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27.7 Peptides. Peptides. Peptides are compounds in which an amide bond links the amino group of one  -amino acid and the carboxyl group of another. An amide bond of this type is often referred to as a peptide bond. H. H. O. O. +. +. –. –. O. O. H 3 N. H 3 N. C. C. C. C. CH 3.

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27 7 peptides
27.7Peptides


Peptides
Peptides

  • Peptides are compounds in which an amide bond links the amino group of one -amino acid and the carboxyl group of another.

  • An amide bond of this type is often referred to as a peptide bond.


Alanine and glycine

H

H

O

O

+

+

O

O

H3N

H3N

C

C

C

C

CH3

H

Alanine and Glycine


Alanylglycine

H

H

O

O

+

H3N

N

O

C

C

C

C

CH3

H

H

Alanylglycine

  • Two -amino acids are joined by a peptide bond in alanylglycine. It is a dipeptide.


Alanylglycine1

H

H

O

O

+

H3N

N

O

C

C

C

C

CH3

H

H

Alanylglycine

C-terminus

N-terminus

Ala—Gly

AG


Alanylglycine and glycylalanine are constitutional isomers

H

H

O

O

+

H3N

N

O

C

C

C

C

CH3

H

H

H

H

O

O

+

H3N

N

O

C

C

C

C

H

H

CH3

Alanylglycine and glycylalanine are constitutional isomers

Alanylglycine

Ala—Gly

AG

Glycylalanine

Gly—Ala

GA


Alanylglycine2

H

H

O

O

+

H3N

N

O

C

C

C

C

CH3

H

H

Alanylglycine

  • The peptide bond is characterized by a planar geometry.


Higher peptides
Higher Peptides

  • Peptides are classified according to the number of amino acids linked together.

  • dipeptides, tripeptides, tetrapeptides, etc.

  • Leucine enkephalin is an example of a pentapeptide.


Leucine enkephalin
Leucine Enkephalin

Tyr—Gly—Gly—Phe—LeuYGGFL


Oxytocin

4

5

3

Ile—Gln—Asn

2

Tyr

Cys—Pro—Leu—GlyNH2

6

7

8

9

1

Cys

S

S

Oxytocin

  • Oxytocin is a cyclic nonapeptide.

  • Instead of having its amino acids linked in an extended chain, two cysteine residues are joined by an S—S bond.

C-terminus

N-terminus


Oxytocin1
Oxytocin

S—S bond

An S—S bond between two cysteines isoften referred to as a disulfide bridge.


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