Download Lecture 28

Amino acids
R-groups
non-polar
polar
acidic
basic
proteins condensation between
carboxylic acids and amines
C
O
+ H N
C
N
OH
carboxylic acid
O
amine
amide
+ H2O
Amides
resonance structure
amides
O
O
C
C
N
N
dipeptide
H
H2N C C
O
OH
H
H2N C C
CH3
H
alanine
glycine
H
O
OH
H2N C C
CH3
O
H
N C C
H H
Ala-Gly
+H2O
O
OH
Polypeptides
“backbone”
_
_
_
_
_
_
H R
H R
H R
_
H N1-C1-C1-N2-C2-C2-N3-C3-C3- OH
O
O
O
peptide bonds
C-terminal
N-terminal
residue
residue
=
=
=
biological activity = structure
4 levels
protein structure
Primary structure
sequence of amino acids
hemoglobin transports O2 and CO2
4 protein chains 300 amino acids
Sickle cell anemia
6th amino acid from
N-terminus
R
Glu
Val
-CH2CH2-CO2H
-CH(CH3)2
water soluble
water insoluble
Primary structure
study evolution
-chain
146 residues
horses - humans = 26
pigs - humans = 10
gorillas - humans = 1
1 successful change / 10,000,000 years
Primary structure - selective hydrolysis
Phe-Val-Asn-Gln-His
Gln-His-Leu-Cys
His-Leu-Cys-Gly-Ser
His-Leu-Val-Glu
Gly-Ser-His-Leu-Val
Leu-Val-Glu-Ala
Phe-Val-Asn-Gln-His
Gln-His-Leu-Cys
His-Leu-Cys-Gly-Ser
Leu-Val-Glu-Ala
Gly-Ser-His-Leu-Val
His-Leu-Val-Glu
Secondary structure
hydrogen bonding backbone groups
=
=
=
H-bond donors
_
_
_
_
_
_
H R
H R
H R
_
H N1-C1-C1-N2-C2-C2-N3-C3-C3- OH
O
O
O
H-bond acceptors
Two main secondary structures:
-helix
-sheet
Alpha helix
Every C=O bonded to N-H 4 residues away
forms a helix
core is backbone
R-groups outside
3.6 amino acids per turn
H
proline
=
C
=
C
O
O
N
no H-bonding
breaks helix
Beta sheet
Every C=O bonded to N-H far apart in 1o structure
on different chains
peptide chains extended
side-by-side
maximal H-bonding for anti-parallel chains
small R-groups above and below the sheet
if not -helix or -sheet
random coil
Secondary structure
some proteins 1o structure amino acid sequence
2o structure -helix
-sheet
-sheet
-helix
silk
collagen bone, teeth triple helices
keratin hair, skin, wool, hooves
cross-linked with disulfide bonds
Disulfide bonds
cysteine -CH2-SH
H
H
N C S-H
H-S C N
C
reduced
C
[O]
H
H
N C S
S C N
C
C
oxidized
Protein function
enzymes biological catalysts
immunoglobulins antibodies
transport hemoglobin
hormones regulation
structural keratin, collagen
motion
actin, myocin
function depends on structure