Download Chap.21_Proteins, part 2

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•
Primary Structure
How important is the exact amino acid sequence?
• See Human insulin.
Human
Cow
Hog
Sh eep
A Chain
p ositions 8-9-10
B Chain
p os ition 30
-Thr-Ser-Ile-A la-Ser-Val-Thr-Ser-Ile-Ala-Gly-Val-
-Thr
-Ala
-Ala
-Ala
Human insulin
Minor A.A. replacements tolerated (sometimes)
Otherwise allergic reactions . . .
NAMING?
start @ N-terminus; e.g. A.A. #1 in insulin is “Gly”
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Primary Structure (cont.)
• Vasopressin and oxytocin - nonapeptides but have quite different
biological functions.
Cys S S Cys Pro Gly NH2
Tyr
A sn
Ph e Gln
Vas op res sin
Cys S S Cys Pro Leu NH2
Tyr
A sn
Ile
Gln
Oxytocin
• Vasopressin is an antidiuretic hormone. (Alcohol inhibits)
• Oxytocin ~ contractions of uterus in childbirth and the muscles of the
breast that aid in the secretion of milk.
Minor replacements  big changes in biology. . .
Primary Sequence matters
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Secondary Structure
Conformations of A.A.s in localized regions of a polypeptide chain.
Two Main Types:
1. a-Helix: polypeptide chain coils into a
Right-handed spiral
2. b-Pleated sheet: two polypeptide chains
align parallel to each other
(chains may be parallel or antiparallel
--------------- (minor)
3. Random coils (orange)
β-Pleated sheet (blue)
α-Helix (green)
4. Extended helix (collagen
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a-Helix
• Figure 21.6.
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•α-Helix The C=O group (peptide bond) is
H- bonded to N-H group other peptide bond,
4 amino acid units away from it.
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• Figure 21.6.
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b-Pleated Sheet
•C=O, N-H groups of peptide bonds from
adjacent chains H-Bond
(intermolecular/intramolecular H-bonds)
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Conformational changes to B sheets (Many diseases)
Mad Cow, Alzheimers etc . . Amyloid plaques
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Tertiary Structure
overall conformation of an entire polypeptide chain.
Stabilized in 4 ways:
1. Covalent bonds, e.g. formation of
disulfide bonds b/w cysteine side chains.
2. H- bonding, e.g. b/w side chains (R groups) e.g. OH groups
of serine and tyrosine.
3.Salt bridges, e.g. -NH3+ group of lysine +
-COO- group of aspartic acid.
4. Hydrophobic interactions, b/w N. polar side chains of
Phenylalanine and leucine. (e.g. oil/vinegar analogy)
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Tertiary Structure
• See figure 21.11
NOTE:
R-groups in A.A.s
play major role in
forming tertiary structure
via 4 bonding types . . .
Otherwise . . .
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Chaperone Proteins
• Facilitate folding of newly synthesized polypeptide chains
 secondary, tertiary structures
Prevent denaturing  loss of biological activity
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22Challenge Question?
• What kind of noncovalent interation occurs between the
side chains (R-groups) of Arginine and glutamic acid?
O
+
H3N
CH
C
O
O-
+
H3N
CH
CH2
CH2
CH2
CH2
CH2
C
NH
O-
C
C
O-
O
NH2+
NH2
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Quaternary Structures
arrangement of polypeptide chains into a noncovalently bonded aggregation.
-chains held together by H-bonds, salt bridges, hydrophobic interactions.
• e.g. Hemoglobin
• Adult hemoglobin: two alpha chains of 141 amino acids
each, and two beta chains of 146 amino acids each.
• Each chain surrounds an iron-containing heme unit.
• Fetal hemoglobin: two alpha chains and two gamma
chains; fetal hemoglobin has a greater affinity for
oxygen than does adult hemoglobin.
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22Hemoglobin
• Figure 21.13 The 4° structure of hemoglobin.
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22Hemoglobin
• Figure 21.14 The structure of heme.
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22Integral Membrane proteins
• Provide structure and perform services for cell
• e.g. transport,
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22Quaternary Structures
• Figure 21.15 Integral membrane protein of
rhodopsin made of a-helices.
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22Quaternary Structure
• Figure 22.18 The b-barrel of an integral membrane
protein of the outer membrane of a
mitochondrion is made of eight b-pleated sheets.
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22Glycoproteins
• proteins with covalently bonded carbos. P.546
• Via O-linked sacharides
• (e.g. mucins, coat membranes
• Respiratory tract)
• Via N-linked saccharides
• (e.g. immunoglobins, IgG,IgA)
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22Denaturation
• process of destroying the native conformation of
a protein by chemical or physical means.
Why Cook Food?
• Some denaturations are reversible, while others permanently
damage the protein.
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