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Transcript
Chapter 16: Amino Acids, Proteins and Enzymes Chem 20 El Camino College 1 Proteins Proteins are polymers made of 20 different αamino carboxylic acids. A single protein molecule contains hundreds or even thousands of amino acid units. An animal body has tens thousands of different proteins 2 Proteins Proteins form components of the body such as muscles, hair, and nails Enzymes are proteins that act as tiny “machines” in cellular processes Hemoglobin is a protein that carries oxygen in the blood Proteins can also act as storage molecules. Some hormones are proteins. 3 α Amino Acids An amino acid contains two functional groups: A carboxylic group, -COOH An amine group –NH2 bonded to the carbon α There are 20 different amino acids in humans R H H C N OH C 4 H O α Amino Acids Under physiological conditions, the –COOH group loses H+ to give –COO- and the –NH2 group gains H+ to give –N H3 ⁺. Thus the amino acid is a dipolar ion, called Zwitterion. 5 Types of Amino Acids AAs are classified as Nonpolar • nonpolar with hydrocarbon side chains • polar with polar or ionic side chains. • acidic with acidic side chains. Acidic • basic with –NH2 side chains. Polar Basic 6 Nonpolar Amino Acids An amino acid is nonpolar when the R group is hydrophobic. 7 Polar Amino Acids An amino acid is polar when the R group is polar ( has OH, SH, -CONH- group) . 8 Acidic and Basic Amino Acids An amino acid is • acidic when the R group is a carboxylic acid. • basic when the R group is an amine. 9 Amino Acids Every amino acid, except glycine, contains at least one chiral center. For example alamine can have two isomers In proteins, only L amino acids are found10 Amino Acids Draw the spatial formulas for serine (R=CH2OH) and lysine (R=CH2CH2CH2CH2NH3+) as they exist in the body HOH2C H H H3NH2CH2CH2CH2C C N O H C H H C O N C H O H H O serine lysine 11 Amino Acids Amino acids can act as buffers by using up H+ ions or OH- ions that are added isoelectric point is the pH at which the positive and negative charges of an amino acid are equal. 12 Peptide Formation Peptide is amide formed between carboxyl group of an amino acid and amino group of the next amino acid. The bond is called peptide bond. By convention the N-terminal amino acid residue is written at the left, and the C-terminal amino acid residue at the right. 13 Peptide Formation The peptide bond is planar (C=O and NH in a single plane), and the bond between C and N does not rotate . 14 15 The Peptide Bond The peptide bond is planar (flat), and the bond between C and N does not rotate H3C H H C 120o 120o N 120o C 120o H O H O N 120o C 120o C O H H H 16 The Peptide Bond The peptide bond exists in another form, obtained by moving the electrons around Different forms are called resonance structures H H N N C : O: :O: : C 17 Draw the tripeptide that forms when serine, glycine, and methionine react. HOH2C H H C N N C H H C O H HOH2C H O H3CSH2CH2C H O C H H H O H3CSH2CH2C O N N C H O O N C H O H C C C C H H H C H H O N C H O H H H 18 Protein Structure Protein structure has 4 components Primary structure (1o) Secondary structure (2o) Tertiary structure (3o) Quaternary structure (4o) 19 Primary Structure of Proteins The primary structure of a protein is • the particular sequence of amino acids. • the backbone of a peptide chain or protein. CH3 CH3 CH CH3 O H3N CH C S CH3 CH O N H CH C N SH CH2 CH2 O CH2 O CH H C N CH C O- H 20 Ala-Leu-Cys-Met 1o Structures • The nonapeptides oxytocin and vasopressin have similar 1o structures. • Only the amino acids at positions 3 and 8 differ. 21 1o Structure of Insulin Insulin • was the first protein to have its 1o structure determined. • has a primary structure of 2 linked polypeptide chains 22 2o Structures The three most common forms of 2o structure are • the alpha helix • the beta pleated sheet • the triple helix 23 2o Structure – Alpha Helix The 2o structure of an alpha helix is • a 3-D arrangement of amino acids in a polypeptide chain • held by interactions between the –N-H gps and C=O gps • a corkscrew shape that looks like a coiled “telephone cord”. 24 2o Structure–Beta Pleated Sheet The 2o structure of a beta pleated sheet • consists of polypeptide chains arranged side by side. • has R groups above and below the sheet. • is typical of fibrous proteins such as silk. 25 2o Structure – Triple Helix The structure of a triple helix is • 3 polypeptide chains woven together. • found in collagen, connective tissue, skin, tendons, and cartilage. 26 3o Structure The 3o structure • the globular structure of a protein. • alpha-helices and beta-sheets wrap around each other to form a 3-D shape. 27 4o Structure The 4o structure • combination of 2 or more protein units. • of hemoglobin consists of 4 polypeptide chains as subunits. 28 Summary of Protein Structure 29 Denaturation Involves: • disruption of 2o, 3o and 4o protein structures. • heat and organic cmpds that break apart interactions between chains. • acids and bases that break interactions between polar R groups and disrupt ionic bonds. • heavy metal ions that disrupt bonds between chains • agitation that stretches peptide chains interactions break. 30 Applications of Denaturation Denaturation of protein occurs when an egg is cooked the skin is wiped with alcohol heat is used to cauterize blood vessels instruments are sterilized in autoclaves. 31