Everything Science Grade 12

2.6 CHAPTER 2. ORGANICMACROMOLECULES

Extension: Charged regions in an amino acid

In an amino acid, the amino group acts as a base because the nitrogenatom

has a pair of unpaired electrons which it can use to bond to a hydrogenion.

The amino group therefore attracts the hydrogen ion from the carboxylgroup,

and ends up having a charge of +1. The carboxyl group from which the hy-

drogen ion has been taken then has a charge of-1. The amino acid glycine can

therefore also be represented as shown in the figure below.

H 3 N+ C

H

H

C

O

O−

glycine

When two amino acid monomers are close together, they may be joinedto each other
by peptide bonds (figure 2.13) to form a polypeptide chain.. The reaction isa conden-
sation reaction. Polypeptides can vary in lengthfrom a few amino acidsto a thousand
or more. The polpeptidechains are then joined toeach other in different ways to form
a protein. It is the sequence of the amino acids in the polymer that gives a protein its
particular properties.

H 2 N C

H

H

C

O

OH

H 2 N C

H

CH 3

C

O

OH

H 2 N C

H

H

C

O

N

H

C

H

CH 3

C

O

OH

+

+ H 2 O

(a)

(b)

Peptide bond

Figure 2.13: Two aminoacids (glycine and alanine) combine to form partof a polypep-
tide chain. The amino acids are joined by a peptide bond between a carbon atom of
one amino acid and a nitrogen atom of the other amino acid.