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2.3.4Topic 2 · Genes and Health

Amino Acids and Peptide Bonds — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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2.3.4 Amino Acids and Peptide Bonds — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the general structure of an amino acid, explain how peptide bonds form and how the R group determines the properties of a protein.Proteins are polymers made of monomers called amino acids.

  • amino group
  • carboxyl group
  • hydrogen atom
  • condensation reaction
  • polypeptide
  • hydrolysis

Section 1 of 9General structure of an amino acid

Proteins are polymers made of monomers called amino acids. There are 20 common amino acids in living organisms. All have the same basic structure: a central carbon atom (the alpha carbon) bonded to four groups:

General structure of an amino acid
Amino group
an amino group (-NH2), which is basic;
Carboxyl group
a carboxyl group (-COOH), which is acidic;
Hydrogen atom
a hydrogen atom;
R group
a variable R group (side chain), which is different in each amino acid.

The R group may be as simple as a single hydrogen atom (glycine) or may contain a ring, a sulfur atom or a charged group. It is the R group that gives each amino acid its particular properties.

Section 2 of 9Categories of R group

TypePropertiesExampleRole in folded protein
Hydrophobic (non-polar)Repelled by waterValine, leucineCluster in the centre of globular proteins
Polar, unchargedForms hydrogen bondsSerineOn surface or forming hydrogen bonds
Charged (acidic or basic)Forms ionic bondsAspartic acid, lysineIonic bonds; on the surface and soluble
Sulfur-containingCan form disulfide bridgesCysteineStrong covalent cross-links

Section 3 of 9Forming a peptide bond

Two amino acids join by a condensation reaction.

Peptide bond
The hydroxyl (-OH) from the carboxyl group of one amino acid and a hydrogen from the amino group of another are removed as water,
  • and a covalent peptide bond (-CO-NH-) forms between the carbon and nitrogen atoms.
Dipeptide
The product of two amino acids is a dipeptide; many joined together make a polypeptide.
A protein consists of one or more polypeptides folded into a specific three-dimensional shape.

The reverse reaction, hydrolysis, adds water to break a peptide bond. This is how digestive proteases break down dietary protein into amino acids. The polypeptide has direction: one end has a free amino group (N-terminus) and the other a free carboxyl group (C-terminus).

Section 4 of 9The importance of the sequence

The order of amino acids in the chain is the primary structure, and it is determined by the base sequence of the gene.

1
Changing one amino acid may change the interactions between R groups and therefore the folded shape and function.
2
With 20 types of amino acid, a chain of just 100 amino acids could be arranged in 20100 ways, giving a huge variety of possible proteins.

Amino acids are also amphoteric

  • at certain pH values the amino group is positively charged and the carboxyl group negatively charged (a zwitterion).
  • This allows proteins to act as buffers, resisting changes in pH.

Section 5 of 9Essential amino acids and tests

Humans can make some amino acids but must obtain essential amino acids from the diet because they cannot synthesise them.

Biuret test
The biuret test detects peptide bonds: add sodium hydroxide, then a few drops of dilute copper(II) sulfate.
A purple or lilac colour indicates protein; blue means no protein.
It detects peptide bonds and so does not detect individual amino acids.
Intensity of colour, measured with a colorimeter, gives a semi-quantitative estimate of concentration.

Section 6 of 9Links

The properties of R groups drive all of the higher levels of protein structure that follow. Hydrophobic interactions, hydrogen bonds, ionic bonds and disulfide bridges between R groups hold a polypeptide in its tertiary structure, and the specific arrangement determines whether a protein will be globular or fibrous.

It is worth practising how to write and recognise the reaction using structures in words.

1
Glycine has R = H, alanine has R = CH3.
2
When glycine and alanine condense, the product glycylalanine contains a -CO-NH- link with the amino end belonging to the first residue and the carboxyl end to the second.
3
Notice that the dipeptide still has a free amino group at one end and a free carboxyl group at the other,
  • so further amino acids can be added at either end, though in cells they are added only at the carboxyl end.
4
The hydrolysis of this bond needs water and a protease enzyme or strong acid and heat in the laboratory.

Remember also the link to nutrition and health.

Links
Dietary protein is digested to amino acids by proteases,
  • which hydrolyse peptide bonds, and the amino acids are absorbed and reassembled into the body's own proteins.
A diet lacking an essential amino acid limits protein synthesis, which is a cause of kwashiorkor in children with a very low protein intake.
In exam answers, always name the bond, the reaction and the molecule released.

Section 7 of 9Exam-style questions

  1. Draw or describe the general structure of an amino acid. [3]3 marks

  2. Describe how two amino acids form a dipeptide. [3]3 marks

  3. A polypeptide has 120 amino acids. State the number of peptide bonds and water molecules released during its formation. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Amino acidThe monomer of proteins: a central carbon with amino group, carboxyl group, hydrogen and R group.
R groupThe variable side chain of an amino acid.
Peptide bondA covalent bond between the carboxyl group of one amino acid and the amino group of the next.
DipeptideTwo amino acids joined by a peptide bond.
PolypeptideA chain of many amino acids.
CondensationA reaction that joins molecules and releases water.
Essential amino acidAn amino acid that cannot be made by the body and must be eaten.
Biuret testA test for peptide bonds that turns from blue to purple.

Section 9 of 9Exam technique

1
Saying that the R group is the same in all amino acids. It is the variable part.
2
Writing that peptide bonds are hydrogen bonds. They are covalent.
3
Counting peptide bonds as equal to the number of amino acids. A chain of n amino acids has n − 1 bonds.
4
Stating that the biuret test detects amino acids. It detects peptide bonds, and so needs at least two linked amino acids.

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