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Amino Acids, Peptides, Amino Acids, Peptides, and Proteins.and Proteins.
Classification of Amino AcidsClassification of Amino Acids
FundamentalsFundamentals
While their name implies that amino acids are While their name implies that amino acids are compounds that contain an —NHcompounds that contain an —NH2 2 group and a group and a
—CO—CO22H group, these groups are actually H group, these groups are actually
present as —NHpresent as —NH33++ and —CO and —CO22
–– respectively. respectively.
They are classified as They are classified as , , , , , , etcetc. amino acids . amino acids according the carbon that bears the nitrogen.according the carbon that bears the nitrogen.
Amino AcidsAmino Acids
NNHH33
++
CCOO22––
an an -amino acid that is an-amino acid that is anintermediate in the biosynthesisintermediate in the biosynthesisof ethyleneof ethylene
++HH33NNCHCH22CHCH22CCOO22
––a a -amino acid that is one of-amino acid that is one ofthe structural units present inthe structural units present incoenzyme Acoenzyme A
++HH33NNCHCH22CHCH22CHCH22CCOO22
–– a a -amino acid involved in-amino acid involved inthe transmission of nervethe transmission of nerveimpulsesimpulses
The 20 Key Amino AcidsThe 20 Key Amino Acids
More than 700 amino acids occur naturally, but More than 700 amino acids occur naturally, but 20 of them are especially important.20 of them are especially important.
These 20 amino acids are the building blocks of These 20 amino acids are the building blocks of proteins. All are proteins. All are -amino acids.-amino acids.
They differ in respect to the group attached to They differ in respect to the group attached to the the carbon. carbon.
These 20 are listed in Table 27.1 (p 1054-1055).These 20 are listed in Table 27.1 (p 1054-1055).
Table 27.1Table 27.1
CC CC
OO
OO––
RRRR
HH
HH33NN++
The amino acids obtained by hydrolysis of The amino acids obtained by hydrolysis of proteins differ in respect to proteins differ in respect to R R (the side chain).(the side chain).
The properties of the amino acid vary as the The properties of the amino acid vary as the structure of structure of RR varies. varies.
Table 27.1Table 27.1
CC CC
OO
OO––
HH
HH
HH33NN++
Glycine is the simplest amino acid. It is the only Glycine is the simplest amino acid. It is the only one in the table that is achiral.one in the table that is achiral.
In all of the other amino acids in the table the In all of the other amino acids in the table the carbon is a stereogenic center.carbon is a stereogenic center.
GlycineGlycine
(Gly or G)(Gly or G)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH33
HH
HH33NN++
AlanineAlanine
(Ala or A)(Ala or A)
Table 27.1Table 27.1
CC CC
OO
OO––
CH(CHCH(CH33))22
HH
HH33NN++
ValineValine
(Val or V)(Val or V)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH22CH(CHCH(CH33))22
HH
HH33NN++
LeucineLeucine
(Leu or L)(Leu or L)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH33CHCHCHCH22CHCH33
HH
HH33NN++
IsoleucineIsoleucine
(Ile or I)(Ile or I)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH33SSCHCH22CHCH22
HH
HH33NN++
MethionineMethionine
(Met or M)(Met or M)
Table 27.1Table 27.1
ProlineProline
CC CC
OO
OO––
CHCH22
HH
HH22NN++
HH22CCCCHH22
(Pro or P)(Pro or P)
Table 27.1Table 27.1
PhenylalaninePhenylalanine
CC CC
OO
OO––
CHCH22
HH
HH33NN++
(Phe or F)(Phe or F)
Table 27.1Table 27.1
TryptophanTryptophan
CC CC
OO
OO––
CHCH22
HH
HH33NN++
NN
HH
(Trp or W)(Trp or W)
Table 27.1Table 27.1
AsparagineAsparagine
CC CC
OO
OO––
HH
HH33NN++
HH22NCCHNCCH22
OO
(Asn or N)(Asn or N)
Table 27.1Table 27.1
GlutamineGlutamine
CC CC
OO
OO––
HH
HH33NN++
HH22NCCHNCCH22CHCH22
OO
(Gln or Q)(Gln or Q)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH22OHOH
HH
HH33NN++
SerineSerine
(Ser or S)(Ser or S)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH33CHOHCHOH
HH
HH33NN++
ThreonineThreonine
(Thr or T)(Thr or T)
Table 27.1Table 27.1
Aspartic AcidAspartic Acid
CC CC
OO
OO––
HH
HH33NN++
OCCHOCCH22
OO
––
(Asp or D)(Asp or D)
Table 27.1Table 27.1
Glutamic AcidGlutamic Acid
CC CC
OO
OO––
HH
HH33NN++
OCCHOCCH22CHCH22
OO
––
(Glu or E)(Glu or E)
Table 27.1Table 27.1
TyrosineTyrosine
CC CC
OO
OO––
CHCH22
HH
HH33NN++
OHOH
(Tyr or Y)(Tyr or Y)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH22SSHH
HH
HH33NN++
CysteineCysteine
(Cys or C)(Cys or C)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH22CHCH22CHCH22CHCH22NNHH33
HH
HH33NN++
LysineLysine
++
(Lys or K)(Lys or K)
Table 27.1Table 27.1
CC CC
OO
OO––
CHCH22CHCH22CHCH22NNHCHCNNHH22
HH
HH33NN++
ArginineArginine
++ NNHH22
(Arg or R)(Arg or R)
Table 27.1Table 27.1
HistidineHistidine
CC CC
OO
OO––
HH
HH33NN++
CHCH22
NHNHNN
(His or H)(His or H)
27.227.2Stereochemistry of Amino AcidsStereochemistry of Amino Acids
Configuration of Configuration of -Amino Acids-Amino Acids
Glycine is achiral. All of the other amino acids Glycine is achiral. All of the other amino acids in proteins have the in proteins have the LL-configuration at their -configuration at their carbon.carbon.
