaccharide peptidoglycan, are key structural compounds. They form fibers that give cells and organisms strength and elasticity. In the cell walls of plants, for example, a collection of about 80 cellulose molecules are cross-linked by hydrogen bonding to produce a tough fiber. These cellulose fibers, in turn, crisscross to form a tough sheet that is able to withstand pulling and pushing forces.  In addition to being tough, structural carbohydrates are durable. Almost all organisms produce enzymes that cleave the α-glycosidic linkages in starch and glycogen molecules, but only a few organisms have enzymes capable of digesting cellulose, chitin, or peptidoglycan. These fibers tend to be insoluble due to the strong interactions between strands consisting of β-1,4- glycosidic linkages. The exclusion of water within these fibers makes their hydrolysis more difficult so they are resistant to degradation and decay.  Ironically, the fact that cellulose is indigestible makes it extremely important for digestive health. The cellulose that you ingest when you eat plants—what biologists call dietary fiber— forms a porous mass that absorbs and retains water. This sponge like mass adds moisture and bulk that helps fecal material move through the intestinal tract more quickly, preventing constipation and other problems. The Role of Carbohydrates in Cell Identity Some polysaccharides exhibit enormous structural diversity, because their component monomers—and the linkages between them— vary a lot. As a result, they are capable of displaying information to other cells through their structure. More specifically, polysaccharides act as an identification badge on the outer surface of the plasma membrane that surrounds a cell.  Carbohydrates attached to lipids and proteins project outward from the cell surface into the surrounding environment. A glycolipid is a lipid that has been glycosylated, meaning it has one or more covalently bonded carbohydrates. A glycoprotein is a protein that is similarly linked to carbohydrates—usually relatively short oligosaccharides. Glycolipids and glycoproteins are key molecules in cell–cell recognition and cell–cell signaling.  Your blood type is determined by the type of oligosaccharides presented on the surface of your blood cells. The A, B, and O types arise from different modifications of the carbohydrates in glycolipids. In addition, each distinct type of cell in a multicellular organism— for example, the nerve cells and muscle cells in your body—displays a different set of glycoproteins on its surface. This identification information helps cells recognize and communicate with each other.  Carbohydrates are at the root of one of the most important events in the life cycle of many multicellular organisms—sexual reproduction. During the 1980s, Paul Wassarman and colleagues investigated the role of glycoproteins in cell–cell recognition between sperm and egg during fertilization. This step guarantees specificity—sperm normally recognize and bind only to eggs of their own species.  In one experiment, the researchers mixed sperm with purified egg-surface glycoproteins and discovered that most of the sperm lost their ability to attach to eggs. Such loss of function is an example of what researchers call competitive inhibition. The glycoproteins had bound to—and thus blocked— the same structure on the sperm that it uses to bind to eggs. This result showed that sperm attach to eggs via egg glycoproteins.  Carbohydrates and Energy Storage The essence of chemical evolution was energy transformations. For example, it was proposed that the energy in sunlight may have been converted into chemical energy and stored in bonds of molecules such as formaldehyde (CH2O). Plants harvest the energy in sunlight and store it in the bonds of carbohydrates by the process known as photosynthesis. CO2 + H2O + sunlight → (CH2O)n + O2 1. The electrons in the C=O bonds of CO2 and the C-O bonds of carbohydrates are held tightly because of oxygen’s high electronegativity. Thus, they have relatively low potential energy. 2. The electrons involved in the C-H bonds of carbohydrates are shared equally because the electronegativity of carbon and hydrogen is about the same. Thus, bonds are weaker and these electrons have relatively high potential energy. 3. Electrons are also shared equally in the carbon–carbon bonds of (C-C) carbohydrates—meaning that they, too, have relatively high potential energy. So, because C-C and C-H bonds have much higher potential energy than C-O bonds have, carbohydrates store much more chemical energy than carbon dioxide does.  The potential energy in bonds is released when they are broken and new, stronger bonds are formed. Both carbohydrates and fats are used as fuel in cells, but fats store twice as much energy per gram compared with carbohydrates.  Starch and glycogen are efficient energy- storage molecules because they polymerize via α-glycosidic linkages instead of the β-glycosidic linkages observed in the structural polysaccharides. The α-linkages in storage polysaccharides are readily hydrolyzed to release glucose, while the structural polysaccharides resist enzymatic degradation.  The most important enzyme involved in catalyzing the hydrolysis of α-glycosidic linkages in glycogen molecules is a protein called phosphorylase. Many of our cells contain phosphorylase, so they can break down glycogen to provide glucose on demand. The enzymes involved in breaking the α-glycosidic linkages in starch are called amylases. Our salivary glands and pancreas produce amylases that are secreted into our mouth and small intestine, respectively. These amylases are responsible for digesting the starch that we eat. Energy Stored in Glucose Is Used to Make ATP When a cell needs energy, reactions break down glucose and capture some of the released energy through synthesis of the nucleotide adenosine triphosphate (ATP). More specifically, the energy that’s released when sugars are processed is used to synthesize ATP from a precursor called adenosine diphosphate (ADP) plus a free inorganic phosphate (Pi) molecule. The overall reaction can be written as follows: (CH2O)n + O2 + ADP + Pi → CO2 + H2O + ATP How much sugar does it take to form ATP? A cell can use the sugar stored in a single candy (about 15 Calories of energy) to produce approximately 3 × 1023 molecules of ATP. Although this sounds like a lot of ATP, an average person would burn through all of this ATP in less than 2 minutes! The energy in ATP drives reactions that are responsible for everything from polymerization to muscle movement.