Organic Chemistry

Section 23.7 Peptide Bonds and Disulfide Bonds 975

oxidation

reduction

polypeptide disulfide bridges

cross-linking portions

of a polypeptide

SH

SH SH

SH

SH

HS S

S

S

S

SS

>Figure 23.7

Disulfide bridges cross-linking

portions of a peptide.

HAIR: STRAIGHT OR CURLY?

Hair is made up of a protein known as keratin.

Keratin contains an unusually large number of

cysteine residues (about 8% of the amino acids), which give it

many disulfide bridges to maintain its three-dimensional struc-

ture. People can alter the structure of their hair (if they feel that

it is either too straight or too curly) by changing the location of

these disulfide bridges. This is accomplished by first applying

a reducing agent to the hair to reduce all the disulfide bridges

straight hair

S

S

S

S

S

S

S

S

S

S

S

S

S S

S

S

SS

curly hair

Insulin, a hormone secreted by the pancreas, controls the level of glucose in the

blood by regulating glucose metabolism. Insulin is a polypeptide with two peptide

chains. The short chain (the A-chain) contains 21 amino acids and the long chain (the

B-chain) contains 30 amino acids. The two chains are held together by two disulfide

bridges. These are interchain disulfide bridges(between the A- and B-chains). In-

sulin also has an intrachain disulfide bridge(within the A-chain).

S S

S S

A-chainGly Ile Val GluGlnCys CysThrSerIleCysSerLeuTyrGlnLeuGluAsnTyrCysAsn

B-chainPhe Val Asn GlnHisLeu CysGlySerHisLeuValGluAlaLeuTyrLeuValCysGlyGluArgGlyPhePheTyr Thr Pro LysAla

an intrachain disulfide bridge

SS

interchain disulfide bridges

insulin

in the protein strands. Then the hair is given the desired shape

(using curlers to curl it or combing it straight to uncurl it), and

an oxidizing agent is applied that forms new disulfide bridges.

The new disulfide bridges maintain the hair’s new shape.

When this treatment is applied to straight hair, it is called a

“permanent.”When it is applied to curly hair, it is called “hair

straightening.”

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