Organic Chemistry

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Amino Acids
& Proteins
27-1
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.
• although -amino acids are commonly written in the
unionized form, they are more properly written in the
zwitterion (internal salt) form.
O
RCHCOH
NH2
-Ami no Acid
O
RCHCO NH3 +
Zwi tteri on
form
27-2
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 have the L-configuration at their carbon.
COOH
N H3 +
CH3
D-Alan i ne
COOH 3N
H
CH3
L-A lan in e
27-3
Amino Acids with Nonpolar side chains
COO- Al an i n e
(Ala, A)
N H3 +
COO- Ph e n yl alan i ne
(Ph e , F)
+
N H3
COO- Gl yci ne
(Gly, G)
N H3 +
COO- Is ol e u ci n e
(Il e, I)
N H3 +
-
COO
N H3 +
S
Le u ci n e
(Le u , L)
COO- Me th i on in e
(Me t, M)
+
N H3
- Prol i ne
COO
N
(Pro, P)
H H
N
H
COO- Tryptoph an
(Trp, W)
N H3 +
COO- Vali n e
(Val , V)
N H3 +
27-4
With Polar side chains
COO-
H2N
O
N H3 +
As paragi n e
(Asn , N)
O
H 2N
COON H3 +
Gl utami n e
(Gln , Q )
HO
COO-
Se ri n e
(Se r, S )
N H3 +
OH
COO- Th reon i n e
(Th r, T)
NH +
3
27-5
With Acidic & Basic Side Chains
-
COO-
O
O
N H3
As partic acid
(Asp, D)
H 2N
+
N H2 +
O
-
COO-Gl utami c aci d
N H + (Glu , E)
O
N
H
3
N
3
-
HS
COO
N H3
+
-
COO
HO
N H3 +
N
H
Cyste in e
(C ys, C )
Tyros in e
(Tyr, Y)
+
H 3N
COO- Argi n in e
(Arg, R)
NH +
COON H3
+
COON H3 +
Hi stidi n e
(His , H)
Lysi n e
(Lys , K)
27-6
Acid-Base properties, Non polar and polar side chains
N onpolar &
polar side
ch ains
alanin e
asp aragine
glutamine
glycine
isoleucine
leucine
methionine
ph enylalanine
proline
serine
threonine
tryp top han
valine
pK a of
pK a of
COOH
2.35
2.02
2.17
2.35
2.32
2.33
2.28
2.58
2.00
2.21
2.09
2.38
2.29
NH3
9.87
8.80
9.13
9.78
9.76
9.74
9.21
9.24
10.60
9.15
9.10
9.39
9.72
+
27-8
Acid-Base Properties, Acidic/Basic Side Chains
Acid ic
Side
Ch ains
aspartic acid
glu tamic acid
cysteine
tyros ine
pK a of
Basic
Side
Ch ains
arginin e
histid ine
lys ine
pK a of
pK a of
COOH NH3
2.10
9.82
2.10
9.47
2.05
10.25
2.20
9.11
pK a of
COOH NH3
2.01
9.04
1.77
9.18
2.18
8.95
+
+
pK a of
Side
Ch ain
3.86
4.07
8.00
10.07
Sid e
Chain
Grou p
carboxyl
carboxyl
su fh yd ryl
ph enolic
pK a of
Side
Ch ain
12.48
6.10
10.53
Sid e
Chain
Grou p
guanid ino
imidazole
1° amin o
27-9
Acidity: -COOH Groups
average pKa of an -carboxyl group is 2.19,
which makes them considerably stronger acids
than acetic acid (pKa 4.76).
 The
• The greater acidity is accounted for by the electronwithdrawing inductive effect of the adjacent -NH3+
group.
RCHCOOH
NH3
+
+
H2 O
RCHCOONH3
+
+
H3 O
pK a = 2.19
+
Note that the NH2 will be protonated at these
low pHs
27-10
Acidity: side chain -COOH
 Due
to the electron-withdrawing inductive effect
of the -NH3+ group, side chain -COOH groups
are also stronger than acetic acid.
• The effect decreases with distance from the -NH3+
group. Compare:
-COOH group of alanine (pKa 2.35)
-COOH group of aspartic acid (pKa 3.86)
-COOH group of glutamic acid (pKa 4.07)
27-11
Acidity: -NH3+ groups
average value of pKa for an -NH3+ group is
9.47, compared with a value of 10.76 for a 1°
alkylammonium ion.
