4. Macromolecules.pdf

Ajman University of Science and Technology College Of Engineering Biomedical Engineering Biology 2181410 The Structure and Function of Large Biological Molecules Dr. Enas AL Zu’abi Overview: The Molecul...

Ajman University of Science and Technology College Of Engineering Biomedical Engineering Biology 2181410 The Structure and Function of Large Biological Molecules Dr. Enas AL Zu’abi Overview: The Molecules of Life All living things are made up of four classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids Macromolecules are large molecules composed of thousands of covalently connected atoms Molecular structure and function are inseparable Concept 5.1: Macromolecules are polymers, built from monomers A polymer is a long molecule consisting of many similar building blocks These small building-block molecules are called monomers Three of the four classes of life’s organic molecules are polymers – Carbohydrates – Proteins – Nucleic acids The Synthesis and Breakdown of Polymers A dehydration reaction occurs when two monomers bond together through the loss of a water molecule Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction Figure 5.2 (a) Dehydration reaction: synthesizing a polymer 1 2 3 Short polymer Unlinked monomer Dehydration removes a water molecule, forming a new bond. 1 2 3 4 Longer polymer (b) Hydrolysis: breaking down a polymer 1 2 3 4 Hydrolysis adds a water molecule, breaking a bond. 1 2 3 Concept 5.2: Carbohydrates serve as fuel and building material Carbohydrates include sugars and the polymers of sugars The simplest carbohydrates are monosaccharides, or single sugars Carbohydrate macromolecules are polysaccharides, polymers composed of many sugar building blocks Sugars Monosaccharides have molecular formulas that are usually multiples of CH2O Glucose (C6H12O6) is the most common monosaccharide Monosaccharides are classified by – The location of the carbonyl group (as aldose or ketose) – The number of carbons in the carbon skeleton Figure 5.3 Aldoses (Aldehyde Sugars) Ketoses (Ketone Sugars) Trioses: 3-carbon sugars (C3H6O3) Glyceraldehyde Dihydroxyacetone Pentoses: 5-carbon sugars (C5H10O5) Ribose Ribulose Hexoses: 6-carbon sugars (C6H12O6) Glucose Galactose Fructose  Though often drawn as linear skeletons, in aqueous solutions many sugars form rings Monosaccharides serve as a major fuel for cells and as raw material for building molecules Figure 5.4 1 6 6 2 5 5 3 4 1 4 1 4 2 2 5 3 3 6 (a) Linear and ring forms 6 5 4 1 3 2 (b) Abbreviated ring structure  A disaccharide is formed when a dehydration reaction joins two monosaccharides This covalent bond is called a glycosidic linkage Figure 5.5 1–4 glycosidic 1 linkage 4 Glucose Glucose Maltose (a) Dehydration reaction in the synthesis of maltose 1–2 glycosidic 1 linkage 2 Glucose Fructose Sucrose (b) Dehydration reaction in the synthesis of sucrose Polysaccharides Polysaccharides, the polymers of sugars, have storage and structural roles The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages Storage Polysaccharides Starch, a storage polysaccharide of plants, consists entirely of glucose monomers Plants store surplus starch as granules within chloroplasts and other plastids The simplest form of starch is amylose Figure 5.6 Chloroplast Starch granules Amylopectin Amylose (a) Starch: 1 m a plant polysaccharide Mitochondria Glycogen granules Glycogen (b) Glycogen: 0.5 m an animal polysaccharide  Glycogen is a storage polysaccharide in animals Humans and other vertebrates store glycogen mainly in liver and muscle cells Structural Polysaccharides The polysaccharide cellulose is a major component of the tough wall of plant cells Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ The difference is based on two ring forms for glucose: alpha () and beta () Figure 5.7 (a)  and  glucose ring structures 4 1 4 1  Glucose  Glucose 1 4 1 4 (b) Starch: 1–4 linkage of  glucose monomers (c) Cellulose: 1–4 linkage of  glucose monomers  Polymers with  glucose are helical Polymers with  glucose are straight In straight structures, H atoms on one strand can bond with OH groups on other strands Parallel cellulose molecules held together this way are grouped into microfibrils, which form strong building materials for plants Figure 5.8 Cell wall Cellulose microfibrils in a plant cell wall Microfibril 10 m 0.5 m Cellulose molecules  Glucose monomer  Enzymes that digest starch by hydrolyzing  linkages can’t hydrolyze  linkages in cellulose Cellulose in human food passes through the digestive tract as insoluble fiber Some microbes use enzymes to digest cellulose Many herbivores, from cows to termites, have symbiotic relationships with these microbes  Chitin, another structural polysaccharide, is found in the exoskeleton of arthropods Chitin also provides structural support for the cell walls of many fungi Figure 5.9 The structure of the chitin monomer Chitin forms the exoskeleton of arthropods. Chitin is used to make a strong and