PROTEINSPages 42 to 46

PROTEINS Chemical composition

CarbonHydrogen OxygenNitrogenSulfur (sometimes)

Monomer/Building BlockAmino Acids (20 different amino acids)

Table 5-1

Fig. 5-UN1

Aminogroup

Carboxylgroup

carbon

Peptidebond

Fig. 5-18

Amino end(N-terminus)

Peptidebond

Side chains

Backbone

Carboxyl end(C-terminus)

(a)

(b)

A peptide bond is the bond joining adjacent

amino acids.

4 LEVELS OF ORGANIZATION OF A PROTEIN Primary - peptide bond joining adjacent

amino acids Secondary - Hydrogen bonding between

nonadjacent amino acids that creates an alpha helix or pleated sheets

Tertiary - bond formation between R-groups; clustering of hydrophobic (nonpolar) or hydrophilic (polar) R-groups, disulfide bridges, ionic bonding, grouping based on pH, etc… that results in 3-dimensional folding

Quaternary – joining of more than one polypeptide (not all proteins have this level)

Fig. 5-21

PrimaryStructure

SecondaryStructure

TertiaryStructure

pleated sheet

Examples ofamino acidsubunits

+H3N Amino end

helix

QuaternaryStructure

Fig. 5-21b

Amino acidsubunits

+H3N Amino end

Carboxyl end125

120

115

110

105

100

95

9085

80

75

20

25

15

10

5

1

Fig. 5-21c

Secondary Structure

pleated sheet

Examples ofamino acidsubunits

helix

Fig. 5-21f

Polypeptidebackbone

Hydrophobicinteractions andvan der Waalsinteractions

Disulfide bridge

Ionic bond

Hydrogenbond

Fig. 5-21g

Polypeptidechain

Chains

HemeIron

Chains

CollagenHemoglobin

IMPORTANCE OF STRUCTURE A slight change in primary structure can

affect a protein’s structure and ability to function

Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin

Fig. 5-22

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Normalhemoglobin(top view)

Primarystructure

Secondaryand tertiarystructures

Quaternarystructure

Function Function

subunit

Molecules donot associatewith oneanother; eachcarries oxygen.

Red bloodcell shape

Normal red bloodcells are full ofindividualhemoglobinmoledules, eachcarrying oxygen.

10 µm

Normal hemoglobin

1 2 3 4 5 6 7

ValHisLeuThrProGluGlu

Red bloodcell shape

subunit

Exposedhydrophobicregion

Sickle-cellhemoglobin

Moleculesinteract withone another andcrystallize intoa fiber; capacityto carry oxygenis greatly reduced.

Fibers of abnormalhemoglobin deformred blood cell intosickle shape.

10 µm

Sickle-cell hemoglobin

GluProThrLeuHisVal Val

1 2 3 4 5 6 7