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20
Amino Acids and Proteins
Study Goals
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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