1. Chapter 19: Proteins
CHEM 1152
Dr. Sheppard

2. Outline Amino Acids Amino Acids as Acids and Bases
Formation of Peptides
Functions of Proteins
Protein Structure: Primary, Secondary, Tertiary and Quaternary Structure
Protein Hydrolysis and Denaturation

3. 19.1 Amino Acids Amino acids: The building blocks of proteins.
Two functional groups:
carboxylic acid group
amino group on the alpha () carbon.
Have different side groups (R) that give each amino acid unique characteristics.

4. Nonpolar Amino Acids
Amino acids are classified as nonpolar when the R groups are H, alkyl, or aromatic
Note three-letter abbreviations

5. Polar Neutral Amino Acids
Amino acids are classified as polar neutral when the R groups are alcohols, thiols, or amides

6. Acidic and Basic Amino Acids
Amino acids are classified as acidic when the R group is a carboxylic acid
Amino acids are classified as basic when the R group is an amine

7. Essential Amino Acids Essential amino acids:
Ten amino acids that are not synthesized by the body
Must be obtained from the diet

8. Essential Amino Acids
Complete proteins contain all 10 essential amino acids (animal products)
Incomplete proteins are deficient in one or more (plant proteins)
Obtained by combining two or more vegetables that provide complementary proteins

9. Learning Check Classify each amino acid as polar or nonpolar:
A. Glycine NH3+—CH2—COO-
CH3 |
CH—OH
B. Threonine NH3+—CH—COO-

10. Learning Check
Classify the following amino acids as hydrophobic (nonpolar), hydrophilic (polar, neutral), acidic, or basic:
A. Lysine R = CH2CH2CH2CH2NH2
B. Aspartate R = CH2CO2H
C. Leucine R = CH2CH(CH3)2
D. Serine R = CH2OH

11. Learning Check
Give the name of the amino acid represented by each of the following three-letter abbreviations:
A. Trp
B. Met
C. Pro
D. Gly

12. 19.2 Zwitterions
Both the –NH2 and the –COOH groups in an amino acid undergo ionization in water
A zwitterion forms that has + and – charge
Melting points of amino acids = very high

13. Isoelectric Point
At the isoelectric point (pI), the + and – charges in the zwitterion are equal (no net charge).
At pH values above the isoelectric point, the amino acid has a net negative value.
At pH values below the isoelectric point, the amino acid has a net positive value.

14. Amino Acids as Acids
In solutions more basic (higher pH) than the pI, the —NH3+ in the amino acid loses a proton to become —NH2
+ OH−
H3N—CH2—COO– H2N—CH2—COO–
Zwitterion Negative ion
at pI Higher pH

15. Amino Acids as Bases
In solution more acidic (lower pH) than the pI, the —COO- in the amino acid gains a proton to become —COOH H
H3N—CH2—COO– H3N—CH2—COOH
Zwitterion Positive ion
at pI Low pH

16. pH and Ionization
Alanine

17. pH and Ionization
Acidic amino acids such as aspartic acid have a second carboxyl group that can donate and accept protons
Ex: the pI for aspartic acid occurs at a pH of 2.8

18. Learning Check CH3 CH3 + | | H3N—CH—COOH H2N—CH—COO– (1) (2)
+ | |
H3N—CH—COOH H2N—CH—COO–
(1) (2)
Which structure represents:
A. Alanine at a pH above its pI?
B. Alanine at a pH below its pI?

19. Learning Check
Would the following ions of serine exist at a pH above, below, or at pI?

20. Learning Check
Write the zwitterion of each of the following amino acids:
A. Phenylalanine B. Methionine

21. 19.3 Reactions of Amino Acids
Two reactions of interest:
Oxidation of cysteine
Peptide bond formation

22. Oxidation of Cysteine Cys side chain = ─CH2─SH
Remember oxidation of thiols to form disulfide bonds (section 13.9)?
R─SH + R─SH R─S─S─R
Two Cys amino acids can react to form a disulfide bond (or disulfide bridge)
[O]

23. Disulfide Bridge/bond

24. Review
Carboxylic acid + amine = ?
Structure of amino acid

25. The Peptide Bond
A peptide bond is an amide bond between the carboxyl group of one amino acid and the amino group of the next amino acid
O CH3 O
|| | ||
H3N—CH2—C—O– H3N—CH—C—O–
O H CH3 O
|| | | ||
H3N—CH2—C—N—CH—C—O– + H2O
peptide bond

26. Peptides Dipeptide = two amino acids; tri = 3; poly = many
Protein = polypeptide with >50 amino acid residues
Peptide is named from the N-terminal end (left; NH3+)
Use -yl endings for the names of the amino acids
The C-terminal amino acid (right; COO-) uses its amino acid name

27. Peptides

28. Learning Check
Use the three-letter abbreviations of the amino acids to write structures for all possible tripeptides that could form from two glycine and one alanine. Name each tripeptide.

29. Learning Check
Write the name of the following tetrapeptide using amino acid names and three-letter abbreviations.

30. Learning Check
Draw the structural formula of each of the following peptides.
A. Methionylaspartic acid
B. Alanyltryptophan

31. 19.5 Characteristics of Proteins
Size:
Varies from 6000 to several million g/mol
Hemoglobin = C2952H4664O832S8Fe4

32. Characteristics of Proteins
Function:

33. Characteristics of Proteins
Function:

34. Example
Classify each of the following proteins according to its function:
A. Insulin, a hormone needed for glucose utilization
B. Antibodies, proteins that disable foreign proteins
C. Casein, milk protein
D. Lipases that hydrolyze lipids

35. Characteristics of Proteins
Classification:
Fibrous vs. globular (based on structural shape)
Fibrous proteins:
Long rods or strings; fibers
Water-insoluble
Connective tissue, hair, skin
Globular proteins:
Spherical
Water-soluble
Synthesis, transport and metabolism in cells

36. Protein Structure Chain of amino acids folded into 3D structure
Levels of structure:
Primary
Secondary
Tertiary
Quaternary