Amino acids and the peptide bond: WASSCE Chemistry

Fish, beans, groundnuts and eggs are body-building foods because they are rich in proteins. Every protein, from the keratin in your hair to the enzymes in your stomach, is a long chain of small units called amino acids, joined by peptide bonds. This lesson covers amino acids and how they link together.

Learning objectives

By the end of this lesson you should be able to:

state the elements present in proteins and name good food sources
describe the general structure of an amino acid and name simple examples
explain why amino acids are amphoteric
distinguish essential and non-essential amino acids
describe the formation of a peptide bond by condensation
explain the terms dipeptide, polypeptide and protein

Proteins in food

Proteins contain carbon, hydrogen, oxygen and nitrogen, and often sulphur (and sometimes phosphorus). The nitrogen distinguishes them from carbohydrates and fats.

Good sources: fish, meat, eggs, milk, beans, groundnuts, soya, cowpeas (black-eyed beans), and leafy vegetables in smaller amounts.

Amino acids

Definition

An amino acid is an organic compound containing both an amino group (–NH₂) and a carboxyl group (–COOH). In the amino acids found in proteins, both groups are attached to the same carbon atom.

General formula: H₂N–CHR–COOH, where R is a side group that differs from one amino acid to another. About 20 different amino acids occur in proteins.

Amino acid	R group	Formula
Glycine (aminoethanoic acid)	–H	H₂NCH₂COOH
Alanine (2-aminopropanoic acid)	–CH₃	H₂NCH(CH₃)COOH
Serine	–CH₂OH	H₂NCH(CH₂OH)COOH
Cysteine	–CH₂SH (contains sulphur)	H₂NCH(CH₂SH)COOH

Amino acids are amphoteric

The –COOH group is acidic and the –NH₂ group is basic, so amino acids react with both acids and bases:

with an alkali: H₂NCH₂COOH + NaOH → H₂NCH₂COONa + H₂O
with an acid: H₂NCH₂COOH + HCl → ClH₃NCH₂COOH (i.e. ⁺H₃NCH₂COOH Cl⁻)

In solid amino acids and in neutral solution, the H⁺ of the –COOH moves to the –NH₂ group, giving a zwitterion, ⁺H₃N–CHR–COO⁻. This ionic form explains why amino acids are crystalline solids with high melting points that dissolve in water.

Essential amino acids

Essential amino acids cannot be made by the body and must come from food (for example lysine, leucine, valine, tryptophan; nine for adults).
Non-essential amino acids can be made in the body from other amino acids.
Animal proteins (fish, eggs, milk) contain all the essential amino acids. Single plant foods may lack some, so mixing cereals and legumes (rice and beans) gives a complete set.

The peptide bond

Two amino acids join: the OH of one carboxyl group and an H of the other's amino group leave as water, forming the –CO–NH– peptide bond of a dipeptide

Figure 1. Two amino acids form a dipeptide by condensation. WASSCE Tutor diagram.

The –COOH of one amino acid reacts with the –NH₂ of another, losing a molecule of water (a condensation reaction). The new link, –CO–NH–, is the peptide bond (an amide link):

\text{H}_2\text{NCH}_2\text{COOH} + \text{H}_2\text{NCH}_2\text{COOH} \rightarrow \text{H}_2\text{NCH}_2\text{CONHCH}_2\text{COOH} + \text{H}_2\text{O}

\begin{gathered}\text{H}_2\text{NCH}_2\text{COOH} + \text{H}_2\text{NCH}_2\text{COOH} \\[6pt] \rightarrow \text{H}_2\text{NCH}_2\text{CONHCH}_2\text{COOH} + \text{H}_2\text{O}\end{gathered}

Two glycine molecules give the dipeptide glycylglycine.

Term	Meaning
Dipeptide	Two amino acids joined by one peptide bond
Polypeptide	A chain of many amino acids (roughly up to 50)
Protein	One or more long polypeptide chains (often hundreds of amino acids) folded into a particular shape

Proteins are therefore natural condensation polymers, with amino acids as the monomers. Because there are 20 amino acids and they can be arranged in any order, the number of possible proteins is enormous.

Worked example

Water lost in forming a peptide. How many water molecules are released when 50 amino acids join into one polypeptide chain? What is the relative molecular mass of a dipeptide made from glycine (Mᵣ 75) and alanine (Mᵣ 89)?

A chain of n amino acids has n − 1 peptide bonds, so 50 amino acids release 49 water molecules.
Dipeptide: $75 + 89 - 18 = 146$ .

In Sierra Leone

Kwashiorkor, a disease of young children who get enough starch but too little protein, causes swollen bellies, thin limbs and poor growth. In Sierra Leone, cheap protein sources such as bonga fish, groundnuts, beans and eggs, added to rice or cassava meals, provide the essential amino acids that starchy staples alone do not.

Common mistake

The peptide bond is –CO–NH–, formed from the carboxyl group of one amino acid and the amino group of the next. It is not formed between two –COOH groups or two –NH₂ groups.

Key points

Proteins contain C, H, O, N (often S); the nitrogen sets them apart from carbohydrates and fats.
Amino acids: H₂N–CHR–COOH, with an amino and a carboxyl group; about 20 occur in proteins (glycine R = H, alanine R = CH₃).
Amino acids are amphoteric and exist as zwitterions.
Essential amino acids must come from food.
Amino acids join by condensation, losing water, to form peptide bonds (–CO–NH–); proteins are condensation polymers.

WASSCE examination focus

WAEC tests this

Expect to write the general formula of an amino acid and name the two functional groups, to show how a peptide bond forms (with structures and water lost), to explain the amphoteric nature of amino acids, and to identify proteins as natural polymers.

Quick check

Which element is present in all proteins but not in carbohydrates?
Name the two functional groups in an amino acid.
Draw (in condensed form) the peptide bond.
Why are amino acids described as amphoteric?

Answers:

Nitrogen.
The amino group (–NH₂) and the carboxyl group (–COOH).
–CO–NH–.
They contain an acidic carboxyl group and a basic amino group, so they react with both acids and bases.

Amino acids and the peptide bond