27-1 amino acids & proteins. 27-2 amino acids amino acid: amino acid: a compound that contains...

27-27-11

Amino Acids Amino Acids

& Proteins& Proteins

27-27-22

Amino AcidsAmino Acids

Amino acid:Amino acid: A compound that contains both an amino group and a carboxyl group.• -Amino acid:-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 zwitterionzwitterion (internal salt) form.

RCHCOH

NH2

RCHCO-

NH3+

-Amino Acid Zwitterion form

OO

27-27-33

Chirality of Amino AcidsChirality 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.

COO-

CH3

HH3N

L-Alanine

COO-

CH3

H NH3+

D-Alanine

27-27-44

Amino Acids with Nonpolar side chainsAmino Acids with Nonpolar side chains

NH3+

COO-

NH3+

COO-

NH3+

COO-

NH3+

COO-

NH3+

COO-S

NH3+

COO-

NH H

COO-

NH3+

COO-

NH

COO-

NH3+

Alanine (Ala, A)

Glycine (Gly, G)

Isoleucine (Ile, I)

Leucine (Leu, L)

Methionine (Met, M)

Phenylalanine (Phe, F)

Proline (Pro, P)

Tryptophan (Trp, W)

Valine (Val, V)

27-27-55

With Polar side chainsWith Polar side chains

NH3+

COO-H2N

O

NH3+

COO-H2N

O

NH3+

COO-HO

NH3+

COO-OH

Asparagine (Asn, N)

Glutamine (Gln, Q)

Serine (Ser, S)

Threonine (Thr, T)

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With Acidic & Basic Side ChainsWith Acidic & Basic Side Chains

NH3+

COO--O

O

NH3+

COO--O

O

NH3+

COO-HS

NH3+

COO-

HO

NH3+

COO-NH

H2N

NH2+

NH3+

COO-N

NH

NH3+

COO-H3N

Cysteine (Cys, C)

Tyrosine (Tyr, Y)

Glutamic acid (Glu, E)

Aspartic acid (Asp, D)

Histidine (His, H)

Lysine (Lys, K)

Arginine (Arg, R)

+

27-27-88

Acid-Base properties, Acid-Base properties, Non polar and polar side chainsNon polar and polar side chains

pKa ofpKa of

valine 2.29 9.72tryptophan 2.38 9.39

9.102.09threonineserine 2.21 9.15

10.602.00prolinephenylalanine 2.58 9.24

9.212.28methionine9.742.33leucine

isoleucine 2.32 9.76glycine 2.35 9.78

9.132.17glutamine8.802.02asparagine9.872.35alanine

Nonpolar &polar side chains NH3

+ COOH

27-27-99

Acid-Base Properties, Acidic/Basic Side ChainsAcid-Base Properties, Acidic/Basic Side Chains

pKa ofpKa ofpKa of

10.079.112.20tyrosine

lysine 2.18 8.95 10.536.109.181.77histidine

glutamic acid 2.10 9.47 4.078.0010.252.05cysteine

aspartic acid 2.10 9.82 3.86

arginine 2.01 9.04 12.48

Side Chain

AcidicSide Chains NH3

+COOH

pKa ofpKa ofpKa ofSide Chain

BasicSide Chains NH3

+COOH

carboxylcarboxylsufhydrylphenolic

guanidinoimidazole1° amino

SideChainGroup

SideChainGroup

27-27-1010

Acidity: Acidity: -COOH Groups -COOH Groups

The average pKa of an -carboxyl group is 2.19, which makes them considerably stronger acids than acetic acid (pKa 4.76).• The greater acidity is accounted for by the electron-

withdrawing inductive effect of the adjacent -NH3+

group.

NH3+

RCHCOOH H2O

NH3+

RCHCOO- H3O++ pKa = 2.19+

Note that the NH2 will be protonated at these low pHs

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Acidity: side chain -COOHAcidity: 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 (ppKKaa 2.35 2.35)

-COOH group of aspartic acid (ppKKaa 3.86 3.86)

-COOH group of glutamic acid (ppKKaa 4.07 4.07)

27-27-1212

Acidity: Acidity: -NH-NH33++ groups groups

The average value of pKa for an -NH3+ group is

9.47, compared with a value of 10.76 for a 1° alkylammonium ion.