flexible surgical thread that decomposes after the wound or incision heals. Concept 5.3: Lipids are a diverse group of hydrophobic molecules Lipids are the one class of large biological molecules that do not form polymers The unifying feature of lipids is having little or no affinity for water Lipids are hydrophobic because they consist mostly of hydrocarbons, which form nonpolar covalent bonds The most biologically important lipids are fats, phospholipids, and steroids Fats Fats are constructed from two types of smaller molecules: glycerol and fatty acids Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon A fatty acid consists of a carboxyl group attached to a long carbon skeleton © 2011 Pearson Education, Inc. Figure 5.10a Fatty acid (in this case, palmitic acid) Glycerol (a) One of three dehydration reactions in the synthesis of a fat  Fats separate from water because water molecules form hydrogen bonds with each other and exclude the fats In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride Figure 5.10b Ester linkage (b) Fat molecule (triacylglycerol)  Fatty acids vary in length (number of carbons) and in the number and locations of double bonds Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds Unsaturated fatty acids have one or more double bonds Figure 5.11 (b) Unsaturated fat (a) Saturated fat Structural formula of a saturated fat molecule Structural formula of an unsaturated fat molecule Space-filling model of stearic acid, a saturated fatty acid Space-filling model of oleic acid, an unsaturated fatty acid Cis double bond causes bending.  Fats made from saturated fatty acids are called saturated fats, and are solid at room temperature Most animal fats are saturated Fats made from unsaturated fatty acids are called unsaturated fats or oils, and are liquid at room temperature Plant fats and fish fats are usually unsaturated  Certain unsaturated fatty acids are not synthesized in the human body These must be supplied in the diet These essential fatty acids include the omega-3 fatty acids, required for normal growth, and thought to provide protection against cardiovascular disease  The major function of fats is energy storage Humans and other mammals store their fat in adipose cells Adipose tissue also cushions vital organs and insulates the body Phospholipids In a phospholipid, two fatty acids and a phosphate group are attached to glycerol The two fatty acid tails are hydrophobic, but the phosphate group and its attachments form a hydrophilic head Figure 5.12 Choline Hydrophilic head Phosphate Glycerol Hydrophobic tails Fatty acids Hydrophilic head Hydrophobic tails (a) Structural formula (b) Space-filling model (c) Phospholipid symbol  When phospholipids are added to water, they self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior The structure of phospholipids results in a bilayer arrangement found in cell membranes Phospholipids are the major component of all cell membranes Figure 5.13 Hydrophilic WATER head Hydrophobic tail WATER Steroids Steroids are lipids characterized by a carbon skeleton consisting of four fused rings Cholesterol, an important steroid, is a component in animal cell membranes Although cholesterol is essential in animals, high levels in the blood may contribute to cardiovascular disease Figure 5.14 Concept 5.4: Proteins include a diversity of structures, resulting in a wide range of functions Proteins account for more than 50% of the dry mass of most cells Protein functions include structural support, storage, transport, cellular communications, movement, and defense against foreign substances Figure 5.15-a Enzymatic proteins Defensive proteins Function: Selective acceleration of chemical reactions Function: Protection against disease Example: Digestive enzymes catalyze the hydrolysis Example: Antibodies inactivate and help destroy of bonds in food molecules. viruses and bacteria. Antibodies Enzyme Virus Bacterium Storage proteins Transport proteins Function: Storage of amino acids Function: Transport of substances Examples: Casein, the protein of milk, is the major Examples: Hemoglobin, the iron-containing protein of source of amino acids for baby mammals. Plants have vertebrate blood, transports oxygen from the lungs to storage proteins in their seeds. Ovalbumin is the other parts of the body. Other proteins transport protein of egg white, used as an amino acid source molecules across cell membranes. for the developing embryo. Transport protein Ovalbumin Amino acids for embryo Cell membrane Figure 5.15-b Hormonal proteins Receptor proteins Function: Coordination of an organism’s activities Function: Response of cell to chemical stimuli Example: Insulin, a hormone secreted by the Example: Receptors built into the membrane of a pancreas, causes other tissues to take up glucose, nerve cell detect signaling molecules released by thus regulating blood sugar concentration other nerve cells. Receptor Signaling protein Insulin High secreted Normal molecules blood