Biochemistry Lecture Notes PDF

Compounds found in living organisms could not BIOCHEMISTRY LECTURE be produced in the laboratory. INTRODUCTION TO BIOCHEMISTRY...

Compounds found in living organisms could not BIOCHEMISTRY LECTURE be produced in the laboratory. INTRODUCTION TO BIOCHEMISTRY German chemist Friedrich Wöhler performed the critical experiment that disproved this belief in 1828 He synthesize an organic compound from BIOCHEMISTRY an inorganic substance. Biochemistry is the chemistry of living organisms. The application of chemistry to the study of biological processes at the cellular and molecular level Biochemistry has become the foundation for understanding all biological processes. It has He synthesized urea by slowly evaporating a provided explanations for the causes of many water solution of ammonium cyanate, which he diseases in humans, animals and plants had prepared by adding silver cyanate to ammonium chloride. HIERARCHY OF STRUCTURES OF OUR BODY FROM SIMPLE TO COMPLEX ORIGIN OF LIFE LEVEL OF ORGANIZATION THE CHEMICAL ELEMENTS OF LIFE - The gases usually postulated to have been present in the atmosphere of the early Earth include NH3, H2S, CO, CO2, CH4, N2, H2, and (in both liquid and vapor forms) H2O. CHEMICAL FOUNDATIONS OF BIOCHEMISTR Y BIOMOLECULES - Complex organic substances - That build up living organisms - It is required for growth and maintenance Most biomolecules can be considered to be derived from the simplest type of organic molecules; hydrocarbon. In early part of the 19th century, there was a widely held belief that includes: KRYSTELLE DAVID 1  MILLER–UREY EXPERIMENT TISSUES, ORGANS, & SYSTEM S Experiments have been - Cells that work together to perform a specific performed in which the function form a tissue. simple compounds of the - Just as cells that work together form a tissue, early atmosphere were tissues that work together form an organ. allowed to react under the - Organs that work together to perform a function varied sets of conditions form a system. Example: circulatory system. that might have been - Plant cells also form tissues, such as the bark of a present on the early Earth. tree. And plant cells work together, forming organs, such as roots and leaves. PROKARYOTIC OR EUKARYOTIC MAJOR CLASSES OF BIOMOLECULES PROKARYOTES Prokaryotes include bacteria & lack a nucleus or membranebound structures called organelles. Prokaryotic cells possess: -cell wall -plasma membrane -genetic material in the nucleoid -cytoplasm -ribosomes -no membrane-bound organelle BIOMOLECULES ARE COMPLEX, BUT ARE MADE UP OF SIMPLER COMPONENTS Prokaryotes are molecules surrounded by a membrane and cell wall. Prokaryotic cells lack characteristic eukaryotic subcellular membrane enclosed "organelles," but may contain membrane systems inside a cell wall. Example Bacteria Archaea EUKARYOTES Eukaryotes include most other cells & have a nucleus and membrane-bound organelles (plants, fungi, & animals. -possess a membrane-bound CELL BASIC OF LIFE nucleus - Cell = smallest unit of life -more complex than prokaryotic cells Cell Function: -five to ten times larger than prokaryotes (diameter) -compartmentalize many cellular functions within Cell work together to perform basic life processes organelles and the endomembrane system. that keep organisms alive Getting rid of body wastes Eukaryotic cells are a type of cell more complex. Making new cells for growth and repair Eukaryotic organisms also have other specialized, Releasing energy from food membrane-bounded structures, called organelles, KRYSTELLE DAVID 2 which are small structures within cells that perform - often called the “powerhouses” or “energy dedicated functions. factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the Example Protists Fungi Plants Animals cell’s main energycarrying molecule - Mitochondria are oval-shaped, double- TWO MAIN TYPES OF EUKARYOTIC CELL membrane organelles. - Animal Cell Endoplasmic Reticulum (ER) - Plant Cell Rough ER: contain ribosome, site of protein EUKARYOTIC STRUCTURE synthesis & modification of protein structure after synthesis Plasma Membrane Smooth ER: involved with lipid synthesis - made up of a phospholipid bilayer with embedded proteins that separates the internal Golgi apparatus contents of the cell from its surrounding environment. - Stacks of flattened sacs - the