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Amino Acids, Peptides, Protein Primary Structure

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Presentation on theme: "Amino Acids, Peptides, Protein Primary Structure"— Presentation transcript:

1 Amino Acids, Peptides, Protein Primary Structure
Chapter 3

2 Amino Acids Basic structural units of proteins
All have 3 common functional grps: -NH2, -COOH, -H Individual aa’s each have diff R grp These 4 grps att’d to a C Is this a chiral C? At neutral pH, exist as dipole (zwitterion) Amino grp as NH3+ Carboxyl grp as COO-

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5 Chiral aC, so have D,L stereo-isomers
L form aa’s polymer-ize  prot’s

6 Side chains vary in size, shape, charge, reactivity, H-bond capacity
Five groups of aa’s, based on R grp similarities Some notes: Glycine – only optically inactive aa Cysteine – highly reactive sulfhydryl grp Histidin – R grp may be proton donor or acceptor at physio pH

7 1) Nonpolar w/ aliphatic R grps
Glycine (Gly, G) Alanine (Ala, A) Proline (Pro, P) Valine (Val, V) Leucine (Leu, L) Isoleucine (Ile, I) Methionine (Met, M)

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9 2) Aromatic R grps Phenylalanine (Phe, F) Tyrosine (Tyr, Y)
Tryptophan (Trp, W) Hydrophobic

10 3) Polar w/ uncharged R grps
Serine (Ser, S) Threonine (Thr, T) Cysteine (Cys, C) Asparagine (Asn, N) Glutamine (Gln, Q)

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12 4) Polar w/ + charged R grps at physio pH
Lysine (Lys, K) Histidine (His, H) Arginine (Arg, R)

13 5) Polar w/ - charged R grps at physio pH
Aspartate (Asp, D) Glutamate (Glu, E)

14 Cysteine/Cystine Reactive SH grp of cys oxidizes  disulfide bond
Forms cystine Hydrophobic mol Impt to protein 3D structure

15 Amino Acid Titration Curves
Aa’s – weak acids Construct titration curves for each REMEMBER: Add OH-, measuring change in pH as titrate w/ OH-. Plot OH- added on x axis vs. pH on y axis Have 2 abstractable H’s, both on grps att’d to a C (bottom p. 81) One on carboxyl grp One on amino grp

16 At midpoint of titration ([OH-]=1 eq), cmpd fully dipolar
2 inflection pts Shape of each inflection sim to inflection seen w/ monoprotic acid (Chpt 2) Each aa has 2 pKa’s At midpoint of titration ([OH-]=1 eq), cmpd fully dipolar No net electrical charge “Isoelectric point” Isoelectric pH = pI Each aa has characteristic pI At any pH<pI, aa has net + charge At any pH>pI, aa has net - charge

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18 pKa1 <<<< pKa2
First H+ released from aa is much more easily given up than second H+ 2 pKa’s = 2 regions of buffering capacity Aa’s w/ ionizable R grps (lys, arg, his) have 3rd pKa

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21 Peptide Bonds Links two aa’s
 Dipeptide Condensation rxn; H2O removed Endothermic rxn Stable under physio cond’s; broken w/ boiling in strong acid/base a carboxyl of aa1 joined to a amino of aa2 In living systems, peptide bond form’n assisted by ribosomes in translation process

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23 Oligopeptide = several aa’s joined by peptide bonds
Polypeptide = many aa’s = small protein Protein commonly MW > 10,000 Aa residue of peptide w/ free amino grp called amino terminus Aa residue of peptide w/ free carboxyl grp = carboxy terminus

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25 At neutral pH, peptides have 1 free NH3+ and 1 free COO-
BUT R grps on each aa may be ionized Each peptide has characteristic pI Peptide ionization = sum of all R grp charges of aa’s which make up the peptide


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