 The
RCHCOO + H2 O
+
NH3
CH3 CHCH3 + H2 O
+
NH3
-
+
RCHCOO + H3 O
NH2
pK a = 9.47
+
CH3 CHCH3 + H3 O
NH2
pK a = 10.60
27-12
Details: The Guanidine Group of Arg
• The basicity of the guanidine group is attributed to the
large resonance stabilization of the protonated form
relative to the neutral form.
NH2
+
:
:
NH2
+
RNH C
:
:
NH2
RNH C
RNH C
+
NH2
NH2
:
:
NH2
H2 O
:
NH2
+
+ H3 O
RN C
pK a = 12.48
:
NH2
27-13
Details: Imidazole Group
• The imidazole group is a heterocyclic aromatic amine.
H
••
N
N+
H
H
NH3
+
+
N
-
COO
NH3 +
••
N
H2 O
COO-
+
H3 O
H
N ot a p art of th e
aromatic sextet;
the proton accep tor
••
N
••
N
H
NH3
+
+
H
O
+
3
COO
-
pKa 6.10
27-14
Useful Recall from buffer solutions
Ka = [H+] [A-] [HA]
/
If Ka = [H+] then [A-] = [HA]
27-15
Isoelectric Point
 Isoelectric
point (pI): pH at which an amino acid,
polypeptide, or protein has a total charge of zero.
• The pI for glycine, for example, falls between the pKa
values for the carboxyl and amino groups.
1
pI = 2
+
( pKa COOH + pKa NH 3 )
= 1 (2.35 + 9.78) = 6.06
2
27-16
Isoelectric Point of glycine continued
Again
1
pI = 2
+
( pKa COOH + pKa NH 3 )
= 1 (2.35 + 9.78) = 6.06
2
pKa = pH-log([conj base]/[acid])
At pH = 6.06
For carboxyl group 2.35 = 6.06 - log ([RCO2-]/[RCO2H])
6.06- 2.35 = 3.71 = log ([RCO2-]/[RCO2H])
([RCO2-]/[RCO2H]) = 103.71 or 99.98% ionized as neg ion.
For amino group 9.78 = 6.06 - log([RNH2]/[RNH3+])
([RNH2]/[RNH3+]) = 10-3.72 or 99.98% protonated.
On average Zero Chg.
27-17
Furthermore…..
A+
B
C-
[A+] = [C-]
27-18
Titration of conjugate acid of glycine with NaOH.
Strongly acid
-CO2H and -NH3+
Buffer solution.
[-CO2H] = [-CO2-]
Isoelectric.
Zero net
charge.
Buffer solution.
[-NH3+]=[-NH2]
Strongly basic
-CO2- and NH2
27-19
Isoelectric Point
N onpolar &
pK a of
pK a of
polar side
+
COOH
NH
ch ains
3
alanin e
2.35
9.87
asp aragine
2.02
8.80
glutamine
2.17
9.13
glycine
2.35
9.78
isoleucine
2.32
9.76
leucine
2.33
9.74
methionine
2.28
9.21
ph enylalanine 2.58
9.24
proline
2.00
10.60
serine
2.21
9.15
threonine
2.09
9.10
tryp top han
2.38
9.39
valine
2.29
9.72
pK a of
Sid e
Chain
----------------------------------------
pI
6.11
5.41
5.65
6.06
6.04
6.04
5.74
5.91
6.30
5.68
5.60
5.88
6.00
If pH is lower than pI then more protonated
molecules. If higher then more negative charge.
27-20
Isoelectric Point
Acidic
Sid e Ch ains
asp artic acid
glutamic acid
cystein e
tyrosin e
Basic
Sid e Ch ains
argin ine
his tidine
lysine
pK a of
+ Sid e
COOH NH3 Chain
2.10
9.82
3.86
2.10
9.47
4.07
2.05
10.25
8.00
2.20
9.11
10.07
pK a of
pK a of
pK a of
+ Sid e
COOH NH3 Chain
2.01
9.04
12.48
1.77
9.18
6.10
2.18
8.95
10.53
pK a of
pI
2.98
3.08
5.02
5.63
pK a of
pI
10.76
7.64
9.74
Three buffered pHs
27-21
Aspartic acid
B
A+
pKa
2.10
pH = pKa
[A+] = [B]
C-
3.86
[B] = [C-]
pI = (2.10 + 3.86)/2
[A+] = [C-]
[D2-] approx 0
D2-
9.82
Note species B
has zero net
charge. pKa1 and
pKa2 control [A+]
and [C-] which
should be equal.
