Chapter 5 The Structure and Function of Macromolecules
Figure 5.1 Overview: The Molecules of Life Another level in the hierarchy of biological organization is reached when small organic molecules are joined together Macromolecules • Are large molecules composed of smaller molecules • Are complex in their structures
Concept 5.1: Most macromolecules are polymers, built from monomers Three of the classes of life’s organic molecules are polymers • Carbohydrates • Proteins • Nucleic acids • A polymer • Is a long molecule consisting of many similar building blocks called monomers
1 HO H 3 2 H HO Unlinked monomer Short polymer Dehydration removes a watermolecule, forming a new bond H2O 1 2 3 4 HO H Longer polymer (a) Dehydration reaction in the synthesis of a polymer Figure 5.2A The Synthesis and Breakdown of Polymers • Monomers form larger molecules by condensation reactions called dehydration synthesis • These are sometimes called condensation rxns because a molecule of water is liberated when a bond is formed.
1 3 HO 4 2 H Hydrolysis adds a watermolecule, breaking a bond H2O 1 2 H HO 3 H HO (b) Hydrolysis of a polymer Figure 5.2B Polymers • Polymers can disassemble by • Hydrolysis (water cleavage)
The Diversity of Polymers • Each class of polymer • Is formed from a specific set of monomers • Although organisms share the same limited number of monomer types, each organism is unique based on the arrangement of monomers into polymers • An immense variety of polymers can be built from a small set of monomers 1 3 2 H HO
Concept 5.2: Carbohydrates serve as fuel and building material • Carbohydrates • Include both sugars and their polymers (starches) • Most carbohydrates have the empirical formula (CH20)n. • Carbohydrates are composed of covalently bonded atoms of carbon, hydrogen, and oxygen.
Sugars • Monosaccharides • The basic unit of a carbohydrate is called a monosaccharide or simple sugar. • Can be used for fuel; The principle source of energy for organisms is glucose, C6H12O6 . Glucose is the form of sugar generally transported in the human body (“blood sugar”). • Monosaccharides can be burned (oxidized) to yield carbon dioxide, water, and energy. • Can be converted into other organic molecules • Can be combined into polymers • Structurally a sugar consists of a carbon backbone of three or more carbon atoms with either an aldehyde or ketonecarbonyl group on one carbon, and hydroxyl groups on each of the other carbons.
Monosaccharides • May be linear • Can form rings (when aqueous)
Triose sugars(C3H6O3) Pentose sugars(C5H10O5) Hexose sugars(C6H12O6) H H H H O O O O C C C C H C OH H C OH H C OH H C OH H C OH H C OH HO C H HO C H Aldoses H H C OH H C OH HO C H H C OH H C OH H C OH Glyceraldehyde H C OH H C OH H Ribose H H Glucose Galactose H H H H C OH H C OH H C OH C O C O C O HO C H H C OH H C OH Ketoses H C OH H C OH H Dihydroxyacetone H C OH H C OH H C OH H Ribulose H Figure 5.3 Fructose • Examples of monosaccharides
A side note: Isomers • a. Structural Isomers: These differ in the arrangement of atoms. For example, glucose and fructose both have the formula C6H12O6 but have different bonding arrangements. • b. Geometric Isomers: These have the same covalent partnership but different spatial arrangements because of the orientation of groups around a double bond, which does not permit free rotation around it. The orientations of the constituent groups are spatially fixed around the double bond. Such arrangements around a double bond are called cis-isomers when the named constituents are oriented on the same side of the double bond, and trans-isomers when the constituents are oriented across from each other. • c. Stereoisomers (Enantiomers): These are molecules that are mirror images of each other. The result is a pair of compounds that are mirror images, right handed and left handed isomers. These are compounds that have the same molecular formula but different three dimensional structures and hence different physical and/or chemical properties. There are 3 types of isomers:
H H H C H H C H H H H H H H (a) Structural isomers H C C C C C H H C H C C H H H H H H H H H X X X C C C C (b) Geometric isomers X H H H CO2H CO2H C C (c) Enantiomers H H NH2 NH2 CH3 CH3 Figure 4.7 A-C ISOMERS • Structural • Geometric • Enantiomers (stereoisomers)
L-Dopa (effective against Parkinson’s disease) D-Dopa (biologically inactive) Figure 4.8 Enantiomers (stereoisomers) • Are important in the pharmaceutical industry (levo, dextro)
Another side note… Drawing organic molecules- Sometimes the drawings showing monosaccharides are simplified to show only the most important parts of the molecules. The same sequence of events diagrammed above could be shown as: Sometimes even the carbons are not labeled, but are assumed to be present at every 'bend or end' of the ring. Simple hexagon diagrams such as those below can serve as highly simplified models of the glucose molecule. Here the hydrogens that complete each molecule are simply 'understood' to be present, and are not included in the diagrams. The diagrams can be still further simplified by showing only the carbons and the carbonyl group [or the oxygen that is built into the ring].