NH3+

RCHCOO-

H2O

NH3+

CH3CHCH3 H2O

NH2

RCHCOO-

NH2

CH3CHCH3

H3O+

H3O+ pKa = 10.60++

+ pKa = 9.47+

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Details: The Guanidine Group of ArgDetails: 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.

CRNH

NH2+

NH2

C

NH2+

NH2

RNH

CRN

NH2

NH2

NH2

CRNH

NH2

H3O+

H2O

+

:

: :

:

::

::

+

pKa = 12.48

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Details: Imidazole GroupDetails: Imidazole Group

• The imidazole group is a heterocyclic aromatic amine.

N

NH

H

COO-

NH3+ N

N

H

H

COO-

NH3+

H2O

H3O+

H

COO-

NH3+N

NH3O+

Not a part of the aromatic sextet;the proton acceptor pKa 6.10+

+

•• +

••

••

••

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Useful Recall from buffer solutionsUseful Recall from buffer solutions

Ka = [H+] [A-]/[HA]

If Ka = [H+] then [A-] = [HA]

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Isoelectric Point Isoelectric Point

Isoelectric point (pI):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.

pI = 12 (pKa COOH + pKa NH3

+)

= 21 (2.35 + 9.78) = 6.06

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Isoelectric Point of glycine continued Isoelectric Point of glycine continued

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.

pI = 12 (pKa COOH + pKa NH3

+)

= 21 (2.35 + 9.78) = 6.06

Again

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Furthermore…..Furthermore…..

+H3N CH C

H

OH

O

+H3N CH C

H

O-

O

H2N CH C

H

O-

O

pH increases

pKa2.35 9.78

pI

[A+] = [C-]

A+ B C-

27-27-1919

Titration of conjugate acid of glycine with NaOHTitration of conjugate acid of glycine with NaOH..

Buffer solution. [-CO2H] = [-CO2

-]

Strongly acid -CO2H and -NH3

+

Isoelectric. Zero net charge.

Buffer solution. [-NH3

+]=[-NH2]

Strongly basic-CO2

- and NH2

27-27-2020

Isoelectric PointIsoelectric Point

6.115.415.656.066.046.045.745.916.305.685.605.886.00

pKa ofpKa ofpKa of

pI

----------------

----------------------------

--------

valine 2.29 9.72tryptophan 2.38 9.39

9.102.09threonineserine 2.21 9.15

10.602.00prolinephenylalanine 2.58 9.24

9.212.28methionine9.742.33leucine

isoleucine 2.32 9.76glycine 2.35 9.78

9.132.17glutamine8.802.02asparagine9.872.35alanine

Side Chain

Nonpolar &polar side chains NH3

+ COOH

If pH is lower than pI then more protonated molecules. If higher then more negative charge.

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Isoelectric PointIsoelectric Point

10.76

2.98

5.023.08

7.649.74

5.63

pKa ofpKa ofpKa ofpI

10.079.112.20tyrosine

lysine 2.18 8.95 10.536.109.181.77histidine

glutamic acid 2.10 9.47 4.078.0010.252.05cysteine

aspartic acid 2.10 9.82 3.86

arginine 2.01 9.04 12.48

Side Chain

AcidicSide Chains NH3

+ COOH

pKa ofpKa ofpKa of

pISide Chain

BasicSide Chains NH3

+ COOH

Three buffered pHs

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Aspartic acidAspartic acid

NH3+

CH

C

H2C

OH

O

CHO

O

NH3+

CH

C

H2C

O-

O

CHO

O

NH3+

CH

C

H2C

O-

O

C-O

O

NH2

CH

C

H2C

O-

O

C-O

O

pKa 2.10 3.86 9.82

A+ B C- D2-

pI = (2.10 + 3.86)/2[A+] = [C-][D2-] approx 0

[A+] = [B]pH = pKa

[B] = [C-]

Note species B has zero net charge. pKa1 and pKa2 control [A+] and [C-] which should be equal.

