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Proteins

Proteins . 16.3 Rx of Amino Acids. Isoelectric Point (IEP). pH where AA is a Zwitterion and overall neutral: positive and negative charges equal each other out. Carboxyl is COO- -1 Amino group is NH3+ +1 Net charge 0

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Proteins

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  1. Proteins 16.3 Rx of Amino Acids

  2. Isoelectric Point (IEP) • pH where AA is a Zwitterion and overall neutral: positive and negative charges equal each other out. • Carboxyl is COO- -1 • Amino group is NH3+ +1 • Net charge 0 Use the textbook (pg 536) and fill the IEP into your AA sheet….

  3. Charge of AA at specific pH’s below IEP at IEP above IEP [H+] [H+] [H+] positive neutral negative

  4. IEP pH ranges • Polar neutral and non-polar AA: IEP 5 - 6 • Acidic AA: IEP ≈ 3(higher [H+] keeps sidechain protonated as COOH • Basic AA: IEP ≈ 7.0-11.0 (lower [H+] prevents sidechain from protonation, stays NH2)

  5. Charge of an AA at specific pH’s • What is the charge of Isoleucine (Ile) at: • pH 4.0 • pH 6.0 • pH 10 IEP (Ile) = 6.0 (non-polar AA) Means it’s neutral at exactly pH=6! a.pH 4: Positive +1 b.pH 6.0: Neutral 0 c. pH10: Negative -1

  6. Acidic AA at different pH’s • What is the charge of Aspartic Acid (Asp) at • pH 2 • pH 2.8 • pH 6 IEP is 2.8 it’s a non-polar AA Means it’s neutral at exactly pH=2.8! a.pH 2: Positive +1 b.pH 2.8: neutral b.pH 6: Negative -1 or negative -2 (We can’t tell: need to know the dissociation constant of both acid groups)

  7. Basic AA at specific pH’s • What is the charge of Arginine (Arg)at • pH 6 • pH 10.8 • pH 11 IEP is 10.8 Means it’s neutral at exactly pH=10.8! a.pH 6: Positive +1 or +2 (don’t know kPa for the two aminogroups) b.pH 10.8: Neutral c. pH11: Negative -1

  8. 16.4 Formation of Peptide

  9. Peptide bonds • Carboxyl and Amino groups react in a Dehydration Rx • Amide is formed called a Peptide bond → Dipeptide • Order of AA is important

  10. Glycine + Alanine: Glycylalanine Alanine + Glycine: Alanylglycine

  11. Naming Peptides • N-terminal AA and center AA: Replace –ine with –yl • C-terminal AA name stays same • Example: Glycine Valine Alanine becomes: Glycylvalylalanine 1.2.3. • Name reflects number and order of AA • Defines structure and function • Name all the other possible versions of how Gly, Val, Ala can be linked together…. Glycylalanylvaline, Valylglycylalanine, Valylalanylglycine Alanylglycylvaline, Alanylvalylglycine

  12. 1o Structure of Peptides • Beginning and end: N- and C-Terminus • Peptide backbone: central core of peptide bonds: • NCCNCCN…..

  13. Peptide Drawing Guide Draw glycylalanylserine • # of AA? 3 • Draw Peptide backbone: 3 x N-C-C N-C-C-N-C-C-N-C-C 3. Attach R-side chains to respective central carbon OH H CH3 CH2 N-C-C-N-C-C-N-C-C

  14. Finish by adding H/O to Peptide Backbone OH H CH3 CH2 H2N-C-C-N-C- C-N-C-COOH H O H H O H H • Finish N/C terminus • Add H to central carbon • Add Carbonyl to Carboxyl carbon • Add H to Nitrogen

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