Chapter 19: Amino Acids and Proteins

1
Chapter 19 Amino Acids and Proteins
aspartame aspartylphenylalanine methyl
ester (Nutrasweet)
2

• Amino Acids
• Structure
• Acid-base properties
• acidity and basicity of the a-groups
• acidity and basicity of side groups
• Isoelectric point
• Polypeptides and Proteins
• Primary structure
• amino acid analysis
• sequence analysis
• Secondary, tertiary, and quaternary structure

3
(No Transcript)
4
I. Amino Acids
A. Structure
5
I. Amino Acids
A. Structure
6
I. Amino Acids
A. Structure
7
I. Amino Acids
B. Acid-base properties
1. acidity and basicity of the a-groups
8
I. Amino Acids
B. Acid-base properties
2. acidity and basicity of side groups
9
Practice Exercise
What is the predominant form of alanine at pH
1.0? 10.0? What is the pI of alanine?
10
II. Polypeptides and Proteins
A. Primary structure
- sequence of amino acids in polypeptide
11
Aside Nutrasweet
12
1 soda 40 g sucrose 225 mg aspartame 96 mg
asp, 126 mg phe, 25 mg CH3OH
no effect on PKUs
1 cup cottage cheese 1674 mg phe 4 oz hamburger
1421 mg phe
ADI (average daily intake) 50 mg/kg
aspartame e.g., 150 lb person 3400 mg, 15
cans/day! Overdose 200 mg/kg aspartame - found
no effect on aspserum - no effect on
HCO2serum
13
II. Polypeptides and Proteins
A. Primary structure
Sequence analysis
14
II. Polypeptides and Proteins
A. Primary structure
1. amino acid analysis complete hydrolysis
15
II. Polypeptides and Proteins
A. Primary structure
2. sequence analysis selective hydrolysis
Cyanogen bromide (BrCN) cleaves C-side of
methionine and Enzymes (peptidases) hydrolyze
peptides at specific amide bonds trypsin
cleaves C-side of lysine or arginine chymotrypsin
cleaves C-side of amino acids with aromatic
groups (phenyalanine, tyrosine, and
tryptophan) pepsin same as chymotrypsin
methionine and leucine ? limited
number of fragments that can be isolated and
sequenced
16
II. Polypeptides and Proteins
A. Primary structure
2. sequence analysis Edman degradation
17
II. Polypeptides and Proteins
A. Primary structure
Sangers determination of the B chain of bovine
insulin
18
Practice Exercise
The polypeptide bradykinin is a tissue hormone
that can function as a potent pain- producing
agent. Amino acid anaylsis indicates a ratio of 3
Pro, 2 Phe, 2 Arg, and one each of Gly and Ser.
Partial hydrolysis gives an assortment of
fragments including Arg-Pro-Pro-Gly, Phe-Arg,
Ser-Pro-Phe, and Gly-Phe-Ser. What is the amino
acid sequence of bradykinin?
19
II. Polypeptides and Proteins
B. Secondary, tertiary, and quaternary structure
Virtually all peptide bonds in proteins have the
trans configuration.
20
II. Polypeptides and Proteins
B. Secondary, tertiary, and quaternary structure

• Secondary structure
• conformations of amino acids in localized
  regions of a polypeptide
• mainly due to H-bonding between N-H and CO of
  neighboringamino acids
• a-helix and b-pleated sheet
• Tertiary structure
• folding patterns and arrangements in a single
  polypeptide chain
• due to disulfide bonds, hydrophobic and
  H-bonding interactions
• Quaternary structure
• arrangements of protein monomers into
  aggregations
• e.g., hemoglobin 2 a-chains of 141 amino acids
  and 2 b-chainsof 146 amino acids grouped into
  one hemoglobin structure
• primarily due to hydrophobic interactions

21
(No Transcript)
22
(No Transcript)
23
(No Transcript)