plasma membrane regulates the passage of - Have a shipping side & a receiving side some substances, such as organic molecules, - Receive & modify proteins made by ER ions, and water, preventing the passage of some - Transport vesicles with modified proteins to maintain internal conditions, while actively pinch off the end bringing in or removing others. Other compounds move passively across the membrane. Lysosomes The Cytoplasm - Contain digestive enzymes - Break down food and worn-out cell parts - comprises the contents of a cell between the - Programmed for cell death (lyse & release plasma membrane and the nuclear envelope. enzymes to break down & recycle cell parts) - consists of 70 to 80 percent water, it has a semi- solid consistency, which comes from the proteins Vacuoles within it. However, proteins are not the only - Responsible for food digestion, osmotic organic molecules found in the cytoplasm. regulation & waste product storage Ribosomes Glucose and other simple sugars, - the cellular structures responsible for protein polysaccharides, amino acids, nucleic acids, synthesis fatty acids, and derivatives of glycerol are found - attached to either the cytoplasmic side of the there too. Ions of sodium, potassium, calcium, plasma membrane or the cytoplasmic side of the and many other elements are also dissolved in endoplasmic reticulum. the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm. THE STRUCTURE AND PROPERTIES OF WATER Nucleus WATER - Surrounded by nuclear envelope - Has nucleolus, and rich in RNA RNA synthesized It is the most abundant substance in living on a DNA template in nucleolus and transport to systems, making up 70% or more of the weight of cytoplasm most organisms. - Near nuclear membrane has chromatin, It is essential constituent of all forms of life aggregate of DNA and proteins. The principal component of most cells in which it is a medium in which all cellular events occur. Mitochondria It has ability to solvate a wide range of organic and inorganic molecules. KRYSTELLE DAVID 3  STRUCTURE OF WATER water and accounts for its relatively high viscosity, surface tension, and boiling point. Water, H2O, is a simple molecule consisting of three atoms. SOME BIOLOGICALLY IMPORTANT H-BONDS Water is a bent molecule with a bond angle of 104.3° Molecular geometry: bent -to minimize repulsion The oxygen atom forms a covalent bond with each of the hydrogen atoms. WATER MOLECULES FORM HYDROGEN BONDS A partially unshielded hydrogen nucleus covalently bound to an electron withdrawing oxygen or nitrogen atom can interact with an unshared electron pair on another oxygen or nitrogen atom to form a hydrogen bond. A hydrogen bond is the electromagnetic attraction PROPERTIES OF WATER created between a partially positively charged hydrogen atom attached to a highly electronegative atom and WATER IS POLAR MOLECULE another nearby electronegative atom. electronegative atom is usually fluorine, oxygen, or nitrogen. Water molecules are polar, with partial positive charges on the hydrogens, a partial negative charge on the oxygen, and a bent overall structure. This is because oxygen is more electronegative, meaning that it is better than hydrogen at attracting electrons. On average, each molecule in liquid water associates through hydrogen bonds with 3.5 others WATER IS AN EXCELLENT SOLVENT Due to its polarity, water is a good solvent Solute: what is being dissolved Solution: the solute in the solvent Water is a poor solvent for nonpolar substances: –nonpolar gases –aromatic rings –aliphatic chains Water has the unique ability to dissolve many polar and Notice that water can serve simultaneously both as a ionic substances. hydrogen donor and as a hydrogen acceptor. Hydrogen Hydrophilic ◦ Water-loving molecules, easily dissolves in bonding profoundly influences the physical properties of water such as polar compounds and ionic KRYSTELLE DAVID 4 Hydrophobic ◦ Water-hating molecules such as non- which is more extensive in water than in polar molecules ◦ Not strong electrostatic attraction ammonia. between intermolecules. It takes much more heat to disrupt the attractions between water molecules than those Amphiphatic ◦ both have a polar and non-polar portion between ammonia molecules molecules ◦ oils WATER HAS COHESIVE AND ADHESIVE PROPERTIES Cohesion is when water molecules