Chapter 20: Proteins

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Chapter 20 Proteins

• CHEM 1152
• Dr. Clower

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Outline

• Functions of Proteins
• Amino Acids
• Amino Acids as Acids and Bases
• Formation of Peptides
• Protein Structure Primary, Secondary, Tertiary
  and Quaternary Structure
• Protein Hydrolysis and Denaturation

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Functions of Proteins

• Proteins perform many different functions

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Example

• Classify each of the following proteins according
  to its function
• A. Insulin, a hormone needed for glucose
  utilization
• B. Antibodies, proteins that disable foreign
  proteins
• C. Casein, milk protein
• D. Lipases that hydrolyze lipids

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Amino Acids

• Amino acids
• The building blocks of proteins.
• Two functional groups
• carboxylic acid group
• amino group on the alpha (?) carbon.
• Have different side groups (R) that give each
  amino acid unique characteristics.
• R side chain
• H2N C COOH General structure of an
• ?-amino acid
• H

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Nonpolar Amino Acids

• Amino acids are classified as nonpolar when the R
  groups are H, alkyl, or aromatic
• Note three-letter abbreviations

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Polar Neutral Amino Acids

• Amino acids are classified as polar neutral when
  the R groups are alcohols, thiols, or amides

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Acidic and Basic Amino Acids

• Amino acids are classified as acidic when the R
  group is a carboxylic acid
• Amino acids are classified as basic when the R
  group is an amine

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Essential Amino Acids

• Essential amino acids
• Ten amino acids that are not synthesized by the
  body
• Must be obtained from the diet

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Essential Amino Acids

• Complete proteins contain all 10 essential amino
  acids (animal products)
• Incomplete proteins are deficient in one or more
  (plant proteins)
• Obtained by combining two or more vegetables that
  provide complementary proteins

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Learning Check

• Classify each amino acid as polar or nonpolar
• A. Glycine NH2CH2COOH
• CH3
• CHOH
• B. Threonine NH2CHCOOH

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Learning Check

• Classify the following amino acids as hydrophobic
  (nonpolar), hydrophilic (polar, neutral), acidic,
  or basic
• A. Lysine R CH2CH2CH2CH2NH2
• B. Aspartic acid R CH2CO2H
• C. Leucine R CH2CH(CH3)2
• D. Serine R CH2OH

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Learning Check

• Give the name of the amino acid represented by
  each of the following three-letter abbreviations
• A. Trp
• B. Met
• C. Pro
• D. Gly

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Zwitterions

• Both the NH2 and the COOH groups in an amino
  acid undergo ionization in water
• A zwitterion forms that has and charge
• At the isoelectric point (pI), the and
  charges in the zwitterion are equal
• NH2CH2COOH H3NCH2COO
• Glycine Zwitterion of
  glycine
• Melting points of amino acids very high

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Amino Acids as Acids

• In solutions more basic (higher pH) than the pI,
  the NH3 in the amino acid loses a proton to
  become NH2
• OH-
• H3NCH2COO H2NCH2COO
• Zwitterion Negative ion
• at pI Higher pH

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Amino Acids as Bases

• In solution more acidic (lower pH) than the pI,
  the COO- in the amino acid gains a proton to
  become COOH
• H
• H3NCH2COO H3NCH2COOH
• Zwitterion Positive ion
• at pI Low pH

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pH and Ionization

• Alanine

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pH and Ionization

• Acidic amino acids such as aspartic acid have a
  second carboxyl group that can donate and accept
  protons
• Ex the pI for aspartic acid occurs at a pH of 2.8

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Electrophoresis

• Laboratory method
• Separates amino acids according to their
  isoelectric points
• The positively charged amino acids move towards
  the negative electrode (cathode)
• The negatively charged amino acids move toward
  the positive electrode (anode)
• An amino acid at its pI (neutral) will not
  migrate in either direction
• When electrophoresis is completed, the amino
  acids are identified as separate bands on the
  filter paper or thin layer plate

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Learning Check

• CH3 CH3
• H3NCHCOOH H2NCH2COO
• (1) (2)
• Which structure represents
• A. Alanine at a pH above its pI?
• B. Alanine at a pH below its pI?

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Learning Check

• Write the zwitterion of each of the following
  amino acids
• A. Phenylalanine B. Methionine

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Learning Check

• Would the following ions of serine exist at a pH
  above, below, or at pI?

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Review

• Carboxylic acid amine ?
• Structure of amino acid

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The Peptide Bond

• A peptide bond is an amide bond between the
  carboxyl group of one amino acid and the amino
  group of the next amino acid
• O CH3
  O
• 
  
• H3NCH2CO H3NCHCO
• O H CH3 O
• 
  
• H3NCH2CNCHCO H2O
• peptide bond

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Peptides

• Dipeptide two amino acids tri 3 poly many
• Peptide is named from the N-terminal end (NH3)
• Use -yl endings for the names of the amino acids
• The C-terminal amino acid (COO-) uses its amino
  acid name

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Learning check

• Write the names and three-letter abbreviations
  of the amino acids in the tripeptides that could
  form from two glycine and one alanine.

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Learning Check

• Write the name of the following tetrapeptide
  using amino acid names and three-letter
  abbreviations.

