>> View the other Key Equations and Concepts in this chapter

Proteins

>> Parts of this equation/concept include:

A -	Identifying Amino Acids
B -	Reaction of Amino Acids to Make Peptide Linkages

Amino acids are characterized by a carbon with single bonds to the following three groups: H, NH₂, and COOH. The fourth bond to that carbon can be any organic group.

There is one naturally occurring amino acid with a secondary amine instead of a primary amine (NH₂).

That amino acid is proline:

>> Example 1

Which of the following are amino acids?

1. 
   
   
   
2. 
   
   
   
3. 

Solution:

1. This is an amino acid. The carboxylic acid is shown in its more drawn out form rather than as COOH.
2. This is not an amino acid. The COOH and NH₂ groups are on different carbons.
3. This is a classic amino acid. CH implies the carbon is bonded to the hydrogen as well and the COOH group and NH₂.

  >> back to the Top of the Page

Amino acids react with other amino acids in a condensation reaction. The reaction produces water (hence, condensation) and links the two amino acids together with a peptide bond. In the reaction the hydroxyl group of the carboxylic acid and a hydrogen of the primary amine form the water. The peptide bond is a carbonyl (carbon doubly bonded to oxygen) carbon singly bonded to nitrogen. The nitrogen is bonded to a hydrogen, the carbonyl, and another carbon.

Since both amino acids have both an amine and carboxylic acid, the reaction can occur in two ways. Amino acid 1 could donate the hydroxyl and amino acid 2 the hydrogen; or amino acid 1 could donate the hydrogen and amino acid 2 the hydroxyl. This means that two amino acids could form two different products. In proteins, order is everything!

The two linked amino acids (a peptide) also have a carboxylic acid group and a primary amine. Thus the peptide can react with another amino acid (at either end!), creating another peptide bond and a three-amino-acid peptide.

>> Example 2

What are the two products (other than water) of the reaction between these two amino acids, alanine and phenylalanine, respectively?

Solution:

1. One product is formed from the amine of alanine and the carboxylic acid group of phenylalanine.

   

   The groups circled react, the nitrogen and one hydrogen remain to give

   
   
   
   

2. The other product reacts the carboxylic acid of alanine with the amine of phenylalanine.

   

   so

   

>> Example 3

What are the two products of the reaction between the following peptide and amino acid?

Solution:

1. In one product the amine of the peptide bonds with the carboxylic acid group of the cysteine.

   
   
   
   

2. The other product combines the amine of cysteine with the COOH group of the peptide.

   

>> View the other Key Equations and Concepts in this chapter