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Amino Acids, Peptides, and Proteins.

Amino Acids, Peptides, and Proteins. Classification of Amino Acids. Fundamentals. While their name implies that amino acids are compounds that contain an —NH 2 group and a —CO 2 H group, these groups are actually present as —NH 3 + and —CO 2 – respectively.

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Amino Acids, Peptides, and Proteins.

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  1. Amino Acids, Peptides, and Proteins.

  2. Classification of Amino Acids

  3. Fundamentals • While their name implies that amino acids are compounds that contain an —NH2 group and a —CO2H group, these groups are actually present as —NH3+ and —CO2– respectively. • They are classified as a, b, g, etc. amino acids according the carbon that bears the nitrogen.

  4. + NH3 – CO2 + – H3NCH2CH2CO2 + – H3NCH2CH2CH2CO2 Amino Acids an a-amino acid that is anintermediate in the biosynthesisof ethylene a a b-amino acid that is one ofthe structural units present incoenzyme A b a g-amino acid involved inthe transmission of nerveimpulses g

  5. The 20 Key Amino Acids • More than 700 amino acids occur naturally, but 20 of them are especially important. • These 20 amino acids are the building blocks of proteins. All are a-amino acids. • They differ in respect to the group attached to the a carbon. • These 20 are listed in Table 27.1 (p 1054-1055).

  6. O H + – H3N O C C R Table 27.1 • The amino acids obtained by hydrolysis of proteins differ in respect to R (the side chain). • The properties of the amino acid vary as the structure of R varies.

  7. O H + – H3N O C C H Table 27.1 • Glycine is the simplest amino acid. It is the only one in the table that is achiral. • In all of the other amino acids in the table the a carbon is a stereogenic center. Glycine (Gly or G)

  8. Table 27.1 O H + – H3N O C C CH3 Alanine (Ala or A)

  9. Table 27.1 O H + – H3N O C C CH(CH3)2 Valine (Val or V)

  10. Table 27.1 O H + – H3N O C C CH2CH(CH3)2 Leucine (Leu or L)

  11. Table 27.1 O H + – H3N O C C CH3CHCH2CH3 Isoleucine (Ile or I)

  12. Table 27.1 O H + – H3N O C C CH3SCH2CH2 Methionine (Met or M)

  13. O H + – H2N O C C CH2 H2C CH2 Table 27.1 Proline (Pro or P)

  14. O H + – H3N O C C CH2 Table 27.1 Phenylalanine (Phe or F)

  15. O H + – H3N O C C CH2 N H Table 27.1 Tryptophan (Trp or W)

  16. O H + – H3N O C C H2NCCH2 O Table 27.1 Asparagine (Asn or N)

  17. O H + – H3N O C C H2NCCH2CH2 O Table 27.1 Glutamine (Gln or Q)

  18. Table 27.1 O H + – H3N O C C CH2OH Serine (Ser or S)

  19. Table 27.1 O H + – H3N O C C CH3CHOH Threonine (Thr or T)

  20. O H + – H3N O C C – OCCH2 O Table 27.1 Aspartic Acid (Asp or D)

  21. O H + – H3N O C C – OCCH2CH2 O Table 27.1 Glutamic Acid (Glu or E)

  22. O H + – H3N O C C CH2 OH Table 27.1 Tyrosine (Tyr or Y)

  23. Table 27.1 O H + – H3N O C C CH2SH Cysteine (Cys or C)

  24. Table 27.1 O H + – H3N O C C + CH2CH2CH2CH2NH3 Lysine (Lys or K)

  25. Table 27.1 O H + – H3N O C C CH2CH2CH2NHCNH2 + NH2 Arginine (Arg or R)

  26. CH2 N NH Table 27.1 O H + – H3N O C C Histidine (His or H)

  27. 27.2Stereochemistry of Amino Acids

  28. – CO2 + H3N H R Configuration of a-Amino Acids • Glycine is achiral. All of the other amino acids in proteins have the L-configuration at their a carbon.

  29. 27.7Peptides

  30. Peptides • Peptides are compounds in which an amide bond links the amino group of one a-amino acid and the carboxyl group of another. • An amide bond of this type is often referred to as a peptide bond.

  31. H H O O + + – – O O H3N H3N C C C C CH3 H Alanine and Glycine

  32. H H O O + – H3N N O C C C C CH3 H H Alanylglycine • Two a-amino acids are joined by a peptide bond in alanylglycine. It is a dipeptide.

  33. H H O O + – H3N N O C C C C CH3 H H Alanylglycine C-terminus N-terminus Ala—Gly AG

  34. H H O O + – H3N N O C C C C CH3 H H H H O O + – H3N N O C C C C H H CH3 Alanylglycine and glycylalanine are constitutional isomers Alanylglycine Ala—Gly AG Glycylalanine Gly—Ala GA

  35. H H O O + – H3N N O C C C C CH3 H H Alanylglycine • The peptide bond is characterized by a planar geometry.

  36. Higher Peptides • Peptides are classified according to the number of amino acids linked together. • dipeptides, tripeptides, tetrapeptides, etc. • Leucine enkephalin is an example of a pentapeptide.

  37. Leucine Enkephalin Tyr—Gly—Gly—Phe—LeuYGGFL

  38. 4 5 3 Ile—Gln—Asn 2 Tyr Cys—Pro—Leu—GlyNH2 6 7 8 9 1 Cys S S Oxytocin • Oxytocin is a cyclic nonapeptide. • Instead of having its amino acids linked in an extended chain, two cysteine residues are joined by an S—S bond. C-terminus N-terminus

  39. Oxytocin S—S bond An S—S bond between two cysteines isoften referred to as a disulfide bridge.

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