27-22
Arginine
A2+
B+
C
D-
pI = (9.04+12.48)/2 =10.76
[B+] = [D-]; [A2+] about 0
27-23
Electrophoresis
 Electrophoresis:
The process of separating
compounds on the basis of their electric charge.
• electrophoresis of amino acids can be carried out
using paper, starch, polyacrylamide and agarose gels,
and cellulose acetate as solid supports.
27-24
Electrophoresis
• A sample of amino acids is applied as a spot on the
paper strip.
• An electric potential is applied to the electrode vessels
and amino acids migrate toward the electrode with
charge opposite their own.
• Molecules with a high charge density move faster than
those with low charge density.
• Molecules at isoelectric point remain at the origin.
• After separation is complete, the strip is dried and
developed to make the separated amino acids visible.
• After derivitization with ninhydrin, 19 of the 20 amino
acids give the same purple-colored anion; proline
gives an orange-colored compound.
27-25
Electrophoresis
• The reagent commonly used to detect amino acid is
ninhydrin.
O
RCHCO + 2
+
NH3
An -amino
acid
O
OH
OH
O
N inh yd rin
-
O
O
N
O
+ RCH + CO2 + H3 O+
O
O
Pu rp le-colored anion
27-26
Polypeptides & Proteins
 In
1902, Emil Fischer proposed that proteins are
long chains of amino acids joined by amide
bonds to which he gave the name peptide bonds.
 Peptide bond: The special name given to the
amide bond between the -carboxyl group of one
amino acid and the -amino group of another.
27-27
Serinylalanine (Ser-Ala)
HOCH2 H
H 2N
O
O
O
S e ri n e
(Se r, S )
H
+
H 2N
O
H
H CH3
Al an i n e
(Ala, A)
pe ptide bon d
HOCH2 H H
O
N
H
H 2N
O
O H CH
3
S e ri n ylalan i n e
(Se r-Al a, (S -A)
27-28
Peptides
• Peptide: The name given to 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 of molecular
weight 5000 g/mol or greater, consisting of one or
more polypeptide chains.
27-29
Writing Peptides
• By convention, peptides are written from the left,
beginning with the free -NH3+ group and ending with
the free -COO- group on the right.
+
H 3N
N-te rminal
ami no aci d
O
C6 H5
O
H
N
C-te rmi nal
ami no aci d
N
OH
O
OH
COOSe r-Phe -As n
27-30
Writing Peptides
• The tetrapeptide Cys-Arg-Met-As
At pH 8 it would be
• At pH 6.0, its net charge is +1.
half ionized.
p Ka 8.00
N-termin al
amin o acid
+
H3 N
SH
H
N
SCH3
O
H
N
N
H
O
O
NH
H2 N
NH2
+
C-terminal
amino acid
O
-
O
NH2
O
pK a 12.48
27-31
Primary Structure
 Primary
structure: The sequence of amino acids
in a polypeptide chain; read from the N-terminal
amino acid to the C-terminal amino acid:
 Amino acid analysis:
• Hydrolysis of the polypeptide, most commonly carried
out using 6M HCl at elevated temperature.
• Quantitative analysis of the hydrolysate by ionexchange chromatography.
27-32
Ion Exchange Chromatography
 Analysis
of a
mixture of amino
acids by ion
exchange
chromatography
27-33
Cyanogen Bromide, BrCN
• BrCN Selectively cleaves of peptide bonds formed by
the carboxyl group of methionine.
cyan ogen b romide is
specific for the cleavage
of this p eptide bond
from the
N-termin al
end
O
O
P N -C-NH CH C NH-PC
CH2
CH2 -S-CH3
from the
C-termin al end
27-34
Cyanogen Bromide, BrCN
O
HN pe pt id e COOH 3 N pe pt id e C-N H
O
+ Br C N
H2 O
s ide ch ain
of me th i on in e S- CH 3
O
H 3 N pe pt id e C-N H
0.1 M HC l
O
O
A s u bs ti tu ted-lacton e of
the am in o acid h om os e rin e
H 3 N pe pt id e COO-
CH 3 S- C N
Me th yl
thi ocyan ate
Mechanism to follow
27-35
Cyanogen Bromide, BrCN
• Step 1: Nucleophilic displacement of bromine.