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ArginineArginine

+H3N CH C

CH2

OH

O

CH2

CH2

NH

C

NH2

NH2+

+H3N CH C

CH2

O-

O

CH2

CH2

NH

C

NH2

NH2+

H2N CH C

CH2

O-

O

CH2

CH2

NH

C

NH2

NH2+

2.01 9.04 12.48pKa

H2N CH C

CH2

O-

O

CH2

CH2

NH

C

NH2

NH

B+A2+ C D-

pI = (9.04+12.48)/2 =10.76[B+] = [D-]; [A2+] about 0

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ElectrophoresisElectrophoresis

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.

27-27-2525


• 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.

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• The reagent commonly used to detect amino acid is ninhydrin.

RCHCO-

O OHO

OOH

NH3+

O

O-

N

O

O

O

RCH CO2 H3O++

An -amino acid

Purple-colored anion

+ +

2+

Ninhydrin

27-27-2727

Polypeptides & ProteinsPolypeptides & 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:Peptide bond: The special name given to the amide bond between the -carboxyl group of one amino acid and the -amino group of another.

27-27-2828

Serinylalanine (Ser-Ala)Serinylalanine (Ser-Ala)

H2N HO

O

HHOCH2

Serine(Ser, S)

H2NO

OH

H CH3

Alanine(Ala, A)

+

H2NN

OH

HOCH2

H

H

CH3O

H O

Serinylalanine(Ser-Ala, (S-A)

peptide bond

27-27-2929

PeptidesPeptides

• Peptide: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:Dipeptide: A molecule containing two amino acids joined by a peptide bond.

• TripeptideTripeptide: A molecule containing three amino acids joined by peptide bonds.

• PolypeptidePolypeptide: A macromolecule containing many amino acids joined by peptide bonds.

• ProteinProtein: A biological macromolecule of molecular weight 5000 g/mol or greater, consisting of one or more polypeptide chains.

27-27-3030

Writing PeptidesWriting 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.

H3N

OH

NH O

HN

COO-

O-

OC6H5O

+

C-terminalamino acid

N-terminalamino acid

Ser-Phe-Asn

27-27-3131

Writing PeptidesWriting Peptides

• The tetrapeptide Cys-Arg-Met-As• At pH 6.0, its net charge is +1.

HN

NH O

HN

O-

OO

O

NH2

SCH3

NH

NH2+H2N

OH3N

SH C-terminalamino acid


pKa 12.48

pKa 8.00

+

At pH 8 it would be half ionized.

27-27-3232

Primary StructurePrimary Structure

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 ion-

exchange chromatography.

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Ion Exchange ChromatographyIon Exchange Chromatography

Analysis of a mixture of amino acids by ion exchange chromatography

27-27-3434

Cyanogen BromideCyanogen Bromide, BrCN, BrCN

• BrCN Selectively cleaves of peptide bonds formed by the carboxyl group of methionine.

PN-C-NH CH C NH-PC

O O

CH2

CH2-S-CH3

cyanogen bromide isspecific for the cleavageof this peptide bond

from theN-terminalend

from theC-terminal end

27-27-3535

Cyanogen Bromide, BrCNCyanogen Bromide, BrCN

O

S-CH3

HNC-NH

OpeptideCOO-

H3N peptideBr C N

H2O

O

OC-NH

O

H3N peptide

H3N peptide COO- CH3S-C N

side chainof methionine

+0.1 M HCl

A substituted -lactone ofthe amino acid homoserine

Methylthiocyanate

Mechanism to follow

27-27-3636


• Step 1: Nucleophilic displacement of bromine.

O

S-CH3

H3N C-NH

O HN COO-

Br C N

O

S-CH3

C N

H3N C-NH

O HN COO-

Br

Cyanogen bromide

a sulfonium ion; a good leaving group

: ::

27-27-3737


• Step 2: Internal nucleophilic displacement of methyl thiocyanate.

O

S-CH3

C N

HN COO-

H3N C-NH

O

O

H3N C-NH

OHN COO-

CH3-S-C N

An iminolactonehydrobromide

+

Methylthiocyanate

:

:

::

27-27-3838


• Step 3: Hydrolysis of the imino group.