stick to each other. Adhesion is when water molecules stick to some other type of substance like plant cell walls. COHESION WATER HAS HIGH HEAT CAPACITY Water molecules have strong cohesive forces due to their ability to form hydrogen bonds with one another. Heat capacity is the amount of heat required to Cohesive forces are responsible for surface tension. raise 1 gram of water by 1 degree Celsius (1 calorie = 4.184 J) Surface Tension It takes a lot of energy to raise the temperature of a certain amount of water by a degree, so water the tendency of a liquid’s surface to resist helps with regulating temperature in the rupture when placed under tension or stress environment. All liquids have a surface tension, but water’s It means water can absorb large quantities of heat surface tension is higher than most. without much change in its own temperature. this The surface tension of water tends to hold a property allows the temperature of water in a pond drop of liquid in a spherical shape to stay relatively constant from day to night, Water molecules at the surface of the liquid regardless of the changing atmospheric experience an unbalanced attraction. As a temperature result, water molecules at the surface tend to be drawn inward. WATER HAS HIGH HEAT OF VAPORIZATION ADHESION large amount of heat is needed to evaporate water because hydrogen bonds must be broken The attraction of molecules of one kind for molecules to change water from liquid to gaseous state. of a different kind. Water is converted from its liquid form to steam when the heat of vaporization is reached Water allows it to stick to substances other Humans (and other animals that sweat) use than itself water’s high heat of vaporization to cool off. Water will make hydrogen bonds with other surfaces such as glass, soil, plant tissues, and WATER HAS HIGH BOILING POINT cotton water adheres to the wall of the vessels, so it Molecular compounds of low molecular mass can travel upward in plants are usually gases or liquids with low boiling adhesion enables water to “climb” upwards points at normal atmospheric pressure. through thin glass tubes (called capillary tubes) Ammonia (NH3) has a molar mass of 17.0 g/mol placed in a beaker of water. and boils at about –33˚C. Water has a molar mass of 18.0 g/mol, but it has a boiling point of 100˚C. The difference between the boiling points of ammonia and water is due to hydrogen bonding, KRYSTELLE DAVID 5  WATER IS LESS DENSE AS A SOLID THAN AS A THE PH SCALE LIQUID An acid is any As water freezes, the molecules form a crystalline substance that structure that spaces the molecules further apart than increases the H+ in liquid water. Most substances contract when they concentration of a freeze, but the opposite is true of water. solution A base is any substance that reduces the H+ concentration of a solution Ice has a lower density than liquid water because the fully hydrogen bonded array in an ice crystal is less densely packed than that in liquid water. Thus, ice cubes and icebergs float. ACIDS, BASES AND PH ACIDIC AND BASIC CONDITIONS AFFECT LIVING Acidic ORGANISMS solutions have A bonded hydrogen atom within a water pH values less molecule can shift between two water than 7 molecules (from one molecule to the other). Basic solutions have pH values greater than 7 Most biological fluids have pH values in the range of 6 to 8 BUFFERS The physiological pH of most living cells must remain close to pH 7 Buffers are substances that RESIST changes in concentrations of H+ and OH– in a solution, therefore they RESIST a change in pH Concentrations of H+ and OH– are equal in pure Most buffers consist of an acid-base pair that water reversibly combines with H+ Adding acids and bases, modifies the concentrations of H+ and OH– Biologists use something called the pH scale to AMINO ACIDS describe whether a solution is acidic or basic (the opposite of acidic; also known as alkaline). KRYSTELLE DAVID 6  AMINO ACIDS 3. acidic-polar, when the side chain contains a carboxylic acid, and The building blocks of proteins 4. Basic-polar when the side chain contains an amino group, is also introduced. This classification system is again based on the structure of the side chain. Their chemical structure influences three- dimensional structure of proteins. Proteins are composed of 20 different amino acid