Download Amino Acids Proteins, and Enzymes

19.6 Primary Structure

The primary structure of a protein is the
sequence of amino acids in the peptide chain
CH3
CH3
H3N
S
CH CH3
SH
CH2
CH3 O
CH O
CH2 O
CH2 O
CH C N
CH C N
CH C N CH C O
H
H
Ala-Leu-Cys-Met
H
Protein
backbone
Insulin
Insulin:
 The first protein to have its
primary structure determined
 51 residues
 2 chains
 2 disulfide bridges
19.7 Secondary Structure

The secondary structure of a protein
indicates the arrangement of the
polypeptide chains in orderly patterns.
1.
Alpha helix
2.
Beta-pleated sheet
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
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
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.
Levels of
Structure


So far…

Primary

Secondary
Next…

Tertiary
19.8 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 bridges
Tertiary Structure

(electrostatic
interactions)
Disulfide Bonds:
Tertiary Structure

The interactions of the R groups give a protein its specific
three-dimensional tertiary structure
Learning Check
Select the type of tertiary interaction as:
1) disulfide
2) salt bridge
A.
Leucine and valine
B.
Cysteine and cysteine
C.
Aspartate and lysine
D.
Serine and threonine
3) H-bonds
4) hydrophobic
19.9 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 in
the blood for transport to the
tissues
Summary of Structural Levels
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
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?
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 salt bridges.
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.
19.10 Reactions of Proteins
Hydrolysis
 Denaturation

Hydrolysis of Amides
Amide hydrolysis (review section 16.8):
O
H
O
H
acid
R
C
N
R
+
H2O
or base
R
C
OH
+
H
N
R
Protein hydrolysis:

Splits the peptide bonds to give smaller peptides and amino acids

Catalyzed by enzymes

Occurs in the digestion of proteins

Occurs in cells when amino acids are needed to synthesize new
proteins and repair tissues
Denaturation



Disruption of bonds
in the native
secondary, tertiary
and quaternary
protein structures
Covalent amide
bonds (primary
structure) are not
affected
Loss of biological
activity with loss of
structure
Applications of
Denaturation

Cooking food containing protein

Wiping the skin with alcohol
(denaturation of bacterial proteins)

Hair permanents
Learning Check
The products of the complete hydrolysis of
the peptide Ala-Ser-Val are:
Learning Check

What structural level of a protein is affected
by denaturation?
End of Chapter 19!