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Amino acids

Amino acids. Lecture 1 Dr. Mamoun Ahram Summer, 2014. Resources. This lecture Campbell and Farrell’s Biochemistry, Chapters 3 (pp.66-76). General structure. (Chiral carbon). The opposite is achiral. Isomers, stereoisomers, enantiomers.

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Amino acids

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  1. Amino acids Lecture 1 Dr. Mamoun Ahram Summer, 2014

  2. Resources • This lecture • Campbell and Farrell’s Biochemistry, Chapters 3 (pp.66-76)

  3. General structure (Chiral carbon) The opposite is achiral

  4. Isomers, stereoisomers, enantiomers • If two molecules have the same number of atoms, they are isomers. • If the isomers have the same atomic connectivity, but differ spatially, they are stereoisomers. • If the stereoisomers are mirror images of each other, they are enantiomers.

  5. Light polarization Optically active compounds can rotate polarized light to the left or the right.

  6. The amino acids that occur in proteins naturally are all of the L form.

  7. Designation of carbons • Side-chain carbon atoms are designated with letters of the Greek alphabet, counting from the -carbon. These carbon atoms are, in turn, the -, -, -, and -carbons. • The terminal carbon atom is referred to as the -carbon

  8. Types of amino acids • There are twenty kinds of amino acids depending on the side chains varying in • Size • Shape • Charge • Hydrogen-bonding capacity • Hydrophobic character • Chemical reactivity

  9. Classification (according to R group)

  10. Glycine Is it chiral?

  11. Non-polar, aliphatic amino acids

  12. Alanine

  13. Valine, leucine, and isoleucine These are essential amino acids in the sense that the body cannot synthesize them.

  14. Methionine

  15. Proline (imino acid) Secondary nitrogen

  16. Polar amino acids

  17. Glycine Is it chiral?

  18. Serine and threonine

  19. Cysteine (Cys, C)

  20. Asparagine and glutamine Amide groups

  21. MSG in food • Monosodium glutamate, or MSG, is a derivative of glutamic acid used as a flavor enhancer. • MSG may cause Chinese restaurant syndrome (chills, headaches, and dizziness).

  22. Aromatic amino acids

  23. Phenylalanine, tyrosine, Tryptophan

  24. Positively-charged amino acids

  25. Lysine and arginine guanidino group

  26. Histidine

  27. Negatively-charged amino acids

  28. Aspartic acid and glutamic acid

  29. Questions • Two amino acids are negatively-charged: • The following amino acid is achiral: • …etc.

  30. Specialized and uncommon amino acids

  31. Biological significance of amino acids • a-nitrogen atom of amino acids is a primary source for many nitrogenous compounds: • Hormones • Neurotransmitters • Biologically active peptides

  32. Tyrosine (1) • is converted into catecholamine neurotransmitters • dopamine • Norepinephrine • Epinephrine • flight or fight

  33. Tyrosine (2) • Tyrosine is converted into • melanin (skin color) • Thyroxine (hormone)

  34. Tyrosine and life • Cheese contain high amounts of tyramine, which mimics epinephrine; for many people a cheese omelet in the morning is a favorite way to start the day.

  35. Tryptophan • Tryptohpan serves as the precursor for the synthesis of Neurotransmitters • serotonin(neurotransmitter-sedative) • melatonin (day-night cycle)

  36. Histamine • Regulates physiological function in the gut • Acts as a neurotransmitter • Causes allergic symptoms (a major causes for asthma) • Contributes to inflammatory response • Causes constriction of smooth muscle

  37. Glutamate • Is a precursor of g-aminobutyricacid (GABA) • Inhibitory neurotransmitter (CNS)

  38. γ- carboxyglutamate (Gla) • The glutamate residues of some clotting factors are carboxylated to form γ- carboxyglutamate (Gla) residues • Vitamin K is essential for the process • This carboxylation is essential for the function of the clotting factors

  39. Arginine • L-arginine is the precursor of nitric oxide (NO)  • NO functions: • Vasodilation, inhibition of vasoconstriction, inhibition of platelet adhesion, inhibition of leukocyte adhesion, antiproliferative action, scavenging superoxide anion (anti-inflammatory) 

  40. Lysine and proline • Both are hydroxylated and are part of collagen structure.

  41. Ionization of amino acids

  42. Why do amino acids get ionized?

  43. Zwitterion and isoelectric point • At physiological pH, amino acids (without ionizable groups) are electrically neutral • Zwitterion: a molecule with two opposite charges and a net charge of zero

  44. Effect of pH

  45. Example 1 (alanine)

  46. Henderson-Hasselbalch Equation • We have calculated the ratio of acid to conjugate base for an -carboxyl group and an -amino group at pH 7.0 • We can do this for any weak acid and its conjugate base at any pH using the Henderson-Hasselbalch equation

  47. Isoelectric Point • The pH where the net charge of a molecules such as an amino acid or protein is zero is known as isoelectric point or pI • For the nonpolar and polar amino acids with two pKa’s, the isoelectric point is calculated by taking the numerical average of the carboxyl group pKa and the a-amino group pKa

  48. Ionization of side chains • Nine of the 20 amino acids have ionizable side chains • These amino acids are tyrosine, cysteine, arginine, lysine, histidine, serine, threonine, and aspartic and glutamic acids • Each side chain has its own pKa values for ionization of the side chains

  49. pI of amino acids Let’s consider pKa of -NH2 = 9 and pKa of –COOH = 2 for all amino acids

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