They are important intermediates in metabolism They can have hormonal and catalytic function. Several genetic disorders are cause in amino acid metabolism errors BASIC STRUCTURE The basic structure of amino acids differ only in the structure of the or the side chain (R-group). Humans are able to synthesize about half of the 20 amino acids found in proteins. These are known as the nonessential amino acids because they do not have to be supplied by our diet. L-isomer is normally found in proteins. Nonessential amino acids include: alanine, Proteins are made up of amino acids *arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, Amino acids are strung together in chains serine, and tyrosine *arginine essential in (PEPTIDES) to make proteins, which then fold on children not in adult themselves and then fold again into whatever structure or enzyme they’re supposed to be. The remaining amino acids, the essential amino acids, are synthesized only by plants and CLASSIFICATION OF AMINO ACIDS microorganisms. It cannot be synthesized by the body and thus must be obtained in the diet. Identifying amino acids as: Essential amino acids include: *arginine (essential in children), Histidine, isoleucine, 1. polar 2. non-polar KRYSTELLE DAVID 7  leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. eggs, dairy, meat, poultry, and fish) and one vegetable protein (soy) contain all of the essential amino acids in the necessary proportions. Dietary deficiencies of the essential amino acids can lead to a number of health problems AMINO ACIDS AS ZWITTERIONS ZWITTERIONS a molecule that contains an equal number of positively and negatively-charged functional groups ZWITTERIONS IN ACIDIC SOLUTIONS has an equal number of —NH + and COO– groups 3 § forms when the H from —COOH in an amino In solutions that are more acidic than the pI, acid transfers to the —NH2 the COO– in the zwitterion accepts a proton the amino acid has a positive charge Glycine, with a pI of 6.0, has a 1+ charge in solutions that have a pH below pH 6.0 ZWITTERIONS IN BASIC SOLUTIONS In solutions that are more basic than the pI, the NH + in the zwitterion loses a proton the amino acid has a negative charge Glycine, with a pI of 6.0, has a 1– charge in solutions ISOELECTRIC POINT (PI) that have a pH above pH 6.0 are the pH at which zwitterions have an overall zero charge of nonpolar and polar (neutral) amino acids exist at pH values from 5.1 to 6.3 KRYSTELLE DAVID 8  PEPTIDE BONDS PI, PH, AND CHARGE Amino acids are linked together by peptide bonds. It is a condensation reaction wherein there is an elimination of water molecule for each bond formed the -OH of the Carboxyl Group (-COOH) of one amino acid and the –H of the Amino Group (-NH2) SUMMARY OF PH, PI, AND IONIZATION of the other amino acid is linked together These charged groups are critical for the formation of peptide bonds during protein synthesis. Understanding the zwitterion form helps elucidate how amino acids link together to form polypeptide chains and ultimately folded proteins with specific functions Amino acids present in peptides, are called aminoacyl residues. To name a peptide: replace the ate or ine suffixes of free amino acids with - yl (eg, alanyl, aspartyl, tyrosyl) The carboxy-terminal (C-Terminal )residue who is NOT involved in a peptide bond will have a ine ending example, Ala-Cys-Val is called Alanyl-Cysteinyl- Valine Three-letter abbreviations are linked by straight lines. Lines are omitted when using single-letter abbreviations. It is the universal custom to write polypeptide chains with the N-terminal residue on the left to C-terminal residue on the right. KRYSTELLE DAVID 9  FUNCTIONS OF PROTEIN STRUCTURE The main structural material for plants is cellulose. For animals, it is structural proteins, which are the chief constituents of skin, bones, hair, and nails. Two important structural proteins are collagen and keratin. CATALYST Virtually all the reactions that take place in living organisms are catalyzed by proteins called enzymes. Without enzymes, the reactions would take place too slowly to be useful. MOVEMENT Every time we crook a finger, climb stairs, or blink an The shortest chains are often simply called peptides, eye, we use our muscles. Muscle expansion and longer ones are polypeptides and still longer ones are contraction are involved