Amino Acids, Peptides, and Proteins.

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Amino Acids, Peptides, and Proteins.
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Classification of Amino Acids
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Fundamentals

• While their name implies that amino acids are
  compounds that contain an NH2 group and a CO2H
  group, these groups are actually present as NH3
  and CO2 respectively.
• They are classified as a, b, g, etc. amino acids
  according the carbon that bears the nitrogen.

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Amino Acids
an a-amino acid that is anintermediate in the
biosynthesisof ethylene
a
a b-amino acid that is one ofthe structural
units present incoenzyme A
b
a g-amino acid involved inthe transmission of
nerveimpulses
g
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The 20 Key Amino Acids

• More than 700 amino acids occur naturally, but 20
  of them are especially important.
• These 20 amino acids are the building blocks of
  proteins. All are a-amino acids.
• They differ in respect to the group attached to
  the a carbon.
• These 20 are listed in Table 27.1 (p 1054-1055).

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Table 27.1

• The amino acids obtained by hydrolysis of
  proteins differ in respect to R (the side chain).
• The properties of the amino acid vary as the
  structure of R varies.

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Table 27.1
Glycine
(Gly or G)

• Glycine is the simplest amino acid. It is the
  only one in the table that is achiral.
• In all of the other amino acids in the table the
  a carbon is a stereogenic center.

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Table 27.1
O
H


O
H3N
C
C
CH3
Alanine
(Ala or A)
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Table 27.1
O
H


O
H3N
C
C
CH(CH3)2
Valine
(Val or V)
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Table 27.1
O
H


O
H3N
C
C
CH2CH(CH3)2
Leucine
(Leu or L)
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Table 27.1
O
H


O
H3N
C
C
CH3CHCH2CH3
Isoleucine
(Ile or I)
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Table 27.1
O
H


O
H3N
C
C
CH3SCH2CH2
Methionine
(Met or M)
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Table 27.1
Proline
(Pro or P)
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Table 27.1
Phenylalanine
(Phe or F)
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Table 27.1
Tryptophan
(Trp or W)
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Table 27.1
Asparagine
(Asn or N)
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Table 27.1
Glutamine
(Gln or Q)
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Table 27.1
O
H


O
H3N
C
C
CH2OH
Serine
(Ser or S)
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Table 27.1
O
H


O
H3N
C
C
CH3CHOH
Threonine
(Thr or T)
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Table 27.1
Aspartic Acid
(Asp or D)
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Table 27.1
Glutamic Acid
(Glu or E)
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Table 27.1
Tyrosine
(Tyr or Y)
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Table 27.1
O
H


O
H3N
C
C
CH2SH
Cysteine
(Cys or C)
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Table 27.1
O
H


O
H3N
C
C

CH2CH2CH2CH2NH3
Lysine
(Lys or K)
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Table 27.1
O
H


O
H3N
C
C
CH2CH2CH2NHCNH2

NH2
Arginine
(Arg or R)
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Table 27.1
O
H


O
H3N
C
C
Histidine
(His or H)
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27.2Stereochemistry of Amino Acids
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Configuration of a-Amino Acids

• Glycine is achiral. All of the other amino acids
  in proteins have the L-configuration at their a
  carbon.

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27.7Peptides
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Peptides

• Peptides are compounds in which an amide bond
  links the amino group of one a-amino acid and the
  carboxyl group of another.
• An amide bond of this type is often referred to
  as a peptide bond.

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Alanine and Glycine
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Alanylglycine

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

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Alanylglycine
N-terminus
C-terminus
AlaGly
AG
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Alanylglycine and glycylalanine are
constitutional isomers
Alanylglycine AlaGly AG
Glycylalanine GlyAla GA
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Alanylglycine

• The peptide bond is characterized by a planar
  geometry.

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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.

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Leucine Enkephalin
TyrGlyGlyPheLeuYGGFL
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Oxytocin
C-terminus
N-terminus

• Oxytocin is a cyclic nonapeptide.
• Instead of having its amino acids linked in an
  extended chain, two cysteine residues are joined
  by an SS bond.

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Oxytocin
SS bond
An SS bond between two cysteines isoften
referred to as a disulfide bridge.