Chapter Outline
22
5P1- 1 Chapter Outline Amino Acids Amino acid classes Stereoisomers Bioactive AA Titration of AA Modified AAAA reactions Peptides Proteins Protein structure Fibrous proteins Globular proteins
description
Chapter Outline. Amino Acids Amino acid classesStereoisomers Bioactive AATitration of AA Modified AAAA reactions Peptides Proteins Protein structure Fibrous proteins Globular proteins. 5.1 Amino Acid: Definition. Generic amino acid at physiological pH: zwitterion form. a C. - PowerPoint PPT Presentation
Transcript of Chapter Outline
5P1-1
Chapter OutlineAmino Acids
Amino acid classes StereoisomersBioactive AA Titration of AAModified AA AA reactions
PeptidesProteins
Protein structureFibrous proteinsGlobular proteins
5P1-2
5.1 Amino Acid: DefinitionAn alpha amino acid is a carboxylic acid
with an amino group on the carbon alpha to the carboxylic acid .
The alpha carbon also has an R group side chain except for glycine which has two Hs.
Generic aminoacid at physiologicalpH: zwitterion form
C
CC
RHNH3
+OO
5P1-3
Definition, cont.If the R group is not H, the AA can exist in two enantiomeric forms (nonsuperimposable mirror image) forms.)
C
C
R1
HNH3+
OO
C
C
R1
H NH3+
O O
Mirror plane carbon
5P1-4
Amino AcidsGeneral form: 1. an amino acid (AA); 2. two AA linked to form the peptide bond.
NC CR1
HNH3
+O
C CR1
HO
OH
C
C
R1
HNH3+
OO
PEPTIDE BONDL-form
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Amino Acids-2Only the L form of amino acids is
commonly found in proteins.Depending on the nature of the R
group, AA are classified into four groups.
nonpolarpolaracidicbasic
5P1-6
AA with nonpolar side chains-1
CC
CH3
HNH3+
OO
alanine, AlaCH2
CC
HNH3+
OO
phenylalanine, Phe
CC
CH2
HNH3+
OO
CHCH3 CH3
leucine,Leu
CC
CHHNH3
+OO
CH2CH3
CH3
isoleucine, I le
5P1-7
AA with nonpolar side chains-2
CC
CHHNH3
+OO
CH3 CH3CH2
CC
HNH3+
OO
CH2S CH3
CC
CH2
HNH2+
OO
CH2
CH2
valine, Val methionine, Met
proline, Pro
CC
CH2
HNH3+
OO
CCHNH
tryptophan, Trp
5P1-8
AA with polar side chains-1
CC
HHNH3
+OO
CH2
CC
HNH3+
OO
OH
CC
CH2
HNH3+
OO
SH
Glycine, Gly Serine, Ser
cysteine, Cys
CC
CHHNH3
+OO
CH3
OH
threonine, Thr
5P1-9
AA with polar side chains-2
glutamine, Gln
tyrosine, Tyr
C NH2O
CC
HNH3+
OO
CH2
C NH2O
CH2
CC
HNH3+
OO
CH2
asparagine, Asn
OHCC
CH2
HNH3+
OO
5P1-10
AA: acidic and basic
glutamic acid, GluC OO
CC
HNH3+
OO
CH2
C OO
CH2
CC
HNH3+
OO
CH2
aspartic ac id, Asp
CH2NH
CH2CH2
CNH2+ NH2
CC
HNH3+
OO
CH2CH2
CH2
CC
HNH3+
OO
CH2
NH3+
NHCH
CH2
CC
HNH3+
OO
CNH +CH
histidine, Hislysine, Lysarginine, Arg
5P1-11
Amino Acid TitrationAt physiological pH, the carboxyl
group of the AA is negatively charged and the amino group is positively charged.
Amino acids without charged side chains are zwitterions and have no net charge. H3
+N-CHR-COO-.A titration curve shows how the
amine and carboxyl groups react with hydrogen ion.
5P1-12
Amino Acid Titration-2At low pH a nonacidic/nonbasic
amino acid is protonated and has the structure below.
H3N+CHRCOOHThe charge behavior of acidic
and basic AAs is more complex.
5P1-13
Titration of Alanine
1
mole base added1 2
pH
5
10
1
NH3+CHR1
CO
OH
NH3+CHR1
CO
O
NH3+CHR1
CO
O
A
B
C
A A=BpK1=2.3
B, pI=pH=6.0
B=CpK2=9.73
C
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Isoelectric pointThe isoelectric point (pI) for an AA
occurs when there is no net charge.For a neutral AA, the pI is calculated
using the equation pK1 + pK2/2Eg.: alanine: 2.34 + 9.7 / 2 = 6.0For acidic or basic AAs, the pI is the
average of the two pKa values bracketing the isoelectric structure.
5P1-15
Isoelectric point-2In general the pI
is the average of the two pKa’s bracketing the isoelectric structure. Eg.: glutamic acid, pI = 3.2
pK1=2.2
pK2=4.3
pK3=9.9
5P1-16
5.2 PeptidesPeptide: a polymer of about 2-100 AAs linked
by the peptide(amide) bond. As the amino group and the carboxyl group link, water is lost.
C CR1
HNH3
+O
O
Peptide bonds
C CR1
HNH3
+O
OC CR1
HNH3
+O
O
C CR1
HNH3
+O
C CR1
HNH
OC CR1
HNH
OO
-H2O -H2O
5P1-17
Peptides-2A peptide is written with the N-terminal
end to the left and the C-terminal end to the right.
H2N-Tyr-Ala-Cys-Gly-COOHName = Tyrosylalanylcysteinylglycine
The peptide bond is rigid and planar due to the resonance contribution shown right. C
CO
NH
C
CC
ON
H
C
+
-
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Peptides-3The peptide bond angles force
specific conformations of proteins and, on extended chains, successive R groups are on opposite sides.
5P1-19
Physiologically Interesting Peptides
CHN
C
CNH
CH2
NH3
+CH2CH2C
O
NH CH COO-
-alanyl-L-histidine
carbon
Common name: carnosinefound in muscle tissue
5P1-20
Physiologically Interesting Peptides
SHCH2
CH CH2CH2C
O
NH CH C
OCOO-
NH3+
NH CH2COO-
-glutamyl-L-cysteineylglycine
Glutathione: the reduced formreduces oxidizing agents by dimerizing toform the disulfide bond with release of 2 H.
5P1-21
Physiologically Interesting Peptides
Tyr-Gly-Gly-Phe-LeuN-terminal AA C-terminal AA
Short form description for a peptide.Leucine enkephalin: a naturalanalgesic found in the brain
5P1-22
Physiologically Interesting Peptides
OxytocinInduces labor and aids in forcing milkfrom the mammaryglands.
Vassopressin has a Phe at position 3 instead of Ile and an Arg at position 8instead of a Leu. Its role is in regulatingblood pressure.
3
8
H3N+-Cys-Tyr-Ile
-Cys-Asn
|Gln |
|S |S
|Pro-Leu-Gly-NH2