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Learning Check

• Draw the structural formula of each of the
  following peptides.
• A. Methionylaspartic acid
• B. Alanyltryptophan

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Primary Structure

• A polypeptide containing 50 or more amino acids
  is called a protein
• The primary structure of a protein is the
  sequence of amino acids in the peptide chain

Ala-Leu-Cys-Met
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Insulin

• Insulin
• The first protein to have its primary structure
  determined
• Human insulin has a primary structure that is
  similar to insulin of pigs and cows

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Secondary Structure

• The secondary structure of a protein indicates
  the arrangement of the polypeptide chains in
  space.
• Alpha helix
• Beta-pleated sheet

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Alpha Helix

• The a-helix is a three-dimensional arrangement of
  the polypeptide chain that gives a corkscrew
  shape like a coiled telephone cord
• The coiled shape of the alpha helix is held in
  place by hydrogen bonds between the amide groups
  and the carbonyl groups of the amino acids along
  the chain

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Beta-Pleated Sheet

• The b-pleated sheet
• Holds proteins in a parallel arrangement with
  hydrogen bonds
• Has R groups that extend above and below the
  sheet
• Is typical of fibrous proteins such as silk

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Learning check

• Indicate the type of structure as
• 1) primary 2) a-helix 3) b-pleated sheet
• A. Polypeptide chains held side by side by H
  bonds.
• B. Sequence of amino acids in a polypeptide
  chain.
• C. Corkscrew shape with H bonds between amino
  acids.

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Levels of Structure

• So far
• Primary
• Secondary
• Next
• Tertiary

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Tertiary Structure

• The tertiary structure
• Gives a specific overall shape to a protein
• Involves interactions and cross links between R
  groups in different areas of the peptide chain
• Is stabilized by
• Hydrophobic and hydrophilic interactions
• Salt bridges (electrostatic interactions)
• Hydrogen bonds
• Disulfide bonds

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Tertiary Structure

• Disulfide Bonds

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Tertiary Structure

• The interactions of the R groups give a protein
  its specific three-dimensional tertiary structure

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Globular Proteins

• Globular proteins
• Have compact, spherical shapes
• Carry out synthesis, transport and metabolism in
  cells
• Ex myoglobin
• 153 amino acids
• 75 helical
• stores and transports oxygen in muscle

Myoglobin
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Fibrous Proteins

• Fibrous proteins
• Long, fiber-like shapes
• Typically structural
• Ex a-keratins
• hair, wool, skin, and nails
• 3 a-helices held together by disulfide bonds
• Ex b-keratins
• Feathers, scales
• large amounts of beta-pleated sheet structure

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Fibrous Proteins

• Ex Collagen
• Connective tissue, skin, tendons, and cartilage
• A triple helix consisting of three alpha helix
  chains
• Contains large amounts glycine, proline,
  hydroxyproline and hydroxylysine that contain
  OH groups for hydrogen bonding

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Learning check

• Select the type of tertiary interaction as
• 1) disulfide 2) ionic 3) H-bonds 4)
  hydrophobic
• A. Leucine and valine
• B. Cysteine and cysteine
• C. Aspartic acid and lysine
• D. Serine and threonine

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Quaternary Structure

• The quaternary structure contains two or more
  tertiary subunits (protein chains)
• Held together by same interactions as tertiary
  structure
• Hemoglobin contains four chains
• The heme group in each subunit picks up oxygen
  for transport in the blood to the tissues

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Summary of Structural Levels

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Learning check

• Identify the level of protein structure
• A. Beta-pleated sheet
• B. Order of amino acids in a protein
• C. A protein with two or more peptide chains
• D. The shape of a globular protein
• E. Disulfide bonds between R groups

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Learning Check

• In myoglobin, about one-half of the 153 amino
  acids have nonpolar side chains.
• A. Where would you expect those amino acids to be
  located in the tertiary structure?
• B. Where would you expect the polar side chains
  to be?
• C. Why is myoglobin more soluble in water than
  silk or wool?

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Learning Check

• State whether the following statements apply to
  primary, secondary, tertiary, or quaternary
  protein structure
• A. Side groups interact to form disulfide bonds
  or ionic bonds.
• B. Peptide bonds join amino acids in a
  polypeptide chain.
• C. Hydrogen bonding between carbonyl oxygen atoms
  and nitrogen atoms of amide groups causes a
  polypeptide to coil.
• D. Hydrophobic side chains seeking a nonpolar
  environment move toward the inside of the folded
  protein.
• E. Protein chains of collagen form a triple
  helix.
• F. A protein contains four tertiary subunits.

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Hydrolysis of Amides

• Amide hydrolysis (review)
• Protein hydrolysis
• Splits the peptide bonds to give smaller peptides
  and amino acids
• Occurs in the digestion of proteins
• Occurs in cells when amino acids are needed to
  synthesize new proteins and repair tissues

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Hydrolysis of a Dipeptide

• In the lab, the hydrolysis of a peptide requires
  acid or base, water and heat
• In the body, enzymes catalyze the hydrolysis of
  proteins

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Denaturation

• Disruption of bonds in the secondary, tertiary
  and quaternary protein structures
• Covalent amide bonds are not affected
• Loss of biological activity with loss of structure

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Applications of Denaturation

• Cooking food containing protein
• Wiping the skin with alcohol (denaturation of
  bacterial proteins)
• Hair permanents

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Learning check

• The products of the complete hydrolysis of the
  peptide Ala-Ser-Val are

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Learning Check

• What structural level of a protein is affected by
  denaturation?

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Learning Check

• Indicate the changes in protein structure for
  each of the following
• A. Tannic acid is placed on a burn
• B. Milk is heated to 60C to make yogurt.
• C. To avoid spoilage, seeds are treated with a
  solution of HgCl2.
• D. Hamburger is cooked at high temperatures to
  destroy E. coli bacteria that may cause
  intestinal illness

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End of Chapter 20!