Download Amino Acids and Peptides: Properties, Stereochemistry, and Biological Significance and more Study notes Biology in PDF only on Docsity! 1 Amino Acids and Peptides Dr. Leisha Mullins Aug. 29, 2003 2 Amino Acids • Amino Acids a compound that contains both an amino group and a carboxyl group § α-Amino acid: an amino acid in which the amino group is on the carbon adjacent to the carboxyl group • All proteins are composed of 20 standard amino acids • Amino acids have acid/ base properties • Amino acids are zwitterionic • At pH 7, the amino group is protonated and the carboxylic group is unprotonated 5 Chirality of Life • Biosynthetic processes produce pure stereoisomers • Chemical processes generally produce racemic mixtures • Most active biological compounds are chiral i.e. only one enantiomer imparts biological activity 20 Standard Amino Acids
TABLE 3.1. Names and Abbreviations of the Common Amino Acids
Amino Acid Three-Letter Abbreviation. One-Letter Abbreviation
Alanine Ala A
Arginine Arg R
Asparagine Asn N
Aspartic acid Asp D
Cysteine Cys Cc
Glutamic acid Glu E
Glutamine Gln Q
Glycine Gly G
Histidine His H
Isoleucine Tle I
Leucine Leu L
Lysine Lys K
Methionine Met M
Phenylalanine Phe iP
Proline Pro P
Serine Ser Ss
Threonine Thr T
Tryptophan Trp WwW
Tyrosine Tyr Y
Valine Val Vv
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7 Classification of Amino acids • The R group, or side chain, determines the structural range and general physical characteristics of the amino acids • The amino acids are generally grouped according to the various characteristics of their R groups • Non-polar amino acids (hydrophobic) • Polar, uncharged amino acids • Polar, charged 10 Polar Charged R Groups: 5 amino acids have charged side chains 11 Nomenclature • All amino acids have 1 and three letter codes • The atoms of the side chain are assigned sequential letters in the Greek alphabet beginning with the carbon next to the carboxyl group 12 Spectroscopic Properties • All amino acids absorb in infrared region • Only Phe, Tyr, and Trp absorb UV • Absorbance at 280 nm is a good diagnostic device for proteins 15 Ionizable Groups in amino acids • Amino Acids are Weak Acids • All amino acids have at least two acid – base groups – α-carboxyl group – α-amino group • Those with ionizable side chains (R groups) have three – Lysine – Arginine – Histidine – Glutamic Acid – Aspartic Acid 16 pKa Values of Free Amino Acids and in Proteins 17 Structure varies with pH • At low pH all ionizable groups are protonated – cationic form • At mid pH range the zwitterionic form dominates • At high pH the ionizable groups are unprotonated – anionic form 20 Peptides and Peptide Bonds • Amino acids polymerize to form long chains called "peptides" 21 Peptides • Individual amino acids are called amino acid residues once they are incorporated into a peptide • Polypeptides chains are described by starting at the N-terminus and proceeding to the C-terminus 22 Peptide Bond • Peptide bonds are stable – prolonged exposure to strong acid or base at elevated temperature required to hydrolyze • Peptide bond has two resonance structures Biologically Active Dipeptides
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(a) (b)
Ccoo™
|
CH, O CH, O
+ | | | ll
H;N—CH—C —N—CH—C —O—CH;
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H
L-Aspartyl-L-phenylalanine (methyl ester)
© 2003 Thomson - Wadsworth
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Biologically Active Tripeptides
NH,’ oO oO
oo I ¥ Il Il _
OCS ASG OE aN —N—CH,—COoo
H CHy H
Sulfhydryl group SH
GSH (Reduced glutathione) (yGlu—Cys—Gly)
SH
(b)
Oxidation
~2H -2¢
2GSH = GSSG
+2H +207
Reduction
Reaction of 2 GSH to give GSSG
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NH,*
“00C —CH—CH, — CH, —
NH,'
II
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n "
GSSG (Oxidized glutathione) (YGlu—Gys—Gly)
nan
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26
27 Biologically Active Pentapeptides Two naturally occuring pain relievers • Leucine Enkephalin Tyr – Gly –Gly- Phe – Leu • Methionine Enkephalin Tyr – Gly –Gly- Phe – Met