in every movement we make. proteins. Muscles are made up of proteins called myosin and actin. Prefixes like di-, tri- or octa- denote peptides with two, three or eight residues TRANSPORT Transport proteins have many functions. For example, hemoglobin, a protein in the blood, carries oxygen from the lungs to the cells in which it is used and carbon dioxide from the cells to the lungs. Other proteins transport molecules across cell membranes. Note: It is important to realize that glycine and alanine could also be linked the other way. They are different HORMONES compounds in all respects, with different properties. Many hormones are proteins, including insulin, Order of amino acids in a peptide or protein is critical to erythropoietin, and human growth hormone. both the structure and function PROTECTION PROTEINS When a protein from an outside source or some other foreign substance (called an antigen) enters the body, PROTEINS the body makes its own proteins (called antibodies) to “protein” is derived from the Greek “proteios” - of first counteract the foreign molecule. This antibody importance production is one of the mechanisms that the body uses to fight disease. Blood clotting is another protective Protein: an energy-yielding nutrient composed of function carried out by a protein, fibrinogen. Without carbon, hydrogen, oxygen, and nitrogen. blood clotting, we would bleed to death from any small wound. Differs from carbohydrates and fats because of the presence of nitrogen. STORAGE The body has at least 30,000 types of protein, each with a different job. Some proteins store materials in the way that starch and glycogen store energy. For example, casein in milk and The building blocks of all protein molecules are amino acids. KRYSTELLE DAVID 10 ovalbumin in eggs store nutrients for newborn mammals SECONDARY STRUCTURE and birds. Ferritin, a protein in the liver, stores iron. Repetitive conformations of the protein REGULATION PROTEINS backbone two most common secondary structures can control the expression of genes, regulating the kind encountered in proteins are the alpha-helix and of proteins synthesized in a particular cell, and the beta-pleated sheet controlling when such manufacture takes place. CLASSIFICATION OF PROTEINS 1. Fibrous proteins which are insoluble in water and are used mainly for structural purposes. Generally composed of long and narrow strands. 2. Globular proteins which are more soluble in water and are used mainly for nonstructural purposes. Generally, have a more compact and rounded shape and have functional roles LEVELS OF STRUCTURE OF PROTEINS Both of these secondary protein structures are stabilized by hydrogen bonding between the carbonyl oxygen atoms and the nitrogen atoms of amino acids in the protein chain Twisting about various bonds in the polypeptide backbone gives proteins a variety of shapes. ALPHA HELIX Bond angles give rise to secondary structures. Then, localized secondary structures help drive a single protein the peptide folding that gives rise to tertiary chain twists in such structure a manner that its shape resembles a PRIMARY STRUCTURE right-handed coiled spring—that is, a consists of the sequence of helix. amino acids that makes up All the amino acid the chain side chains point the particular sequence of outward from the helix. amino acids on the chain The shape of the helix is maintained by enables the whole chain to numerous intramolecular hydrogen bonds that fold and curl in such a way exist between the backbone C=O and -NH as to assume its final shape groups. KRYSTELLE DAVID 11  There is a hydrogen bond between the C=O Keratin, a fibrous protein of hair, fingernails, oxygen atom of each peptide bond and the -NH horns, and wool, is one protein that does have hydrogen atom of another peptide bond four a predominantly a-helix structure amino acid residues Silk is made of fibroin, another fibrous protein, farther along the which exists mainly in the b-pleated sheet chain. form. These hydrogen bonds are in just the TERTIARY STRUCTURE right position to The overall 3-D conformation of a polypeptide chain, cause the molecule including the interactions of the side chains and the to maintain a helical position of every atom in the polypeptide. shape. Each –NH points upward and each – C=O points downward, roughly BETA PLEATED SHEET the orderly