Proteins
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Proteins as Natural Polyamides (Supplement)
Proteins are natural polyamides, formed from amino acid monomers by condensation polymerisation. Every amino acid has the same general structure:
H₂N-CHR-COOH
where R represents a different side-chain in each different amino acid. Every amino acid has the same amino (-NH₂) and carboxyl (-COOH) groups attached to the same carbon atom - only the R group varies from one amino acid to the next.
The Structure of Proteins (Supplement)
When amino acids join together, the -COOH group of one amino acid reacts with the -NH₂ group of the next, releasing a water molecule and forming an amide (peptide) linkage:
-CO-NH-
This is repeated many times to build up a long protein chain, with a different R group hanging off each repeat unit:
-[NH-CHR-CO]-[NH-CHR-CO]-[NH-CHR-CO]-
Because each amino acid can have a different R group, proteins (unlike nylon) are built from many different monomers in a specific sequence, not just one or two repeating monomers.
Worked Example
- Question (based on a real exam question): a diagram shows part of a protein's peptide bond, -C(=X)-Z(H is attached to Z, labelled Y)-. Identify X, Y and Z.
- Answer: X = oxygen (the C=O of the peptide bond); Y = hydrogen (attached to the nitrogen); Z = nitrogen (the N of the -CO-NH- linkage).
- Question (based on a real exam question): a diagram shows part of a protein made from 4 amino acid residues joined by 3 peptide bonds, with 2 different types of side-chain shown. (i) How many different types of amino acid monomer are needed? (ii) How many water molecules are produced when this part of the protein forms?
- Answer: (i) 2 (matching the 2 different side-chains shown); (ii) 3 - one water molecule is released for every peptide bond formed, and 4 amino acids joined in a chain form 3 bonds.
Common Mistakes
- ❌ Assuming a protein is built from only one repeating monomer, like poly(ethene) or nylon
- ✓ a protein can be built from up to 20 different naturally-occurring amino acids, each with a different R group - the exact sequence of different amino acid monomers is what gives each protein its identity
- ❌ Forgetting that a water molecule is released every time a peptide bond forms
- ✓ protein formation is a condensation polymerisation - for every peptide (amide) linkage formed between two amino acids, exactly one water molecule is released, so the number of water molecules produced is always one fewer than the number of amino acids joined
Interactive revision notes, videos and practice questions load below.