BCH 4053 Biochemistry I
Fall 2001
Dr. Michael Blaber


Lecture 8

Proteins are linear polymers of amino acids, Architecture of protein molecules, The many biological functions of proteins


Amino acids are the monomeric building blocks of protein molecules

The joining of two amino acids results in the formation of a "dipeptide"

The peptide bond (as drawn above) looks like it is a single bond, and therefore, free to rotate. However, the peptide bond has partial double bond character due to a resonance structure:

The resonance structures have the following consequences:

"Proteins" may consist of a single polypeptide, or a complex of two or more polypeptides

Subunit organization

Meaning

Example

ab

The protein is composed of two separate polypeptide chains, and each has a different amino acid sequence (a heterodimer)

Insulin

Chain 'A' has 21 amino acids
Chain 'B' has 30 amino acids

a2

The protein is composed of two separate polypeptide chains, and they are identical (a homodimer)

Lambda phage Cro repressor

Each monomer has 66 amino acids

abg

The protein is composed of three separate polypeptide chains, and each is uniquely different (a heterotrimer)

Chymotrypsin

Chain a has 13 amino acids
Chain
b has 132 amino acids
Chain
g has 97 amino acids

a2b2

The protein is composed of four separate polypeptide chains. However, these can be described as two sets of identical pairs of polypeptide chains (a dimer of homodimers)

g-globulin

Chain a has 214 amino acids
Chain
b has 446 amino acids

 

Amino acid compositions of polypeptides

Peptide bonds can be hydrolyzed by strong acid (typically 6N HCl, boiling overnight). This method can allow the quantitation of the relative proportions and amounts of each amino acid in a polypeptide. There are a few problems with this method:

Amino Acid

Amino acid concentration from hydrolysis
(n moles)

Normalized Concentration
(moles)

Probable
Composition

Ala

570

12.7

13

Cys

58

1.3

1

Asx

400

8.9

9

Glx

690

15.3

15

Phe

300

6.7

7

Gly

520

11.6

12

His

410

9.1

9

Ile

275

6.1

6

Lys

725

16.1

16

Leu

700

15.6

16

Met

110

2.4

2

Pro

180

4.0

4

Arg

120

2.7

3

Ser

240

5.3

5

Thr

97

2.2

2

Val

255

5.7

6

Tyr

45

1.0

1

Total amino acids: 127

 

Architecture of Protein Molecules

There are three very general categories of proteins in the body:

 

Levels of Protein Structure

There are four "levels" of protein structure: primary, secondary, tertiary and quaternary

click here for a vrml file of a helix (need cosmo player plugin)

 

 

 

The Biological Functions of Proteins


© 2001 Dr. Michael Blaber