alignment of protein chains is maintained by intermolecular or intramolecular hydrogen bonds. TERTIARY STRUCTURES ARE STABILIZED FIVE The Beta-sheet structure can occur between WAYS molecules when polypeptide chains run parallel (all N-terminal ends on one side) or antiparallel 1. COVALENT BOND. The covalent bond most often (neighboring Nterminal ends on opposite sides). involved in stabilization of the tertiary structure of B-Pleated sheets can also occur proteins is the disulfide bond intramolecularly, when the polypeptide chain 2. HYDROGEN BOND. Tertiary structures are makes a U-turn, forming a hairpin structure, and stabilized by hydrogen bonding between polar the pleated sheet is antiparallel. groups on side chains or between side chains and the peptide backbone. 3. SALT BRIDGES, also called electrostatic attractions, occur between two amino acids with ionized side chains—that is, between an acidic amino acid (-COO-) and a basic amino acid (-NH3+) side chain. The two are held together by simple ion–ion attraction. 4. HYDROPHOBIC INTERACTIONS In aqueous solution, globular proteins usually turn their polar groups outward, toward the aqueous solvent, and their nonpolar groups inward, away from the water molecules. The nonpolar groups prefer to interact with each other, excluding water from these regions. Although this type of interaction is weaker than hydrogen bonding or salt bridges, it usually acts over large surface areas, so that the interactions are collectively strong enough to stabilize a loop or some other tertiary structure formation. 5. METAL ION COORDINATION Two side chains with the same charge would normally repel each other, but they can also be linked via a metal ion. For KRYSTELLE DAVID 12  example, two glutamic acid side chains (-COO-) HEME GROUP would both be attracted to a magnesium ion (Mg2+), forming a bridge. This is one reason the A prosthetic group consists of a metal ion, human body requires certain trace minerals—they Fe(II), and an organic part, protoporphyrin IX are necessary components of protein. Binding site for Oxygen PROTEIN QUATERNARY STRUCTURE The spatial CHAPERONS relationship and interactions Proteins that help between subunits other proteins to fold in a protein that into the biologically has more than one active conformation polypeptide chain and enable partially The subunits are denatured proteins packed and held to regain their together by biologically active hydrogen bonds, conformation salt bridges, and hydrophobic MYOGLOBIN: AN EXAMPLE OF PROTEIN interactions— the STRUCTURE same forces that Myoglobin was the operate within first protein for tertiary structures which the complete tertiary structure HEMOGLOBIN was determined by A tetramer, consisting of four polypeptide X-ray chains, two α -chains, and two β-chains crystallography the terms α and β have nothing to do with the a- The complete helix and the bpleated sheet; rather they just myoglobin molecule refer to two different polypeptide chain subunits consists of a single The overall structure of hemoglobin is α2β2 in polypeptide chain Greek-letter notation of 153 amino acid The a-chain is 141 residues long, and the b-chain residues and includes a prosthetic group, the is 146 residues long; for comparison, the heme group. myoglobin chain is 153 residues long Myoglobin has eight a-helical regions and no b- pleated sheet regions KRYSTELLE DAVID 13  a globular protein unfolds, precipitation or coagulation takes place the tertiary structure is disrupted and the protein is no longer biologically active Myoglobin has the function of oxygen storage in heart and skeletal muscle One Oxygen molecule in one myoglobin Hemoglobin is oxygen transport, It must be able both to bind strongly to oxygen and to release oxygen easily, depending on conditions. Four molecule of Oxygen bind to one hemoglobin PROTEION DENATURATION Denaturation of a protein occurs when the interactions of residues that stabilize secondary, tertiary or quaternary structures are disrupted, which destroys the shape and renders the protein biologically inactive. Does not affect the peptide bonds between amino acids. DENATURATION OF PROTEINS, HEAT Proteins are denatured when heated above 50 °C. The heat disrupts the hydrogen bonds and hydrophobic interactions between nonpolar residues. does not change the nutritional value