Polymers of Monosaccharides • Disaccharides are formed by joining two monosaccharides together. The two monosaccharides are linked by a reaction called a dehydration synthesis or condensation reaction. • Are joined by a glycosidic linkage
(a) Dehydration reaction in the synthesis of maltose. The bonding of two glucose units forms maltose. The glycosidic link joins the number 1 carbon of one glucose to the number 4 carbon of the second glucose. Joining the glucose monomers in a different way would result in a different disaccharide. CH2OH CH2OH CH2OH CH2OH O O O O H H H H H H H H 1–4glycosidiclinkage HOH HOH HOH HOH 4 1 H H H H OH OH O H OH HO HO OH O H H H H OH OH OH OH H2O Glucose Maltose Glucose CH2OH CH2OH CH2OH CH2OH O O O O 1–2glycosidiclinkage H H H H H H HOH HOH 2 1 H H HO H HO H Dehydration reaction in the synthesis of sucrose. Sucrose is a disaccharide formed from glucose and fructose.Notice that fructose,though a hexose like glucose, forms a five-sided ring. (b) OH H O O HO CH2OH HO CH2OH H OH H H OH H OH OH H2O Glucose Sucrose Fructose Figure 5.5 Examples of disaccharides
Polysaccharides Condensation reaction/dehydration synthesis: the joining of two smaller organic compounds resulting in the formation of a larger organic molecule and the release of a water molecule. The condensation reaction, a synthesis reaction, is important because it is the reaction that puts together polymers from monomer units. Synthesis reactions require energy to complete. Hydrolytic cleavage (hydrolysis): With the addition of water, the splitting of a large organic molecule into two smaller organic molecules. Hydrolysis reactions liberate energy. Hydrolytic cleavage, or hydrolysis, is the opposite of a dehydration or condensation reaction. For example, in the human digestive system, sucrose (disaccharide) is split into glucose and fructose (two monosaccharides).
Polysaccharides • Polysaccharides: are polymers of sugars • Serve many roles in organisms • Three examples of polysaccharides are starch, glycogen, and cellulose. A polysaccharide consists of three or more (usually hundreds of) monosaccharides, joined together by condensation reactions.
Chloroplast Starch 1 m Amylose Amylopectin (a) Starch: a plant polysaccharide Figure 5.6 Two different types of starches • Starch • Is a polymer consisting entirely of glucose monomers • Starch is the storage polysaccharide in plants and is an important reservoir for energy. • There are two common types of (plant) starch:
Starches 1) Amylose: the simplest starch, consisting of unbranched chains of hundreds of glucose molecules. Note: the [alpha] 1,4 glycosidic bond, (the glucose units are connected to the first and fourth carbons) 2) Amylopectin: large molecule consisting of short glucose chains with other glucose chains branching off of the main chain. • Note: the glucose units are linked by both [alpha] 1,4 AND [alpha] 1,6 bonds!
Giycogen granules Mitochondria 0.5 m Glycogen Figure 5.6 (b) Glycogen: an animal polysaccharide “animal” starch • Glycogen • Consists of linked, highly branched, glucose monomers • Is the major storage form of glucose in animals
Glycogen • Glycogen is the main energy storage polysaccharide in animals • Glycogen is composed of branching glucose chains, with more branches than amylopectin. • It is found in the liver and muscles and acts as the temporary storage form of glucose. The liver removes the excess glucose from the bloodstream, converts the glucose monomers to glycogen via condensation reactions, and stores it as glycogen. • When vertebrates need glucose for energy, glycogen is converted by hydrolysis back to glucose. • In glycogen, or animal starch, the glucose units are again joined by [alpha] 1,4 linkages to produce long chains, but side chains are linked to the main chain by [alpha] 1,6 linkages
Structural Polysaccharides • Cellulose is a structural polysaccharide and is the major building material made by plants. • It is the most abundant organic material on earth. • Cellulose is made up of long, straight glucose molecules. Cellulose is called a structural polysaccharide because it gives the plant cell its shape, is not soluble, and is very strong. • Cellulose is flexible when the plant cell is young. As the cell grows, the cellulose becomes thicker and more rigid.
