1. Chapter 19 Biomolecules: Amino Acids, Peptides, and Proteins 19.1 Structures of Amino Acids 19.2 Isoelectric Points 19.3 Peptides and Proteins 19.4 Peptide Synthesis 19.5 Classification of Proteins 19.6 Protein Structure

2. Amino acids ( 氨基酸 ): Alanine ( 丙氨酸 ) Peptides ( 肽 ): Many Chains with fewer than 50 amino acids Amide bonds Protein: ( 蛋白质 ) For larger chains

3. 19.1 Structures of Amino Acids Dipolar ionZwitterion ( 偶极分子 ) Amino acids are amphoteric ( 两性离子 ): In base solution: In acid solution:

4. 20α- amino cids found in protein. 19 of the 20amino acids are primary amines; Proline ( 脯氨酸 ) is a secondary amine. P476,Table 15.1 Differ only in the nature of side chain. Glycine ( 甘氨酸 ) (S)-Alanine L- Alanine (S)-Phenylalanine L-Phenylalanine (s)-Serine L-Serine L-Glycer- aldehyde

5. Aspatic acid ( 天冬氨酸 ) Glutamic acid ( 谷氨酸 ) Acidic Lysine ( 赖氨酸 ) arginine ( 精氨酸 ) Histidine ( 组氨酸 ) basic The others are neutral amino acids. 19.2 Isoelectric Points ( 等电点 ) pH Low pH (protonated) High pH (deprotonated)

6. Isoelectric Points (pI): At some intermediate pH, the concentration of the dipolar ion is maximal, the concen- trations of the anions and cations are equal. pI pH=5.0~5.6 Neutral amino acid: acidic amino acid: pI at lower pH basic amino acid:pI at higher pH Separation of a mixture of amino acids by Electrophoresis ( 电泳 ). -+ Lysine pI=9.74 Glycine pI=5.97 Aspartic acid pI=2.77 Paper strip at pH=5.97

7. C-terminal Amino acids polymers linked together by amide bonds. Peptide bond 19.3 Peptides and Proteins Alainine(Ala)Serine(Ser) Alanylserine(Ala-Ser) ( 丙氨酰 - 丝氨酸 )( 丙 - 丝二肽 ) Dipeptide ( 二肽 ) Tripeptide ( 三肽 ) Proteins: MW: >10000 N-terminal

8. 19.4 Peptide Synthesis 1.Protection of the amino group by BOC group: Ex.: Synthesis of Alanine-Leucine: Alanine Di-tert-butyldicarbonateBoc-Ala 2. Protection of the carboxyl group by treatment with methanol or benzyl alcohol: BOC group can be removed by CF 3 COOH The groups can easily be removed by NaOH(aq).

9. 3. The two protected amino acids are coupled using DCC. Dicyclohexylcarbodiimide (DCC) ( 二环己基碳二亚胺 ) 4. Removal of the BOC group. 5. Removal of ester group.

10. 19.5 Classification of Proteins 19.5.1. Classification according to the composition 1. Simple proteins: Blood serum albumin ( 血清蛋白 ), Globin ( 球蛋白 ). 2. Conjugated proteins ( 结合蛋白 ) : Glycoprotein ( 糖蛋白 ), Hemoglobin ( 血红蛋白 ). 19.5.2. Classification according to the three-dimensional shape 1. Fibrous proteins: Collagen ( 胶原蛋白 ), keratin ( 角蛋白 ). Tough and insoluble in water.

11. 2. Globular proteins: Hemoglobin, Insulin ( 胰岛素 ), Enzymes ( 酶 ). Soluble in water. 19.6 Protein Structure The primary structure of a protein: The amino acid sequence. The secondary structure of a protein: The segments of the peptide backbone ( 肽链 ). The tertiary structure of a protein: The three-dimensional shape of the entire protein molecule coiled.

12. Two kinds of The secondary structure of a protein: 1. α-Helix (α- 螺旋 ) α-Keratin (α- 角蛋白 )

13. 2. β-Pleated sheet （ β- 折叠型 ) The β-Pleated sheet structure present in silk fibroin ( 蚕丝蛋白 )