Transcript Chapter 14-part 1

Chapter 14
Proteins
Proteins
Proteins serve many functions, including the following:
◦ 1. Structure: Collagen and keratin are the chief
constituents of skin, bone, hair, and nails.
◦ 2. Catalysts: Virtually all reactions in living systems are
catalyzed by proteins called enzymes.
◦ 3. Movement: Muscles are made up of proteins called
myosin and actin.
◦ 4. Transport: Hemoglobin transports oxygen from the
lungs to cells; other proteins transport molecules across
cell membranes.
◦ 5. Hormones: Many hormones are proteins, among them
insulin, oxytocin, and human growth hormone.
Proteins
◦ 6. Protection: Blood clotting involves the protein
fibrinogen; the body used proteins called antibodies to
fight disease.
◦ 7. Storage: Casein in milk and ovalbumin in eggs store
nutrients for newborn infants and birds. Ferritin, a protein
in the liver, stores iron.
◦ 8. Regulation: Certain proteins not only control the
expression of genes, but also control when gene expression
takes place.
 Proteins are divided into two types:
◦ Fibrous proteins
◦ Globular proteins
Proteins

Proteins are divided into two types:
◦ Fibrous proteins: insoluble in water and are used mainly
for structural purposes
◦ long fibers or sheets formed by parallel polypeptide chains
◦ dominated mostly by secondary structure
◦ mostly water insoluble
◦ great strength and/or stretchiness from affects of regular H-bonds
◦ examples:
◦ collagen in connective tissue
◦ actin and myosin in muscle tissue
Proteins

Globular proteins: more or less soluble in water and are
used for nonstructural purpose
◦ folded into complex 3-D irregular spherical shape
◦ dominated mostly by tertiary structure
◦ mostly water soluble
◦ functions determined by 3-D shape
◦ examples:
◦ enzymes such as amylase
◦ hormones such as insulin
◦ transport such as hemoglobin
◦ protective, such as immunoglobulins
Amino Acids
Amino acid: A compound that contains both an amino
group and a carboxyl group.
◦ -Amino acid: An amino acid in which the amino group
is on the carbon adjacent to the carboxyl group.
Table 14.1 The 20 amino acids commonly found in proteins
Chirality of -Amino Acids
With the exception of glycine, all protein-derived amino acids
have at least one stereocenter (the -carbon) and are chiral.
◦ The vast majority of -amino acids have the L-configuration
at the -carbon.
Chirality of -Amino Acids
A comparison of the configuration of L-alanine and Dglyceraldehyde (as Fischer projections):
Protein-Derived -Amino Acids
Nonpolar side chains. Each ionizable group is shown in the
form present in highest concentration at pH 7.0).
Protein-Derived -Amino Acids

Polar side chains (at pH 7.0)
Protein-Derived -Amino Acids
Acidic and basic side chains (at pH 7.0)
Protein-Derived -Amino Acids
1. For 19 of the 20, the -amino group is primary; for proline,
it is secondary.
2. With the exception of glycine, the -carbon of each is a
stereocenter.
3. Isoleucine (left) and threonine (right) contain a second
stereocenter.
Ionization vs. pH
The net charge on an amino acid depends on the pH of the
solution in which it is dissolved.
◦ If we dissolve an amino acid in water, it is present in the
aqueous solution as its zwitterion.
Ionization vs. pH

To summarize
pH = 0
:
a zwitterion
pH = 7
pH = ~ 14
Isoelectric Point (pI)

Isoelectric point, pI:
The pH at which
the majority of
molecules of a
compound in
solution have no
net charge.
Nonpolar &
polar side chains
alanine
asparagine
cysteine
glutamine
glycine
isoleucine
leucine
methionine
phenylalanine
proline
serine
threonine
tyrosine
tryptophan
valine
pI
6.01
5.41
5.07
5.65
5.97
6.02
5.98
5.74
5.48
6.48
5.68
5.87
5.66
5.88
5.97
Acidic
pI
Side Chains
aspartic acid 2.77
glutamic acid 3.22
Basic
pI
Side Chains
10.76
arginine
histidine
7.59
lysine
9.74
What determines the
characteristic of amino acid
Cystine
The -SH (sulfhydryl) group of cysteine is easily oxidized to an -S-S(disulfide).
Hair is made up by a protein called karetin that contains a large number
of cysteine residues
Phe, Trp, and Tyr
The amino acids phenylalanine, tryptophan, and tyrosine have
aromatic rings on their side chains.
Tryptophan is the precursor to the neurotransmitter serotonin.
Tyr and Phe
Phenylalanine and tyrosine are precursors to norepinephrine and
epinephrine, both of which are stimulatory.
Other Amino Acids
Figure 14.3 Hydroxylation
(oxidation) of proline, lysine,
and tyrosine, respectively
and iodination for tyrosine,
give these uncommon amino
acids.
Peptides
In 1902, Emil Fischer proposed that proteins are long chains of
amino acids joined by amide bonds.
◦ Peptide bond (peptide linkage): The special name given to the
amide bond between the -carboxyl group of one amino acid
and the -amino group of another.
Peptides
◦ Peptide: A short polymer of amino acids joined by peptide
bonds; they are classified by the number of amino acids in
the chain.
◦ Dipeptide: A molecule containing two amino acids joined by
a peptide bond.
◦ Tripeptide: A molecule containing three amino acids joined
by peptide bonds.
◦ Polypeptide: A macromolecule containing many amino acids
joined by peptide bonds.
◦ Protein: A biological macromolecule containing at least 30 to
50 amino acids joined by peptide bonds.
◦ The individual amino acid units are often referred to as
“residues”.
Peptide Bond

A peptide bond is typically written as a carbonyl group bonded to
an N-H group. Linus Pauling, however, discovered that there is
about 40% double bond character to the C-N bond and that a
peptide bond between two amino acids is planar, which Pauling
explained using the concept of resonance.
Peptide bond
Writing Peptides
By convention, peptides are written from the left to right,
beginning with the free -NH3+ group and ending with the free COO- group.
◦ C-terminal amino acid: The amino acid at the end of the chain
having the free -COO- group.
◦ N-terminal amino acid: The amino acid at the end of the chain
having the free -NH3+ group.
Writing Peptide Bond
Example

Show how to form the dipeptide Gly-Val

Draw the tetrapeptide Ala-Thr-Asp-Asn and indicate the
peptide bond