H3 N
O
HN
C-NH
-
O
COO
O
H3 N
-
HN
COO
C-NH
O
Br
:
:S-CH3
Br
C N
Cyanogen bromide
a sulfon ium
ion; a good
leaving group
:S-CH3
C N
27-36
Cyanogen Bromide, BrCN
• Step 2: Internal nucleophilic displacement of methyl
thiocyanate.
H3N
C-NH
:
O
COO-
HN
O
O
: S-CH3
C N
H3N
C-NH
HN
COO-
:
An i mi n olacton e
h ydrobrom ide
+ CH3-S-C N
:
O
Me th yl
thi ocyan ate
27-37
Cyanogen Bromide, BrCN
• Step 3: Hydrolysis of the imino group.
O
H3 N
C-N H
HN
COOH2 O
O
O
H3 N
C-N H
O
O
H 3N
COO-
A s u bs ti tu ted -lacton e of
the am in o acid h om ose rin e
27-38
Enzyme Catalysis
A
group of protein-cleaving enzymes called
proteases can be used to catalyze the hydrolysis
of specific peptide bonds.
Enzyme
Catalyzes Hydrolysis of Pep tide Bond
Formed b y Carboxyl Group of
Tryps in
Argin ine, lysine
Chymotrypsin Phenylalanine, tyrosin e, tryptoph an
27-39
Edman Degradation
 Edman
degradation: Cleaves the N-terminal
amino acid of a polypeptide chain.
N-terminal
amino acid
R
+
H3 N
NH
O
-
COO
+ S=C=N-Ph
Phenyl isothiocyan ate
R
HN
S
O + H2 N
COO-
N
Ph
A p henylthiohydantoin
27-40
Edman Degradation
• Step 1: Nucleophilic addition to the C=N group of
phenylisothiocyanate and proton tautomerization
R
NH
:
H2 N
O
R
O
HN
:
:
Ph N C S
-
COO
Ph N
H
S
NH
COO-
A d erivative of
N-phen ylth iourea
27-41
Edman Degradation
• Step 2: Nucleophilic addition of sulfur to the C=O of
the adjacent amide group and acid catalysis.
R
HN
Ph N
H+
O
R
O–H
HN
NH
COOH
S
+
Ph-N
H
H
S
NH
COOH H
+
H+
R
HN
O
Ph-N
+
+ H3 N
COOH
S
A thiazolinone
27-42
Edman Degradation
• Step 3: Isomerization of the thiazolinone ring.
R
HN
Ph-N
R
O
S
+ H-Nu
substitution
O
HN
Ph N
SH
keto-en ol
tautomerism
Nu
A th iazolin on e
R
R
O
HN
S
NH
Ph
Nu
- H-Nu
HN
O
N
Ph
A p henylthiohydantoin
S
27-43
DNFB tagging of N terminal AA
27-44
Carboxypeptidase cleavage of C terminal AA
 Treatment
of peptide with carboxypeptidase
cleaves the peptide linkage adjacent to the free
alpha carboxyl group. It may then be identified.
27-45
Primary Structure, example
Deduce the 1° structure of this pentapeptide
Expe rim e n tal Proce du re Ami n o Acid C omposi ti on
pe n tape ptide Arg, G lu , Hi s, Ph e , S er
Edman De gradation Gl u
GluHydrol ysi s - C h ymotryps in
Fragme n t A Gl u , His , Ph e
Glu-His-Phe
Fragme n t B Arg, S e r
Hydrol ysi s - Tryps in
Fragme n t C Arg, G lu , Hi s, Ph e Glu-His-Phe-Arg-Ser
Fragme n t D
Ser
using
Enzyme
Catalyzes Hydrolysis of Pep tide Bond
Formed b y Carboxyl Group of
Tryps in
Argin ine, lysine
Chymotrypsin Phenylalanine, tyrosin e, tryptoph an
27-46
Polypeptide Synthesis
 The
problem in protein synthesis is how to join
the -carboxyl group of aa-1 by an amide bond to
the -amino group of aa-2, and not vice versa.