O

H3N C-NH

OHN COO-

H2O

OH3N C-NH

O

O H3N COO-

A substituted -lactone ofthe amino acid homoserine

27-27-3939

Enzyme CatalysisEnzyme Catalysis

A group of protein-cleaving enzymes called proteases can be used to catalyze the hydrolysis of specific peptide bonds.

Phenylalanine, tyrosine, tryptophanChymotrypsin

Arginine, lysineTrypsin

Catalyzes Hydrolysis of Peptide Bond Formed by Carboxyl Group ofEnzyme

27-27-4040

Edman DegradationEdman Degradation

Edman degradation:Edman degradation: Cleaves the N-terminal amino acid of a polypeptide chain.

S=C=N-Ph

H2N COO-HN

NO

R

SPh

H3NNH

R

O

COO- +

+


A phenylthiohydantoin

Phenyl isothiocyanate

+

27-27-4141


• Step 1: Nucleophilic addition to the C=N group of phenylisothiocyanate and proton tautomerization

H2NNH

R

O

COO-

Ph N C S

HN

S

RO

NH COO-

Ph NH

A derivative of N-phenylthiourea

:: :

27-27-4242


• Step 2: Nucleophilic addition of sulfur to the C=O of the adjacent amide group and acid catalysis.

HN

S

RO

NH COOHPh N

H

H

HN

S

R

O

Ph-N

HN

S

R

Ph-N

O–H

NH

H

COOH

H+

H3N COOH

H+

+

A thiazolinone

+

+

+

27-27-4343


• Step 3: Isomerization of the thiazolinone ring.

HN

S

R

O

Ph-N

+ H-Nu

R

HN

N SH Nu

O

Ph

R

HN

S NH

Ph

Nu

O - H-NuHN

N

R

O

SPh

A thiazolinone

A phenylthiohydantoin

keto-enoltautomerism

substitution

27-27-4444

DNFB tagging of N terminal AADNFB tagging of N terminal AA

F

NO2

NO2

H2N

R

O

DNFB

HN

R

NO2

O2N O

hydrolysisHN

R

OH

NO2

O2N O

plus unlabeled AA

27-27-4545

Carboxypeptidase cleavage of C terminal AACarboxypeptidase 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.

27-27-4646

Primary Structure, examplePrimary Structure, example

Deduce the 1° structure of this pentapeptide

pentapeptideEdman Degradation

Hydrolysis - Chymotrypsin

Fragment AFragment B

Hydrolysis - TrypsinFragment CFragment D

Arg, Glu, His, Phe, SerGlu

Glu, His, PheArg, Ser

Arg, Glu, His, Phe

Ser

Experimental ProcedureAmino Acid Composition

Phenylalanine, tyrosine, tryptophanChymotrypsin

Arginine, lysineTrypsin

Catalyzes Hydrolysis of Peptide Bond Formed by Carboxyl Group ofEnzyme

using

Glu-

Glu-His-Phe

Glu-His-Phe-Arg-Ser

27-27-4747

Polypeptide SynthesisPolypeptide 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.

aa1

H3NCHCO-

O

aa2

H3NCHCO-

O

aa1 aa2

H3NCHCNHCHCO-

OOH2O

? +++

++

27-27-4848

Polypeptide Synthesis StrategyPolypeptide Synthesis Strategy

• Protect the -amino group of aa-1.• Activate the -carboxyl group of aa-1.• Protect the -carboxyl group of aa-2.

+

+

form peptide bond

protectinggroup

activatinggroup

protectinggroup

O O

aa2aa1

Z-NHCHC-Y H2NCHC-X

Z-NHCHCNHCHC-X H-Y

O Oaa1 aa2

27-27-4949

Amino-ProtectionAmino-Protection

• convert them to amides.

O

(CH3)3COCOCOC(CH3)3

O O

(CH3)3COC-

O

PhCH2OC-

O

PhCH2OCCl

Di-tert-butyl dicarbonate

Benzyloxycarbonylchloride

tert-Butoxycarbonyl (BOC-) group

Benzyloxycarbonyl(Z-) group

27-27-5050

Amino-ProtectionAmino-Protection

• Treatment of an amino group with either of these reagents gives a carbamate (an ester of the monoamide of carbonic acid).

