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20 Amino Acids and Proteins Study Goals • • • • • • Classify proteins by their functions in the cells. Draw the structures of amino acids. Draw the zwitterion forms of amino acids at the isoelectric point, and at pH levels above and below the isoelectric point. Write the structural formulas of di- and tripeptides. Identify the structural levels of proteins as primary, secondary, tertiary, and quaternary. Describe the effects of denaturation on the structure of proteins. Chapter Outline 20.1 20.2 20.3 20.4 20.5 20.6 20.7 Functions of Proteins Amino Acids Amino Acids as Acids and Bases Formation of Peptides Career Focus: Rehabilitation Specialist Protein Structure: Primary and Secondary Levels Health Note: Natural Opiates in the Body Protein Structure: Tertiary and Quaternary Levels Health Note: Essential Amino Acids Health Note: Protein Structure and Mad Cow Disease Health Note: Sickle-Cell Anemia Protein Hydrolysis and Denaturation Explore Your World: Denaturation of Milk Protein Chapter Summary and Demonstrations 1. Amino Acids Amino acids are described as the building blocks of proteins. Structures, names, R groups, and polarity in water are discussed. The amphoteric properties of amino acids are described in terms of isoelectric points, zwitterion forms and changes in acid and base. Amino acids are categorized as essential or nonessential. 2. Peptide Bonds and Protein Structure The peptide bond is described as the amide bond between amino acids in the primary structure of a protein. The structural formulas of di- and tripeptides are written. The secondary forms of protein structure include the alpha helix, pleated sheet and collagen. The interaction of side groups to form the cross-links of tertiary structure is discussed. The breakdown in the secondary and tertiary structural levels is described as part of a discussion on denaturation of proteins. The discussion includes agents that bring about denaturation such as high temperatures, acidic or basic conditions, and organic solvents. Chapter 20 Laboratory Suggestions 36 Amino Acids The side (R) groups in amino acids are identified and used to determine if an amino acid will be acidic, basic, or neutral; hydrophobic or hydrophilic. Paper chromatography separates amino acids and an unknown amino acid in a mixture. Rf values for amino acids and unknown are calculated and the unknown identified. A. Amino Acids B. Chromatography of Amino Acids Laboratory Skills to Demonstrate Preparation of paper for amino acid chromatography Use of ninhydrin spray 37 Peptide and Proteins Students identify the structural patterns of proteins and observe denaturation. Milk is acidified to the isoelectric point to coagulate the milk protein casein. Chemical tests are used to identify proteins and amino acids. A. Peptide Bonds B. Structure of Proteins C. Denaturation of Proteins D. Isolation of Casein (Milk Protein) E. Color Tests for Proteins