Cellulose • Cellulose is indigestible to humans because the linkages are 1-4 beta linkages, and our enzymes can only break down 1-4 alpha linkages because the shapes are different. • Cellulose is the so-called "fiber" in our diets. • Some bacteria, protists, fungi, and lichens can break down cellulose. For example, bacteria and protists found in the stomachs of termites and grazing animals break down the cellulose in the grass and wood to provide the animal with glucose
About 80 cellulose molecules associate to form a microfibril, the main architectural unit of the plant cell wall. Cellulose microfibrils in a plant cell wall Microfibril Cell walls 0.5 m Plant cells OH OH CH2OH CH2OH O O O O OH OH OH OH O O O O O OH CH2OH OH CH2OH Cellulose molecules CH2OH OH CH2OH OH O O O O OH OH OH OH Parallel cellulose molecules are held together by hydrogen bonds between hydroxyl groups attached to carbon atoms 3 and 6. O O O O O OH CH2OH OH CH2OH CH2OH CH2OH OH OH O O O O OH OH OH OH O O O A cellulose molecule is an unbranched glucose polymer. O O OH CH2OH OH CH2OH Figure 5.8 • Glucose monomer Cellulose • Is a major component of the tough walls that enclose plant cells
Figure 5.9 Cellulose is difficult to digest • Cows have microbes in their stomachs to facilitate this process
CH2OH O OH H H OH H H H NH O C CH3 OH (b) Chitin forms the exoskeleton of arthropods. This cicada is molting, shedding its old exoskeleton and emerging in adult form. (c) Chitin is used to make a strong and flexible surgical thread that decomposes after the wound or incision heals. (a) The structure of the chitin monomer. Figure 5.10 A–C Other structural polysaccharides • Chitin, another important structural polysaccharide • Is found in the exoskeleton of arthropods • Chitin is very soft but is combined with CaCO3 (calcium carbonate or limestone) to become hard. Most animals cannot digest chitin • Can be used as surgical thread Pectin and carrageenan: These are extracted from algae. Pectin and carrageenan are put into food items such as jellies, jams, yogurt, icecream , and milkshakes to give them a jelly-like or creamy consistency.
LIPIDS • Concept 5.3: Lipids are a diverse group of hydrophobic molecules • Are the one class of large biological molecules that do not consist of polymers • Lipids are a diverse group of molecules defined by their solubility rather than by their structures (Share the common trait of being hydrophobic) • Lipids dissolve in nonpolar solvents such as chloroform, ether, and benzene. • There are 5 classes of lipids: triglycerides, phospholipids, glycolipids, steroids, and waxes.
Triglycerides: Fats and Oils • Fat: a lipid that is solid at room temperature.Oil: a lipid that is a liquid at room temperature. • Glycerol: Fatty acids usually have an even number of carbons, differ in the length of the carbon chain, and may contain double covalent bonds.
Triglycerides: Fats and oils • A triglyceride is composed of one glycerol molecule and three fatty acid molecules. • The synthesis of a triglyceride occurs when a glycerol molecule joins with three (of the seventy different) fatty acids. Fat molecule (triacylglycerol)
Stearic acid Figure 5.12 (a) Saturated fat and fatty acid Saturated fatty acids • Have the maximum number of hydrogen atoms possible • Have no double bonds Animal fats are usually saturated fats and solidify at room temperature.
Unsaturated fatty acids Figure 5.12 Oleic acid cis double bond causes bending (b) Unsaturated fat and fatty acid • Have one or more double bonds between carbons • This structure means that they have fewer hydrogens than the saturated fats; these are called unsaturated fats. • Unsaturated fats can be found in plants (olive oil, peanut oil, corn oil) more commonly than animals • Usually liquids at room temperature. • We can't make unsaturated fats, so we need to eat small amounts of unsaturated fats. • Polyunsaturated fats have more than one double bond.
Functions of Fats • Triglycerides (fats and oils) are a concentrated source of energy. • When the fat is combined with oxygen, the fats release a large amount of energy, more than twice as much per gram as carbohydrates. • Seeds store triglycerides, animals store energy as fat for lean seasons or migration or insulation, humans store fat under the skin and around internal organs. • Fat serves for insulation and flotation. • Storage fat serves as padding in your fingers and your bottom.
PHOSPHOLIPIDS: Important components of cell membranes • Have only two fatty acids • Have a phosphate group instead of a third fatty acid (attached to the glycerol) • Consists of a hydrophilic “head” and hydrophobic “tails”
Importance of phospholipids • The structure of phospholipids • Results in a bilayer arrangement found in cell membranes
Other Biologically Important Groups of Lipids Glycolipids: The third carbon in the glycerol molecule isn't bound to a phosphate group. Instead, it is bonded to a short carbohydrate chain (1-15 monosaccharides). The carbohydrate head is hydrophilic; thus glycolipids behave in the same way as phospholipids. They are also important components of the cell membrane.
Other Biologically Important Groups of Lipids Waxes: Waxes are similar in structure to triglycerides, but instead of glycerol there is a long chain alcohol. Because of their hydrophobic quality, waxes are found in many living things that need to conserve water. Insects have waxy cuticles, plants have wax on their leaves, fruit skins and petals have wax as an outer covering.
