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Chapter 19: Proteins

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Title: Chapter 19: Proteins


1
Chapter 19 Proteins
  • CHEM 1152
  • Dr. Sheppard

2
Outline
  • Amino Acids
  • Amino Acids as Acids and Bases
  • Formation of Peptides
  • Functions of Proteins
  • Protein Structure Primary, Secondary, Tertiary
    and Quaternary Structure
  • Protein Hydrolysis and Denaturation

3
19.1 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.

4
Nonpolar Amino Acids
  • Amino acids are classified as nonpolar when the R
    groups are H, alkyl, or aromatic
  • Note three-letter abbreviations

5
Polar Neutral Amino Acids
  • Amino acids are classified as polar neutral when
    the R groups are alcohols, thiols, or amides

6
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

7
Essential Amino Acids
  • Essential amino acids
  • Ten amino acids that are not synthesized by the
    body
  • Must be obtained from the diet

8
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

9
Learning Check
  • Classify each amino acid as polar or nonpolar
  • A. Glycine NH3CH2COO-
  • CH3
  • CHOH
  • B. Threonine NH3CHCOO-

10
Learning Check
  • Classify the following amino acids as hydrophobic
    (nonpolar), hydrophilic (polar, neutral), acidic,
    or basic
  • A. Lysine R CH2CH2CH2CH2NH2
  • B. Aspartate R CH2CO2H
  • C. Leucine R CH2CH(CH3)2
  • D. Serine R CH2OH

11
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

12
19.2 Zwitterions
  • Both the NH2 and the COOH groups in an amino
    acid undergo ionization in water
  • A zwitterion forms that has and charge
  • Melting points of amino acids very high

13
Isoelectric Point
  • At the isoelectric point (pI), the and
    charges in the zwitterion are equal (no net
    charge).
  • At pH values above the isoelectric point, the
    amino acid has a net negative value.
  • At pH values below the isoelectric point, the
    amino acid has a net positive value.

14
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

15
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

16
pH and Ionization
  • Alanine

17
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

18
Learning Check
  • CH3 CH3
  • H3NCHCOOH H2NCHCOO
  • (1) (2)
  • Which structure represents
  • A. Alanine at a pH above its pI?
  • B. Alanine at a pH below its pI?

19
Learning Check
  • Would the following ions of serine exist at a pH
    above, below, or at pI?

20
Learning Check
  • Write the zwitterion of each of the following
    amino acids
  • A. Phenylalanine B. Methionine

21
19.3 Reactions of Amino Acids
  • Two reactions of interest
  • Oxidation of cysteine
  • Peptide bond formation

22
Oxidation of Cysteine
  • Cys side chain -CH2-SH
  • Remember oxidation of thiols to form disulfide
    bonds (section 13.9)?
  • R-SH R-SH R-S-S-R
  • Two Cys amino acids can react to form a disulfide
    bond (or disulfide bridge)

O
23
Disulfide Bridge/bond
24
Review
  • Carboxylic acid amine ?
  • Structure of amino acid

25
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

26
Peptides
  • Dipeptide two amino acids tri 3 poly many
  • Protein polypeptide with gt50 amino acid
    residues
  • Peptide is named from the N-terminal end (left
    NH3)
  • Use -yl endings for the names of the amino acids
  • The C-terminal amino acid (right COO-) uses its
    amino acid name

27
Peptides
28
Learning Check
  • Use the three-letter abbreviations of the amino
    acids to write structures for all possible
    tripeptides that could form from two glycine and
    one alanine. Name each tripeptide.

29
Learning Check
  • Write the name of the following tetrapeptide
    using amino acid names and three-letter
    abbreviations.

30
Learning Check
  • Draw the structural formula of each of the
    following peptides.
  • A. Methionylaspartic acid
  • B. Alanyltryptophan

31
19.5 Characteristics of Proteins
  • Size
  • Varies from 6000 to several million g/mol
  • Hemoglobin C2952H4664O832S8Fe4

32
Characteristics of Proteins
  • Function

33
Characteristics of Proteins
  • Function

34
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

35
Characteristics of Proteins
  • Classification
  • Fibrous vs. globular (based on structural shape)
  • Fibrous proteins
  • Long rods or strings fibers
  • Water-insoluble
  • Connective tissue, hair, skin
  • Globular proteins
  • Spherical
  • Water-soluble
  • Synthesis, transport and metabolism in cells

36
Protein Structure
  • Chain of amino acids folded into 3D structure
  • Levels of structure
  • Primary
  • Secondary
  • Tertiary
  • Quaternary
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