• A carbamate is stable to dilute base but can be removed by treatment with HBr in acetic acid.

PhCH2OCNH-peptide

OHBr

CH3COOHPhCH2Br CO2 H3N-peptide

A Z-protectedpeptide

+ ++

Benzylbromide

Unprotectedpeptide

PhCH2OCClO

CH3

H3NCHCO-

O 1. NaOH

2. HCl, H2OCH3

PhCH2OCNHCHCOHO O

Alanine N-Benzyloxycarbonylalanine(Z-Ala)

++

Benzyloxycarbonylchloride (Z-Cl)

27-27-5151

Amino-Protecting GroupsAmino-Protecting Groups

The benzyloxycarbonyl group is also removed by hydrogenolysis.• The intermediate carbamic acid loses carbon dioxide

to give the unprotected amino group.

PhCH2OCNH-peptide

OH2

PdPhCH3 CO2 H2N-peptide

A Z-protectedpeptide

Unprotectedpeptide

Toluene+++

27-27-5252

Carboxyl-Protecting Groups. EstersCarboxyl-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

27-27-5353

Peptide Bond FormationPeptide 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.

1,3-Dicyclohexylcarbodiimide (DCC)

+

N,N' -dicyclohexylurea (DCU)

C NN

H

N NC

H

O

H2O

In bringing about formation of a peptide bond, DCC acts a dehydrating agent.

27-27-5454

Peptide Bond FormationPeptide Bond Formation

Carboxyl-protectedaa2

Amino-protectedaa1

++CHCl3

Z-NHCHC-OH H2 NCHCOCH3

Amino and carboxyl protected dipeptide

+Z-NHCHC-NHCHCOCH3

DCC

DCU

R1 R2

R1 R2

O O

O O

27-27-5555

Solid-Phase SynthesisSolid-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.

27-27-5656


The solid support used in the Merrifield solid phase synthesis.

27-27-5757

27-27-5858


• Merrifield synthesized the enzyme ribonuclease, a protein containing 124 amino acids

27-27-5959

Peptide Bond GeometryPeptide 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.

27-27-6060


• 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.

C

C

N

H

C

C

COO

C N

H

+

-: :

:

: : :

27-27-6161


• 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

C

O

C N

H

• • ••

••

CC

O

C N

H• • ••

••

s-trans s-cis

27-27-6262

Secondary StructureSecondary Structure

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.

27-27-6363


• Hydrogen bonding between amide groups.

27-27-6464


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-Helix:: A type of secondary structure in which a

section of polypeptide chain coils into a spiral, most commonly a right-handed spiral.

27-27-6565

The The -Helix-Helix

• The polypeptide chain is repeating units of L-alanine.

H bonds (C=O…H) parallel to axis of helix

27-27-6666


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.

27-27-6767


An -helix of repeating units of L-alanine • A ball-and-stick model

viewed looking down the axis of the helix.

• A space-filling model viewed from the side.

27-27-6868

-Pleated Sheet-Pleated Sheet

The antiparallel -pleated sheet consists of adjacent polypeptide chains running in opposite directions:• 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.

27-27-6969

-Pleated Sheet-Pleated Sheet

• -pleated sheet with three polypeptide chains running in opposite directions (antiparallel).

27-27-7070

Tertiary StructureTertiary Structure

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.

27-27-7171

Tertiary StructureTertiary Structure

• A ribbon model of myoglobin.

27-27-7272

Quaternary StructureQuaternary Structure

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:Hydrophobic effect: The tendency of nonpolar groups to cluster together in such a way as to be shielded from contact with an aqueous environment.

27-27-7373


• The quaternary structure of hemoglobin. The -chains in yellow, the heme ligands in red, and the Fe atoms as white spheres.

27-27-7474


• 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.

ProteinNumber ofSubunits

Insulin

Hemoglobin

Alcohol dehydrogenase

Lactate dehydrogenase

Aldolase

Glutamine synthetaseTobacco mosaic virus protein disc

6

4

2

4

4

1217

27-1 amino acids & proteins. 27-2 amino acids amino acid: amino acid: a compound that contains...

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