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Chapter Outline

Chapter Outline. Amino Acids Amino acid classes Stereoisomers Bioactive AA Titration of AA Modified AA AA reactions Peptides Proteins Protein structure Fibrous proteins Globular proteins. 5.1 Amino Acid: Definition. Generic amino acid at physiological pH: zwitterion form. a C.

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Chapter Outline

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  1. Chapter Outline • Amino Acids • Amino acid classes Stereoisomers • Bioactive AA Titration of AA • Modified AA AA reactions • Peptides • Proteins • Protein structure • Fibrous proteins • Globular proteins

  2. 5.1 Amino Acid: Definition Generic amino acid at physiological pH: zwitterion form aC • An 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.

  3. Definition, cont. a carbon Mirror plane • If the R group is not H, the AA can exist in two enantiomeric forms (nonsuperimposable mirror image) forms.)

  4. Amino Acids • General form: 1. an amino acid (AA); 2. two AA linked to form the peptide bond. L-form

  5. Amino Acids-2 • Only 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. • nonpolar • polar • acidic • basic

  6. AA with nonpolar side chains-1

  7. AA with nonpolar side chains-2

  8. AA with polar side chains-1

  9. AA with polar side chains-2

  10. AA: acidic and basic

  11. Amino Acid Titration • At 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.

  12. Amino Acid Titration-2 • At low pH a nonacidic/nonbasic amino acid is protonated and has the structure below. • H3N+CHRCOOH • The charge behavior of acidic and basic AAs is more complex.

  13. Titration of Alanine C B=C pK2=9.73 B, pI=pH=6.0 A=B pK1=2.3 A 1

  14. Isoelectric point • The 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 + pK2/2 • Eg.: alanine: 2.34 + 9.7 / 2 = 6.0 • For acidic or basic AAs, the pI is the average of the two pKa values bracketing the isoelectric structure.

  15. Isoelectric point-2 • In general the pI is the average of the two pKa’s bracketing the isoelectric structure. Eg.: glutamic acid, pI = 3.2 pK3=9.9 pK2=4.3 pK1=2.2

  16. 5.2 Peptides • Peptide: 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.

  17. Peptides-2 • A peptide is written with the N-terminal end to the left and the C-terminal end to the right. • H2N-Tyr-Ala-Cys-Gly-COOH • Name = Tyrosylalanylcysteinylglycine • The peptide bond is rigid and planar due to the resonance contribution shown right.

  18. Peptides-3 • The peptide bond angles force specific conformations of proteins and, on extended chains, successive R groups are on opposite sides.

  19. Physiologically Interesting Peptides b carbon Common name: carnosine found in muscle tissue

  20. Physiologically Interesting Peptides Glutathione: the reduced form reduces oxidizing agents by dimerizing to form the disulfide bond with release of 2 H.

  21. Physiologically Interesting Peptides Short form description for a peptide. Leucine enkephalin: a natural analgesic found in the brain Tyr-Gly-Gly-Phe-Leu C-terminal AA N-terminal AA

  22. Physiologically Interesting Peptides 3 8 Oxytocin Induces labor and aids in forcing milk from the mammary glands. Vassopressin has a Phe at position 3 instead of Ile and an Arg at position 8 instead of a Leu. Its role is in regulating blood pressure.

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