of proteins but makes them more digestible. High temperatures are also used to disinfect surgical instruments and gowns by denaturing The loss of secondary and tertiary structures in a protein the proteins of any bacteria present occurs when conditions change, such as increasing the temperature. DENATURATION OF PROTEINS: DETERGENTS making the pH very acidic or basic. Detergents such as sodium dodecyl sulphate denature adding certain organic compounds or heavy protein by associating with Non-Polar groups(-R,NP) of metal ions. proteins thus interfering normal interactions. adding mechanical agitation. When the interactions between the residues are disrupted, KRYSTELLE DAVID 14  DENATURATION OF PROTEINS, REDUCING AGENT Reducing agents such as 2-mercaptoethanol can break the –S-S- disulfide bonds, reducing them to -SH groups. The process of permanent waving and straightening of curly hair are example. The protein keratin, which makes up human hair, contains a high percentage of disulfide bonds. DENATURATION OF PROTEINS, ACIDS AND BASES DENATURATION OF PROTEINS, AGITATION Proteins can be denatured by changing the pH, which The whipping of cream and the beating of egg whites are examples of using mechanical agitation to denature breaks hydrogen bonds. proteins. The whipping action stretches the polypeptide disrupts ionic bonds and salt bridges. chains until the stabilizing interactions are disrupted Tannic acid, a weak acid used in burn ointments, is applied to the site of the burn to coagulate DENATURATION OF PROTEINS, SUMMARY proteins. It forms a protective cover and prevents further loss of fluid from the burn. DENATURATION OF PROTEINS, ORGANIC COMPOUNDS Organic compounds such as ethanol and isopropyl alcohol act as disinfectants by exchanging the bacterial protein’s hydrogen bonds to water with their own. disrupting the side chain intramolecular hydrogen bonding. PROTEIN DENATURATION CAN BE REVERSIBLE OR An alcohol swab is used to clean wounds or to IRREVERSIBLE prepare the skin for an injection because the alcohol passes through the cell walls and coagulates the proteins inside the bacteria DENATURATION OF PROTEINS, HEAVY METAL IONS Heavy metal ions such as Ag+, Pb2+, and Hg2+ denature proteins by forming bonds with ionic residues or reacting with –SH groups They form salt bridges, as in -S-Hg2+-S- This very feature is taken advantage of in the antidote for oral heavy metal poisoning: raw egg whites and milk. PROTEIN STRUCTURE DI SEASES The egg and milk proteins are denatured by the metal SICKLE CELL ANMEMIA ions, forming insoluble precipitates in the stomach. These must be pumped out or removed by inducing Normal adult human hemoglobin has two alpha chains vomiting. In this way, the poisonous metal ions are and two beta chains in sickle cell, the hemogblobin is removed from the body. callled hbs. differs from the normal type only in the beta chains and only in one position on these two KRYSTELLE DAVID 15  chains: the glutamic acid in the sixth position of normal Hb is replaced with a valine residue in Hb This change affects only two positions in a molecule containing 574 amino acid residues, yet it is enough to produce a very serious disease Red blood cells carrying HbS behave normally when there is an ample oxygen supply. When the oxygen pressure decreases, however, the red blood cells become sickle- shaped. This malformation occurs in the capillaries. As a result of this change in shape, the cells may clog the capillaries. The body’s defenses destroy the clogging cells, and the loss of the blood cells causes anemia. This change at only a single position of a chain consisting of 146 amino acids is severe enough to cause a high death rate. There is no known cure for sickle cell anemia but can be controlled by treatment of Hydroxyurea. PRION PROTEINS Cause by a conformational change of protein forming beta-sheet amyloid prions are small proteins found in nerve tissue, although their exact function remains a mystery. When prions undergo conformational change, they can cause diseases such as mad cow disease and scrapie in sheep During the conformational change, the a-helical content of the normal prion protein unfolds and reassembles in the b-sheet conformation. It causes spongiform encephalitis, a b-Amyloid plaques that is also linked to Alzheimer’s Disease. KRYSTELLE DAVID 16