HH33NN++
HH
RR
COCO22––
27.727.7PeptidesPeptides
PeptidesPeptides
Peptides are compounds in which an amide Peptides are compounds in which an amide bond links the amino group of one bond links the amino group of one -amino acid -amino acid and the carboxyl group of another.and the carboxyl group of another.
An amide bond of this type is often referred to An amide bond of this type is often referred to as a peptide bond.as a peptide bond.
Alanine and GlycineAlanine and Glycine
CHCH33
OO
CC++
HH
CC OO––
HH33NN
OO
CC
HH
HH
CCHH33NN++
OO––
AlanylglycineAlanylglycine
CHCH33
OO
CCHH33NN++
HH
CC
OO
CCNN
HH
HH
CC OO––
HH
Two Two -amino acids are joined by a peptide bond -amino acids are joined by a peptide bond in alanylglycine. It is a in alanylglycine. It is a dipeptidedipeptide..
AlanylglycineAlanylglycine
CHCH33
OO
CCHH33NN++
HH
CC
OO
CCNN
HH
HH
CC OO––
HH
Ala—GlyAla—Gly
AGAG
N-terminusN-terminus C-terminusC-terminus
Alanylglycine and glycylalanine are Alanylglycine and glycylalanine are constitutional isomersconstitutional isomers
CHCH33
OO
CCHH33NN++
HH
CC
OO
CCNN
HH
HH
CC OO––
HH
HH
OO
CCHH33NN++
HH
CC
OO
CCNN
HH
CHCH33
CC OO––
HH
AlanylAlanylglycineglycineAlaAla——GlyGly
AAGG
GlycylGlycylalaninealanineGlyGly——AlaAla
GGAA
AlanylglycineAlanylglycine
CHCH33
OO
CCHH33NN++
HH
CC
OO
CCNN
HH
HH
CC OO––
HH
The peptide bond is The peptide bond is characterized by a characterized by a planar geometry.planar geometry.
Higher PeptidesHigher Peptides
Peptides are classified according to the number Peptides are classified according to the number of amino acids linked together.of amino acids linked together.
dipeptides, tripeptides, tetrapeptides, etc.dipeptides, tripeptides, tetrapeptides, etc.
Leucine enkephalin is an example of a Leucine enkephalin is an example of a pentapeptide.pentapeptide.
Leucine EnkephalinLeucine Enkephalin
Tyr—Gly—Gly—Phe—LeuTyr—Gly—Gly—Phe—LeuYGGFLYGGFL
OxytocinOxytocin
Oxytocin is a cyclic nonapeptide.Oxytocin is a cyclic nonapeptide.
Instead of having its amino acids linked in an Instead of having its amino acids linked in an extended chain, two cysteine residues are extended chain, two cysteine residues are joined by an joined by an S—S S—S bond.bond.
N-terminusN-terminus
C-terminusC-terminusIle—Gln—AsnIle—Gln—Asn
TyrTyr
CysCys SS SS
Cys—Pro—Leu—GlyNHCys—Pro—Leu—GlyNH22
11
22
3344 55
66 77 88 99
OxytocinOxytocin
S—S bond
An S—S bond between two cysteines isAn S—S bond between two cysteines isoften referred to as a often referred to as a disulfide bridgedisulfide bridge..