sugar blood sugar Contractile and motor proteins Structural proteins Function: Movement Function: Support Examples: Motor proteins are responsible for the Examples: Keratin is the protein of hair, horns, undulations of cilia and flagella. Actin and myosin feathers, and other skin appendages. Insects and proteins are responsible for the contraction of spiders use silk fibers to make their cocoons and webs, muscles. respectively. Collagen and elastin proteins provide a fibrous framework in animal connective tissues. Actin Myosin Collagen Muscle tissue Connective 100 m tissue 60 m  Enzymes are a type of protein that acts as a catalyst to speed up chemical reactions Enzymes can perform their functions repeatedly, functioning as workhorses that carry out the processes of life Polypeptides Polypeptides are unbranched polymers built from the same set of 20 amino acids A protein is a biologically functional molecule that consists of one or more polypeptides Amino Acid Monomers Amino acids are organic molecules with carboxyl and amino groups Amino acids differ in their properties due to differing side chains, called R groups Figure 5.UN01 Side chain (R group)  carbon Amino Carboxyl group group Figure 5.16 Nonpolar side chains; hydrophobic Side chain (R group) Glycine Alanine Valine Leucine Isoleucine (Gly or G) (Ala or A) (Val or V) (Leu or L) (Ile or I) Methionine Phenylalanine Tryptophan Proline (Met or M) (Phe or F) (Trp or W) (Pro or P) Polar side chains; hydrophilic Serine Threonine Cysteine Tyrosine Asparagine Glutamine (Ser or S) (Thr or T) (Cys or C) (Tyr or Y) (Asn or N) (Gln or Q) Electrically charged side chains; hydrophilic Basic (positively charged) Acidic (negatively charged) Aspartic acid Glutamic acid Lysine Arginine Histidine (Asp or D) (Glu or E) (Lys or K) (Arg or R) (His or H) Amino Acid Polymers Amino acids are linked by peptide bonds A polypeptide is a polymer of amino acids Polypeptides range in length from a few to more than a thousand monomers Each polypeptide has a unique linear sequence of amino acids, with a carboxyl end (C-terminus) and an amino end (N-terminus) Figure 5.17 Peptide bond New peptide bond forming Side chains Back- bone Amino end Peptide Carboxyl end (N-terminus) bond (C-terminus) Protein Structure and Function A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape Figure 5.18 Groove Groove (a) A ribbon model (b) A space-filling model  The sequence of amino acids determines a protein’s three-dimensional structure A protein’s structure determines its function Figure 5.19 Antibody protein Protein from flu virus Four Levels of Protein Structure The primary structure of a protein is its unique sequence of amino acids Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain Tertiary structure is determined by interactions among various side chains (R groups) Quaternary structure results when a protein consists of multiple polypeptide chains Figure 5.20a Primary structure Amino acids Amino end Primary structure of transthyretin Carboxyl end  Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word Primary structure is determined by inherited genetic information Figure 5.20b Secondary Tertiary Quaternary structure structure structure  helix Hydrogen bond  pleated sheet  strand Transthyretin Hydrogen Transthyretin protein bond polypeptide  The coils and folds of secondary structure result from hydrogen bonds between repeating constituents of the polypeptide backbone Typical secondary structures are a coil called an  helix and a folded structure called a  pleated sheet Figure 5.20c Secondary structure  helix Hydrogen bond  pleated sheet  strand, shown as a flat arrow pointing toward the carboxyl end Hydrogen bond  Tertiary structure is determined by interactions between R groups, rather than interactions between backbone constituents These interactions between R groups include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions Strong covalent bonds called disulfide bridges may reinforce the protein’s structure Figure 5.20e Tertiary structure Transthyretin polypeptide Figure 5.20f Hydrogen bond Hydrophobic interactions and van der Waals interactions Disulfide bridge Ionic bond Polypeptide backbone  Quaternary structure results when two or more polypeptide chains form one macromolecule Collagen is a fibrous protein consisting of three polypeptides coiled like a rope Hemoglobin is a globular protein consisting of four polypeptides: two alpha and two beta chains Figure 5.20g Quaternary structure Transthyretin protein (four identical polypeptides) Figure 5.20h Collagen Figure 5.20i Heme Iron  subunit  subunit  subunit  subunit Hemoglobin What Determines Protein Structure? In addition to primary structure, physical and chemical conditions can affect structure Alterations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel This loss of a protein’s native structure is called denaturation A denatured protein is biologically inactive Figure 5.22 tu Normal protein Denatured protein