O
+ O +
H3 NCHCO + H3 NCHCO
aa2
aa1
?
O
+ O
H3 NCHCNHCHCO + H2 O
aa1
aa2
27-47
Polypeptide Synthesis Strategy
• Protect the -amino group of aa-1.
• Activate the -carboxyl group of aa-1.
• Protect the -carboxyl group of aa-2.
prote ctin g
grou p
acti vatin g
grou p
prote ctin g
grou p
O
O
Z-N HCHC- Y + H2 N CH C-X
a a2
a a1
form peptide
bon d
O
O
Z-N HCHCN HCHC-X + H- Y
a a1
a a2
27-48
Amino-Protection
• convert them to amides.
O
PhCH2 OCCl
Benzyloxycarb on yl
ch loride
O O
( CH3 ) 3 COCOCOC(CH3 ) 3
D i-t ert-b utyl dicarbonate
O
PhCH2 OCBenzyloxycarb on yl
(Z-) group
O
(CH3 ) 3 COCt ert -Bu toxycarbonyl
(BOC-) grou p
27-49
Amino-Protection
• Treatment of an amino group with either of these
reagents gives a carbamate (an ester of the
monoamide of carbonic acid).
O
O
O
1.
NaOH
+
PhCH2 OCNHCHCOH
+ H3 NCHCO 2. HCl, H O
2
CH3
CH3
Benzyloxycarbonyl
Alanine
N-Benzyloxycarbonylalanine
chloride (Z-Cl)
(Z-Ala)
O
PhCH2 OCCl
• A carbamate is stable to dilute base but can be
removed by treatment with HBr in acetic acid.
O
PhCH2 OCNH-peptide
A Z-pro tected
peptid e
HBr
CH3 COOH
+
PhCH2 Br + CO2 + H3 N-peptide
Benzyl
bromide
Unprotected
peptide
27-50
Amino-Protecting Groups
 The
benzyloxycarbonyl group is also removed by
hydrogenolysis.
• The intermediate carbamic acid loses carbon dioxide
to give the unprotected amino group.
O
PhCH2 OCNH-peptide + H2
A Z-pro tected
peptid e
Pd
PhCH3 + CO2 + H2 N-pept ide
Toluene
Unprotected
peptide
27-51
Carboxyl-Protecting Groups. Esters
 Carboxyl
groups are most often protected as
methyl, ethyl, or benzyl esters.
• Methyl and ethyl esters are prepared by Fischer
esterification, and removed by hydrolysis in aqueous
base under mild conditions.
• Benzyl esters are removed by hydrogenolysis; they
are also removed by treatment with HBr in acetic acid
27-52
Peptide Bond Formation
 The
reagent most commonly used to bring about
peptide bond formation is DCC.
• DCC is the anhydride of a disubstituted urea and,
when treated with water, is converted to DCU.
N C N
+ H2 O
1,3-D icy clo hexyl carbod ii mide
(D CC)
O
N C N
In bringing about
formation of a peptide
bond, DCC acts a
dehydrating agent.
H
H
N,N' -d icy clo hexy lu rea
(D CU)
27-53
Peptide Bond Formation
O
O
CHCl 3
Z-N HCHC- OH + H2 NCHCOCH3 + DCC
R1
R2
Ami n o-prote cte d C arboxyl-protected
aa1
aa2
O
O
Z-N HCHC- NHCH COCH3 + DCU
R1
R2
Ami n o an d carboxyl
prote cte d dipe pti de
27-54
Solid-Phase Synthesis
 Bruce
Merrifield, 1984 Nobel Prize for Chemistry
• Solid support: a type of polystyrene in which about 5%
of the phenyl groups carry a -CH2Cl group.
• The amino-protected C-terminal amino acid is bonded
as a benzyl ester to the support beads.
• The polypeptide chain is then extended one amino
acid at a time from the N-terminal end.
• When synthesis is completed, the polypeptide is
released from the support beads by cleavage of the
benzyl ester.
27-55
Solid-Phase Synthesis
 The
solid support used in the Merrifield solid
phase synthesis.
27-56
27-57
Solid-Phase Synthesis
• Merrifield synthesized the
enzyme ribonuclease, a
protein containing 124 amino
acids
27-58
Peptide Bond Geometry
• The four atoms of a peptide bond and the two alpha
carbons joined to it lie in a plane with bond angles of
120° about C and N.
