27.19 Secondary Structures of Peptides and Proteins - PowerPoint PPT Presentation

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27.19 Secondary Structures of Peptides and Proteins

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... a b sheet of protein chains composed of alternating glycine and alanine residues. ... seen with amino acids having small side chains (glycine, alanine, serine) ... – PowerPoint PPT presentation

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Title: 27.19 Secondary Structures of Peptides and Proteins


1
27.19Secondary Structuresof Peptides and
Proteins
2
Levels of Protein Structure
  • Primary structure the amino acid sequence plus
    disulfide links
  • Secondary structure conformational relationship
    between nearest neighbor amino acids
  • a helix pleated b sheet

3
Levels of Protein Structure
The a-helix and pleated b sheet are both
characterized by
  • planar geometry of peptide bond
  • anti conformation of main chain
  • hydrogen bonds between NH and OC

4
Pleated b Sheet
  • Shown is a b sheet of protein chains composed of
    alternating glycine and alanine residues.
  • Adjacent chains are antiparallel.
  • Hydrogen bonds between chains.
  • van der Waals forces produce pleated effect.

5
Pleated b Sheet
  • b Sheet is most commonly seen with amino acids
    having small side chains (glycine, alanine,
    serine).
  • 80 of fibroin (main protein in silk) is
    repeating sequence of GlySerGlyAlaGlyAla.
  • b Sheet is flexible, but resists stretching.

6
a Helix
  • Shown is an a helix of a protein in which all of
    the amino acids are L-alanine.
  • Helix is right-handed with 3.6 amino acids per
    turn.
  • Hydrogen bonds are within a single chain.
  • Protein of muscle (myosin) and wool (a-keratin)
    contain large regions of a-helix. Chain can be
    stretched.

7
27.20Tertiary Structureof Peptides and Proteins
8
Tertiary Structure
  • Refers to overall shape (how the chain is folded)
  • Fibrous proteins (hair, tendons, wool) have
    elongated shapes
  • Globular proteins are approximately spherical
  • most enzymes are globular proteins
  • an example is carboxypeptidase

9
Carboxypeptidase
  • Carboxypeptidase is an enzyme that catalyzes the
    hydrolysis of proteins at their C-terminus.
  • It is a metalloenzyme containing Zn2 at its
    active site.
  • An amino acid with a positively charged side
    chain (Arg-145) is near the active site.

10
Carboxypeptidase
Disulfide bond
Zn2
Arg-145
N-terminus
C-terminus
tube model
ribbon model
11
What happens at the active site?

O
O
H2N


C

H3N
peptide
NHCHC

Arg-145
C
O
H2N
12
What happens at the active site?

O
O
H2N


C

H3N
peptide
NHCHC

Arg-145
C
O
H2N
  • The peptide or protein is bound at the active
    site by electrostatic attraction between its
    negatively charged carboxylate ion and
    arginine-145.

13
What happens at the active site?

O
O
H2N


C

H3N
peptide
NHCHC

Arg-145
C
O
H2N
  • Zn2 acts as a Lewis acid toward the carbonyl
    oxygen, increasing the positive character of the
    carbonyl carbon.

14
What happens at the active site?

O
O
H2N


C

H3N
peptide
NHCHC

Arg-145
C
O
H2N
  • Water attacks the carbonyl carbon. Nucleophilic
    acyl substitution occurs.

15
What happens at the active site?
H2N

Arg-145
C
16
27.21Coenzymes
17
Coenzymes
  • The range of chemical reactions that amino acid
    side chains can participate in is relatively
    limited.
  • acid-base (transfer and accept
    protons) nucleophilic acyl substitution
  • Many other biological processes, such as
    oxidation-reduction, require coenzymes,
    cofactors, or prosthetic groups in order to occur.

18
Coenzymes
  • NADH, coenzyme A and coenzyme B12 are examples of
    coenzymes.
  • Heme is another example.

19
Heme
  • Molecule surrounding iron is a type of porphyrin.

20
Myoglobin
Heme
  • Heme is the coenzyme that binds oxygen in
    myoglobin (oxygen storage in muscles) and
    hemoglobin (oxygen transport).

21
27.22Protein Quaternary StructureHemoglobin
22
Protein Quaternary Structure
  • Some proteins are assemblies of two or more
    chains. The way in which these chains are
    organized is called the quaternary structure.
  • Hemoglobin, for example, consists of 4 subunits.
  • There are 2 a chains (identical) and 2 b chains
    (also identical).
  • Each subunit contains one heme and each protein
    is about the size of myoglobin.
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