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Part 4
–Chapter 19
• Proteins:
• Structure:
•
•
•
•
primary
Secondary
Tertiary
Quaternary
• Loss of protein structure
• Tests for proteins (lab)
• Sections: 19.4-19.5
Proteins Structures: Primary
• Proteins are polypeptides of 50 or more
amino acids that has biological activity.
• Each protein in our cells has a unique
sequence of amino acids that determines its
3-D structural and biological function.
• The primary structure of a protein is the
particular sequence of the amino acid held
together by peptide bonds..
Proteins Structures:
Primary
• Isulin’s primary structure is two polypeptide
chains.
• There are 21 amino acids in Chain A and 30
in chain B.
Protein Structures:
Secondary
• The secondary structure of a protein describes the type
of structure that forms when amino acids form H-bonds
within a single polypeptide chain or between multiple
polypeptides chains.
• The three most common secondary structures are:
– Alpha helix
– Beta-pleated sheets
– Triple helix
• In alpha helix, H-bonds form between the hydrogen
atoms of the N-H groups in the amide bonds, and the
oxygen atoms in the carbonyl groups of the amide bonds
that are four amino acids away in the next turn of the
alpha helix.
Protein Structures:
Secondary
• In a β-pleated sheet, polypeptide chains are held
together side by side by hydrogen bonds that form
between oxygen atoms of the carbonyl group in one
section of the polypeptide chain, and the hydrogen atom
in the N-H groups of the amide bond in a nearby section
of the polypeptide chain.
• The hydrogen bonds holding the sheets tightly in place
account for the strength and durability of fibrous proteins
such as silk.
• Amino acids in α-helix structures typically have shorter R
group attached and amino acids in β-pleated sheet have
longer R groups attached.
Protein Structures:
Secondary
Collagen
• Collagen, which is the most abundant protein in the
body, makes up 25-35% of all protein in vertebrates.
• It is found in:
–
–
–
–
–
–
–
Connective tissue
Blood vessels
Skin
Tendons
Ligaments
The cornea of the eye
cartilage.
• Its strong structure is a result of three a-helices
woven together like a braid to form a triple helix.
Protein Structures: Tertiary
• The tertiary structure of a protein involves attractions and repulsions
between the R groups of the amino acids in the polypeptide chain.
• As interactions occur between different parts of the peptide chain,
segments of the chain twist and bend until the protein acquires a
specific three-dimensional shape.
A Phone cord is similar to the α-helix
secondary structure.
The phone cord is likely to fold up on itself.
The tertiary structure of a protein refers to the
way the secondary structure folds back upon
itself or twist around to form a 3-D structure.
The secondary coil structure is still there, but
the tertiary tangle has been superimposed on
it.
Protein Structure: Tertiary
•Example: myoglobin
•The helix is folded up
on itself.
•The blue tube is drawn
in to more easily see
the helix outline.
•Bonding between the
side R groups are
responsible for holding
the tertiary structure
together
Protein Structure:
Quaternary
• When several polypeptide chains called subunits bind to
form a larger complex, it is referred to as a quaternary
structure.
• Many proteins are biologically active as tertiary
structures, but some proteins require two or more tertiary
structures to be biologically active.
• Quaternary structures are held together by bonding
between the side chain, R, groups on the amino acids.
Protein Structure:
Quaternary
•Example: Hemoglobin
•Each color is a different
tertiary structure
Protein Structure
Loss of Protein Structure
• There are two different way that proteins can lose their structure.
• Both involve the loss of biological function, which means it no longer
acts as it did in its natural state.
1.
2.
If the secondary, tertiary, or quaternary structure, but not primary is
altered or disrupted it is referred to as denaturation.
If the primary structure is broken down it is known as hydrolysis. The
proteins are ultimately converted into amino acids.
• In denaturation, the weak hydrogen, electrostatic, and disulfide
bonds are broken.
• The bonds are weak enough that denaturation can occur from
something as gentle as pouring a protein.
• It can also occur from heating, whipping, and ultraviolet radiation, or
from using strong acids and bases, alcohol, or detergent.
– Examples: The egg albumin in egg whites are denatured and coaulates
when they are whipped with an egg beater.
Tests for Proteins and Amino
Acids
•
Xanthoproteic Test: If the protein contains a benzene ring, it will react with
concentrated nitric acid used in the test to give a yellow product . Proteins
in the skin react with nitric acid to “stain” your skin yellow for a few days.
Tryptophan
Phenylalanine
•
Tyrosine
Biuret Test: Copper (II) sulfate are added to an alkaline (basic) solution that
contains peptides or proteins. A violet color is produced when at least two
peptide bonds are present in the peptide or protein. This means that the
test will not be positive for amino acids or dipeptides.
H
H
+
N
H
O
H
CH3 O
C
C
N
C
H
H
H
C
O-