• Model of the zwitterion form of Gly-Gly viewed from
two perspectives to show the planarity of the six
atoms of the peptide bond and the preferred s-trans
geometry.
27-59
Peptide Bond Geometry
• To account for this geometry, Linus Pauling proposed
that a peptide bond is most accurately represented as
a hybrid of two contributing structures.
• The hybrid has considerable C-N double bond
character and rotation about the peptide bond is
restricted.
:
:
-
:O
C
:
C
C
:O :
C
N
H
C
+
C
N
H
27-60
Peptide Bond Geometry
• Two conformations are possible for a planar peptide
bond.
• Virtually all peptide bonds in naturally occurring
proteins studied to date have the s-trans conformation.
•
•
C
O
••
H
•
•
••
O
••
C
C
••
N
C
H
s-t rans
N
C
C
s-cis
27-61
Secondary Structure
 Secondary
structure: The ordered arrangements
(conformations) of amino acids in localized
regions of a polypeptide or protein.
 To determine from model building which
conformations would be of greatest stability,
Pauling and Corey assumed that:
1. All six atoms of each peptide bond lie in the same
plane and in the s-trans conformation.
2. There is hydrogen bonding between the N-H group of
one peptide bond and a C=O group of another peptide
bond as shown in the next screen.
27-62
Secondary Structure
• Hydrogen bonding between amide groups.
27-63
Secondary Structure
 On
the basis of model building, Pauling and
Corey proposed that two types of secondary
structure should be particularly stable:
• -Helix.
• Antiparallel -pleated sheet.
 -Helix:
A type of secondary structure in which a
section of polypeptide chain coils into a spiral,
most commonly a right-handed spiral.
27-64
The -Helix
• The polypeptide chain is repeating units of L-alanine.
H bonds (C=O…H) parallel to axis of helix
27-65
The -Helix
 In
a section of -helix:
• There are 3.6 amino acids per turn of the helix.
• Each peptide bond is s-trans and planar.
• N-H groups of all peptide bonds point in the same
direction, which is roughly parallel to the axis of the
helix.
• C=O groups of all peptide bonds point in the opposite
direction, and also parallel to the axis of the helix.
• The C=O group of each peptide bond is hydrogen
bonded to the N-H group of the peptide bond four
amino acid units away from it.
• All R- groups point outward from the helix.
27-66
The -Helix
-helix of repeating
units of L-alanine
 An
• A ball-and-stick model
viewed looking down
the axis of the helix.
• A space-filling model
viewed from the side.
27-67
-Pleated Sheet
antiparallel -pleated sheet consists of
adjacent polypeptide chains running in opposite
directions:
 The
• Each peptide bond is planar and has the s-trans
conformation.
• The C=O and N-H groups of peptide bonds from
adjacent chains point toward each other and are in the
same plane so that hydrogen bonding is possible
between them.
• All R- groups on any one chain alternate, first above,
then below the plane of the sheet, etc.
27-68
-Pleated Sheet
• -pleated sheet with three polypeptide chains running
in opposite directions (antiparallel).
27-69
Tertiary Structure
 Tertiary
structure: The three-dimensional
arrangement in space of all atoms in a single
polypeptide chain.
• Disulfide bonds between the side chains of cysteine
play an important role in maintaining 3° structure.
27-70
Tertiary Structure
• A ribbon model of myoglobin.
27-71
Quaternary Structure
 Quaternary
structure: The arrangement of
polypeptide chains into a noncovalently bonded
aggregation.
• The major factor stabilizing quaternary structure is the
hydrophobic effect.
 Hydrophobic
effect: The tendency of nonpolar
groups to cluster together in such a way as to be
shielded from contact with an aqueous
environment.
27-72
Quaternary Structure
• The quaternary structure of hemoglobin. The -chains
in yellow, the heme ligands in red, and the Fe atoms as
white spheres.
27-73
Quaternary Structure
• If two polypeptide chains, for example, each have one
hydrophobic patch, each patch can be shielded from
contact with water if the chains form a dimer.
Pro tein
N umber o f
Subun its
Al coho l dehyd rog enase 2
Al dol ase
4
Hemog lo bin
4
Lactate d eh ydrog enase 4
Insul in
6
Glu tamine sy nthetase 12
Tobacco mosai c vi rus
17
protei n d isc
27-74
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