Steroids Steroids are not really that structurally similar to fatty acids or lipids. Since they are hydrophobic, however, they are called lipids. All steroids have four linked carbon rings. Steroids have a tail and many have an -OH group.
H3C CH3 CH3 CH3 CH3 HO Figure 5.15 Steroids • Lanolin- Commercially refined from sheep's wool. Humans have a small amount of lanolin in the hair and skin; lanolin helps give these structures flexibility. • Cholesterol- A major constituents of the cell membrane. When bombarded with ultraviolet light, it rearranged into vitamin D. When modified slightly, it makes sex hormones.
Proteins • Concept 5.4: Proteins can be folded into many shapes, resulting in a wide range of functions • Proteins have many roles inside the cell • Proteins are large, complex organic molecules that are made of smaller monomer units, amino acids. • Proteins are naturally occurring biological molecules that are composed of amino acid monomers linked together through dehydration (condensation) reactions. • Amino Acids are the building blocks (monomers) of proteins. • There are 20 different amino acids.
Table 5.1 An overview of protein functions
Basic Structure of an Amino Acid Each amino acid has a carbon with four different groups attached. (1) Amine group, NH2 , (basic, can accept H+ and thus have a positive charge). (2) Carboxyl group, -COOH, acidic, can donate H+ and thus have a negative charge (-COO-) (3) Hydrogen (4) R group: (“variable” group) The R group is the portion of the amino acids that is different in each amino acid. In the amino acid glycine, the R group is replaced with an H atom.
CH3 CH3 CH3 CH CH2 CH3 CH3 H CH3 H3C CH3 CH2 CH O O O O O H3N+ C H3N+ C H3N+ H3N+ C C C C C C H3N+ C C O– O– O– O– O– H H H H H Valine (Val) Leucine (Leu) Isoleucine (Ile) Glycine (Gly) Alanine (Ala) Nonpolar CH3 CH2 S H2C CH2 O NH CH2 C C H2N CH2 CH2 O– CH2 O O O H H3N+ H3N+ C C C C H3N+ C C O– O– O– H H H Phenylalanine (Phe) Proline (Pro) Methionine (Met) Tryptophan (Trp) Figure 5.17 20 different amino acids make up proteins
R Groups The R group of the amino acid determines the physical and chemical properties of the protein. R groups can be nonpolar, polar, acidic, or basic.
OH NH2 O C NH2 O C OH SH CH2 CH3 OH Polar CH2 CH CH2 CH2 CH2 CH2 O O O O O O H3N+ C H3N+ C H3N+ C C H3N+ C C H3N+ C C C C C H3N+ C O– O– O– O– O– O– H H H H H H Glutamine (Gln) Tyrosine (Tyr) Asparagine (Asn) Cysteine (Cys) Serine (Ser) Threonine (Thr) Basic Acidic NH3+ NH2 NH+ O– O –O O CH2 C NH2+ C C NH Electrically charged CH2 CH2 CH2 CH2 CH2 O O CH2 CH2 C CH2 C H3N+ C H3N+ C O O– O– CH2 C H3N+ CH2 C H O H O– C C H3N+ CH2 H O O– C C H3N+ H O– H Lysine (Lys) Histidine (His) Arginine (Arg) Glutamic acid (Glu) Aspartic acid (Asp)
R Groups They can also be the site of the addition of prosthetic groups, inorganic groups (vitamins, minerals) that are essential for the functioning of the protein. These prosthetic groups often determine the protein's function, as in hemoglobin. Minerals in our diets are often essential parts of prosthetic groups; for example, iron (Fe2+) in our diet is essential for the synthesis of the heme group the prosthetic group in hemoglobin.
Biological Sources and Utilization of Amino Acids How do the cells in the body obtain amino acids? Many foods contain proteins; the proteins are broken down into small pieces called peptides. Peptides are small (about 30 amino acids long) and are carried in the blood vessels. When a cell is actively making proteins, peptides are taken into the cell, broken down, and the constituent amino acids are reconfigured into a protein.
Peptidebond OH SH CH2 CH2 CH2 H H H C C H C C N C OH H C OH N N DESMOSOMES H O H O H O (a) H2O OH DESMOSOMES DESMOSOMES Side chains SH OH Peptidebond CH2 CH2 CH2 H H H N OH C C C C C H C N N Backbone H H O O H O Amino end(N-terminus) Carboxyl end(C-terminus) Figure 5.18 (b) Amino Acid Polymers • Amino acids • Are linked by peptide bonds OH
Determining the Amino Acid Sequence of a Polypeptide • Proteins have a three dimensional configuration which is determined by the amino acid sequence. • The amino acid sequences of polypeptides • Were first determined using chemical means • Can now be determined by automated machines http://en.wikipedia.org/wiki/Frederick_Sanger