Announcements & Agenda (04/23/07)
Pick Up Grade Sheets
Exam 3 back later this week
Class in VWF 102 on Wed!
Today
 Amino acids (16.1-16.3)
 Peptides (16.4)
 Protein Structure (16.5)
1
I Thought Exam 3 was…
1.
2.
3.
4.
5.
Piece of Cake
Easier than I Expected
Mostly What I Expected
Much Harder than I Expected
Uh Oh…
2
Ch 16: Amino Acids,
Proteins, & Enzymes!
Baaaaaa…….
3
Functions of Proteins
Proteins perform many different functions in the body.
Function of proteins determined by amino acids used
and how they are put together in 2-D and 3-D
4
Proteins are made of Amino Acids
20 Amino acids
• are the building blocks of proteins.
• contain a carboxylic acid group and an amino group on
the alpha () carbon.
• are ionized in solution.
• each contain a different side group (R).
R
side chain
R
│
+ │
H2N—C —COOH
H3N—C —COO−
│
│
H
H ionized form
5
“Essential” Amino Acids
• must be obtained
from the diet.
• not synthesized by
the body.
• are in meat and
diary products.
• are missing (one
or more) in grains
and vegetables.
6
Types of Amino Acids
Nonpolar
Polar
4 main kinds:
• nonpolar (hydrophobic)
with hydrocarbon side
chains.
• polar (hydrophilic) with
polar or ionic side chains.
• acidic (hydrophilic) with
acidic side chains.
• basic (hydrophilic) with
–NH2 side chains.
Acidic
Basic
Be able to recognize these 4 kinds, no need to memorize
all 20 for the Final Exam!!!!!!!!!!!!!!!!!
7
Nonpolar Amino Acids
An amino acid is nonpolar when the R group is
H, alkyl, or aromatic.
8
Polar Amino Acids
An amino acid is polar when the R group is an
alcohol, thiol, or amide.
9
Acidic & Basic Amino Acids
An amino acid is
• acidic when the R group is a carboxylic acid.
• basic when the R group is an amine.
10
Fischer Projections of Amino Acids
• are chiral except glycine
• have Fischer projections that are stereoisomers.
• ONLY “L” amino acids used in proteins.
COOH
COOH
H2N
H
CH3
L-Alanine
H
NH2
CH3
D-Alanine
COOH
H2N
H
CH2SH
L-Cysteine
COOH
H
NH2
CH2SH
D-Cysteine
11
Zwitterions
A zwitterion
• has charged −NH3+ and COO– groups.
• forms when both the –NH2 and the –COOH groups
in an amino acid ionize in water.
• has equal + and – charges at the isoelectric point
(pI).
O
║
NH2—CH2—C—OH
glycine
O
║
+
H3N—CH2—C—O–
zwitterion of glycine
12
pH and ionization
OH–
H+
+
H3N–CH2–COOH
positive ion
low pH
+
H3N–CH2–COO–
zwitterion
pI
H2N–CH2–COO–
negative ion
high pH
13
16.4
Formation of Peptides
14
The Peptide Bond
• is an amide bond.
• forms between the carboxyl group of one amino acid
and the amino group of the next amino acid.
O
+
||
H3N—CH2—C—O– +
CH3 O
|
||
+
H3N—CH—C—O–
O H CH3 O
|| | |
||
+
H3N—CH2—C—N—CH—C—O–
peptide bond
15
Formation of a Dipeptide
16
Naming Dipeptides (Tri… etc.)
(NO NEED TO MEMORIZE NAMING RULES)
A dipeptide
• is named from the free amine (NH3+) using a yl ending for the name.
• names the last amino acid with the free
carboxyl group (COO-) by its amino acid name.
17
Tour of Protein Structure…
18
Primary Structure of Proteins
• sequence of amino acids
• different proteins have different sequences
• the backbone of a peptide chain or
protein.
CH3
CH3
S
CH CH3
SH
CH2
CH O
CH2 O
CH2 O
H3N CH C N
CH C N
CH C N
CH C O-
H
H
H
CH3 O
Ala─Leu─Cys─Met
19
Oxytocin (pitocin) V. du Vigneaud
Nobel prize in chemistry 1955
Uterine contracting and milk letdown hormone
20
Primary Structures
• The nonapeptides oxytocin and vasopressin
have similar primary structures.
• Only the amino acids at positions 3 and 8 differ.
21
Primary Structure of Insulin
Insulin
• was the first protein to have
its primary structure
determined.
• has a primary structure of
two polypeptide chains linked
by disulfide bonds.
• has a chain A with 21 amino
acids and a chain B with 30
amino acids.
22
Modification of insulin
23
Newer drugs for diabetics
http://www.rxlist.com/cgi/generic/byetta.ht
m
http://www.diabetesmonitor.com/symlin.ht
m
http://health.dailynewscentral.com/content/
view/0002300/38/
Glitazone
http://www.gpnotebook.co.uk/cache/x2002
0419151758067650.htm
24
Insulin can be administered by…
1. Inhalation
2. Injection
3. oral
25
Secondary Structure Elements
• a 3-D arrangement of amino
acids in a polypeptide chain.
• result from intermolecular forces
such as hydrogen bonding
• Several types of secondary
structure
•
•
•
Alpha helices
Beta sheets
Triple helices
26
Beta Pleated Sheet
•
•
•
•
polypeptide chains side by side.
hydrogen bonds between chains.
has R groups above and below the sheet.
is typical of fibrous proteins such as silk.
27
Secondary Structure – Triple Helix
The secondary
structure of a triple
helix is
• three polypeptide
chains woven
together.
• typical of collagen,
connective tissue,
skin, tendons, and
cartilage.
28
Tertiary Structure
• overall 3-D shape.
• determined by
attractions &
repulsions between
side chains of amino
acids
29
Crosslinks in Tertiary Structures
involve attractions
and repulsions
between the side
chains of the
amino acids in the
polypeptide chain.
30
Quaternary Structure
• combination of 2 or
more protein units.
• Example: hemoglobin
consists of 4 subunits.
• stabilized by the same
interactions found in
tertiary structures.
31
Summary of Protein Structure
32
Denaturation
• the disruption of bonds in the secondary, tertiary
and quaternary protein structures.
• heat and organic compounds: break apart H
bonds and disrupt hydrophobic interactions.
• acids and bases: break H bonds between polar
R groups and disrupt ionic bonds.
• heavy metal ions: react with S-S bonds to form
solids (among many other things)
• agitation such as whipping that stretches
peptide chains until bonds break.
33
Applications of Denaturation
• cooking.
• the skin is wiped
with alcohol.
• heat is used to
cauterize blood
vessels.
• instruments are
sterilized in
autoclaves.
34