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# Chapter 3b

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### Chapter 3b

1. 1. <ul><li>學習目標 (p.78-85): </li></ul><ul><ul><li>熟悉胺基酸在水溶液中的化學性質； </li></ul></ul><ul><ul><li>瞭解胺基酸如何聚合形成蛋白質； </li></ul></ul><ul><ul><li>瞭解蛋白質的各級結構及其特性。 </li></ul></ul>
2. 2. Amino acid in H 2 O <ul><li>Ampholyte (amphorteric) </li></ul><ul><li>Zwitterion (zwitterionic) </li></ul><ul><li>Dipolar ion </li></ul>Fig 3-9, p.79
3. 3. Chemical properties of A.A. <ul><li>Can act as an acid or a base – zwitterion </li></ul><ul><li>Min. 2 proton yielding groups per a.a. </li></ul>p. 79 C COO - H R NH 3 + Net charge: + 1 - 1 0 R C COO - H NH 2 H + C COOH H R NH 3 + H + H + H +
4. 4. K a : dissociation constant <ul><li>Review Henderson-Hasselbalch Equation (p. 60) </li></ul>p  = - log  兩邊倒數，取 log At K eq, [HA] = [A - ]  At K eq , pH = p K a CH 3 COOH H + + CH 3 COO - HA H + + A - K eq = = K a [H + ] [A - ] [HA] = log [H + ] [A - ] [HA] 1 K a log
5. 5. Titration curve of Gly (I) <ul><li>Titrate 0.1 M Gly with 0.1 M KOH </li></ul>Fig 3-10, p.79
6. 6. Titration curve of Gly (II) <ul><li>The pKa of a.a. with no ionizing R group. </li></ul><ul><li>Two buffer zones </li></ul><ul><li>Net charge and pH relationship  isoelectric point or pH (pI) </li></ul>Fig 3-10 pI = ½ (pK 1 + pK 2 )
7. 7. What is isoelectric point (pI)? <ul><li>pI = The pH at which the net charges on an amino acid equal zero ( 等電點 ) </li></ul><ul><li>At its pI, the amino acid will no longer move in an electrical field </li></ul>pH 14 pH 1 AA - AA + AA ++++++++ -------- Lys + pI=9.7 Arg + pI=10.8 Glu - pI=3.2 Asp - pI=2.7
8. 8. Titration curve of Glu <ul><li>An ionizable R group ( carboxyl ) </li></ul>Fig 3-12a, p.81 pI = ½ (pK 1 + pK R )
9. 9. Titration curve of His <ul><li>An ionizable R group ( imidazole ) </li></ul><ul><li>pK R near 7 </li></ul>Fig 3-12b, p.81 pI = 7.59
10. 10. Amino Acid  Monomer  Polymer Peptide, Polypeptide, Protein
11. 11. The Peptide Bond <ul><li>Two amino acids </li></ul><ul><li>Removal of one water molecule ( condensation ) </li></ul><ul><li>Formation of the CO-NH </li></ul>Fig 3-13, p.82 (condensation) (hydrolysis)
12. 12. Peptide and Polypeptide <ul><li>Amino acid sequence: aa 1 -aa 2 -aa 3 … </li></ul><ul><li>Each unit is called a residue </li></ul><ul><li>Dipeptide, tripeptide, …, Oligopeptide (12-20 residues), polypeptide </li></ul>a.a. 1 a.a. 2 a.a. 3 ….. Peptide Chain N-terminal C-terminal
13. 13. Peptide chain Fig 3-14, p.82 Ser – Gly – Tyr – Ala - Leu
14. 14. NutraSweet (aspartame) <ul><li>Dipeptide (made of 2 amino acids) </li></ul><ul><li>A.A. sequence: Aspartate + phenylalanine </li></ul><ul><li>Artificial sweetener </li></ul>p. 83
15. 15. 箭牌潔白無糖口香糖
16. 16. NutraSweet (aspartame)
17. 17. Chemical properties of peptides <ul><li>Determined by </li></ul><ul><li>Free  -amino </li></ul><ul><li>Free  -carboxyl </li></ul><ul><li>Nature and number of ionizable R groups </li></ul>Fig 3-15, p.83
18. 18. “ Protein” <ul><li>One or more polypeptide chains </li></ul><ul><li>One polypeptide chain - a mono meric protein </li></ul><ul><li>More than one - multi meric protein </li></ul><ul><ul><li>Homo multimer - one kind of chain </li></ul></ul><ul><ul><li>Hetero multimer - two or more different chains </li></ul></ul><ul><ul><ul><li>Hemoglobin, for example, is a heterotetramer </li></ul></ul></ul><ul><ul><ul><ul><li>It has two  chains and two  chains </li></ul></ul></ul></ul>p. 86-87  1  2  1  2
19. 19. Table 3-1 modified <ul><li>For 20 a.a. </li></ul><ul><ul><li>Average M r ~ 138 </li></ul></ul><ul><li>The smaller a.a. predominate in most proteins. </li></ul><ul><ul><li>Average M r ~ 128 </li></ul></ul><ul><li>Average M r of an a.a. in a protein </li></ul><ul><ul><li>110 (= 128-18) </li></ul></ul>p.84, right column Amino acid 　 M r Occurence % Gly 75 7.2 Ala 89 7.8 Val 117 6.6 Leu 131 9.1 Ile 131 5.3 Met 149 2.3 Phe 165 3.9 Tyr 181 3.2 Trp 204 1.4 Ser 105 6.8 Pro 115 5.2 Thr 119 5.9 Cys 121 1.9 Asn 132 4.3 Gln 146 4.2 Lys 146 5.9 His 155 2.3 Arg 174 5.1 Asp 133 5.3 Glu 147 6.3 Average 136.8 129.0
20. 20. Conjugated proteins <ul><li>Prosthetic group : non-amino acid part </li></ul><ul><li>Table 3-4 (p. 85) </li></ul><ul><li>Lipid + protein = lipo protein </li></ul><ul><li>Sugar + protein = glyco protein </li></ul><ul><li>Metal + protein = metallo protein </li></ul><ul><li>Hemoglobin (heme = iron porphyrin) </li></ul><ul><li>Alcohol dehydrogen ase (Zn) </li></ul>