Chapter 16: Amino Acids, Proteins and Enzymes

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Chapter 16 Amino Acids, Proteins and Enzymes

• Chem 20
• El Camino College

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Proteins

• Proteins are polymers made of 20 different
  a-amino carboxylic acids.
• A single protein molecule contains hundreds or
  even thousands of amino acid units.
• An animal body has tens thousands of different
  proteins

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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.

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a Amino Acids

• An amino acid contains two functional groups
• A carboxylic group, -COOH
• An amine group NH2 bonded to the carbon a
• There are 20 different amino acids in humans

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a 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.

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Types of Amino Acids

• AAs are classified as
• nonpolar with hydrocarbon side chains
• polar with polar or ionic side chains.
• acidic with acidic side chains.
• basic with NH2 side chains.

Nonpolar Polar
Acidic
Basic
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Nonpolar Amino Acids

• An amino acid is nonpolar when the R group is
  hydrophobic.

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Polar Amino Acids

• An amino acid is polar when the R group is polar
  ( has OH, SH, -CONH- group) .

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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.

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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 found
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Amino Acids

• Draw the spatial formulas for serine (RCH2OH)
  and lysine (RCH2CH2CH2CH2NH3) as they exist in
  the body

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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.

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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.

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Peptide Formation

• The peptide bond is planar (CO and NH in a
  single plane), and the bond between C and N does
  not rotate
• .

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The Peptide Bond

• The peptide bond is planar (flat), and the bond
  between C and N does not rotate

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The Peptide Bond

• The peptide bond exists in another form, obtained
  by moving the electrons around
• Different forms are called resonance structures

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Draw the tripeptide that forms when serine,
glycine, and methionine react.
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Protein Structure

• Protein structure has 4 components
• Primary structure (1o)
• Secondary structure (2o)
• Tertiary structure (3o)
• Quaternary structure (4o)

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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.

• Ala-Leu-Cys-Met

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1o Structures

• The nonapeptides oxytocin and vasopressin have
  similar 1o structures.
• Only the amino acids at positions 3 and 8 differ.

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1o Structure of Insulin

• Insulin
• was the first protein to have its 1o structure
  determined.
• has a primary structure of 2 linked polypeptide
  chains

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2o Structures

• The three most common forms of 2o structure are
• the alpha helix
• the beta pleated sheet
• the triple helix

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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 CO
  gps
• a corkscrew shape that looks like a coiled
  telephone cord.

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2o StructureBeta 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.

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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.

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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.

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4o Structure

• The 4o structure
• combination of 2 or more protein units.
• of hemoglobin consists of 4 polypeptide chains as
  subunits.

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Summary of Protein Structure
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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.

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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.