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Chapter 11. Protein Structure and Function

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Title: Chapter 11. Protein Structure and Function


1
Chapter 11. Protein Structure and Function
  • These are biopolymers that are constructed from a
    limited set of amino acids.
  • They are the most plentiful organic substances in
    the cell.
  • About half of the dry mass of a cell is composed
    of proteins.
  • They serve a wide range of functions.

2
Protein function
  • Enzymes biological catalysts.
  • Immuno- antibodies of immune system.
  • globulins
  • Transport move materials around -
    hemoglobin for O2.
  • Regulatory hormones, control of metabolism.
  • Structural coverings and support -
  • skin, tendons, hair, bone.
  • Movement muscle, cilia, flagella.

3
Amino acids
  • All proteins are composed of amino acids.
  • Twenty common amino acids.
  • All are ?-amino acids.
  • Except for proline, primary amino- group is
    attached to the ? carbon - the carbon just after
    the acid group.

? carbon
H R-C-COOH
NH2
General Structure
4
Amino acids
  • Because both acid and base groups are present, an
    amino acid can form a /- ion.
  • H H
  • R-C-COOH R-C-COO-

  • NH2 NH3
  • The position of the equilibrium is based on pH
    and the type of amino acid. Called a zwitterion.

5
Some amino acid examples
H3C H \ HC-C-COO-
/ H3C NH3
H CH3-C-COO-
NH3
valine
alanine
H
CH3
-S-CH2-CH2-C-COO-
NH3
methionine
tryptophan
6
Some amino acid examples
H
HO-CH2-C-COO-
NH3
H H-C-COO-
NH3
serine
glycine
O H
H2N-C-CH2-C-COO-
NH3
O H
-O-C-CH2-CH2-C-COO-

NH3
asparagine
glutamic acid
7
Abbreviations
  • glycine Gly G
  • alanine Ala A
  • valine Val V
  • leucine Leu L
  • isoleucine Ile I
  • methionine Met M
  • phenylalanine Phe F
  • tryptophan Trp W
  • proline
  • Pro
  • P

8
Groups of Amino Acids
  • Hydrophobic
  • Polar, neutral
  • Negatively charged
  • positively charged

9
Primary protein structure
  • Proteins are polymers made up of amino acids.
  • Peptide bond - how the amino acids are
  • linked together to make
  • a protein.

H H2NCCOOH
R
H H2NCCOOH
R
H O H2N -
C - C - R
H N - C - COOH H
R

H2O
10
Four levels of protein structure
  • Primary structure
  • The sequence of amino acids in a protein.
  • Secondary structure
  • Way that chains of amino acids are coiled or
    folded - (?-helix, ?-sheet, random coil).
  • Tertiary structure
  • Way ?-helix, ?-sheet, random coils fold and coil.
  • Quaternary structure
  • Way that two or more peptide chains pack
    together.

11
Primary structure
  • All proteins have the same covalent backbone.
  • Part of a protein.

12
Secondary structure
  • Long chains of amino acids commonly fold or curl
    into a regular repeating structure.
  • Structure is a result of hydrogen bonding between
    amino acids within the protein.
  • Common secondary structures are
  • ? - helix
  • ? - pleated sheet
  • Secondary structure adds new properties to a
    protein like strength, flexibility, ...

13
?-helix
One common type of secondary structure. Propertie
s of an ?-helix include strength and
low solubility in water. Originally proposed
by Pauling and Corey in 1951.
14
a-helix
15
?-helix
Every amide hydrogen and carbonyl oxygen is
involved in a hydrogen bond.
16
Collagen
  • Family of related proteins.
  • About one third of all protein in humans.
  • Structural protein
  • Provides strength to bones, tendon, skin, blood
    vessels.
  • Forms triple helix - tropocollagen.

17
Tropocollagen
18
?-Pleated sheets
  • Another secondary structure for protein.
  • Held together by hydrogen bonding between
    adjacent sheets of protein.

19
?-Pleated sheets
  • Silk fibroin - main protein of silk is an example
    of a ? pleated sheet structure.

Composed primarily of glycine and alanine. Stack
like corrugated cardboard for extra strength.
20
Beta sheet
21
Tertiary structure of proteins
  • Fibrous proteins
  • insoluble in water
  • form used by connective tissues
  • silk, collagen, ?-keratins
  • Globular proteins
  • soluble in water
  • form used by cell proteins
  • 3-D structure - tertiary

22
Tertiary structure of proteins
  • Results from interaction of side chains.
  • The protein folds into a tertiary structure.
  • Possible side chain interactions
  • Similar solubilities
  • Ionic attractions
  • Electrostatic attraction between ? and ?-
    sidechains
  • Covalent bonding

23
Tertiary structureof proteins
Sulfide crosslink
Hydrophobic interaction
- S - S -
-COO- H3N-
Hydrogen bonding
Salt bridge
24
Quaternary structureof proteins
  • Many proteins are not single peptide strands.
  • They are combinations of several proteins
  • - aggregate of smaller globular proteins.
  • Conjugated protein - incorporate another type of
    group that performs a specific function.
  • prosthetic group

25
Quaternary structureof proteins
Aggregate structure This example shows
four different proteins and two
prosthetic groups.
26
Hemoglobin and myoglobin
  • Hemoglobin
  • oxygen transport protein of red blood cells.
  • Myoglobin
  • oxygen storage protein of skeletal muscles.
  • As with the cytochrome example, both proteins use
    heme groups. It acts as the binding site for
    molecular oxygen.

27
Heme
  • myoglobin
  • 1 heme group
  • hemoglobin
  • 4 heme groups

28
Myoglobin
Heme
29
Hemoglobin
2 ? chains
4 heme
2 ? chains
30
Oxygen Transport
31
Example - cytochrome C 550
Heme structure Contains Fe2 Used
in metabolism.
Aggregate of proteins and other structures.
32
Sickle cell anemia
  • Defective gene results in production of mutant
    hemoglobin.
  • Still transports oxygen but results in deformed
    blood cells - elongated, sickle shaped.
  • Difficult to pass through capillaries. Causes
    organ damage, reduced circulation.
  • Affects 0.4 of African-American.

33
Comparison of normal andsickle cell hemoglobin
Normal
Sickle
34
Summary of protein structure
primary
secondary
tertiary
quaternary
35
Denaturation of Proteins
  • The loss of secondary, tertiary, and quaternary
    structures
  • 1) pH extremes.
  • 2). Heat -
  • 3). Mechanical Agitation (foaming)
  • 4). Detergents
  • 5). Organic Solvents
  • 6). Inorganic Salts -
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