Describe and draw the structure of proteins as

Organic Chemistry – Polymers: Proteins

What is a Protein?

Think of a protein as a super‑strong LEGO® set made from tiny blocks called amino acids. Each block has a unique shape and colour, and when you snap them together in a specific order, you build a functional machine – the protein. 🧩

Amino Acids – The Building Blocks

There are 20 standard amino acids. Each has a central carbon (the α‑carbon) bonded to:

  • an amino group (–NH₂)

  • a carboxyl group (–COOH)

  • a hydrogen atom (H)

  • and a unique side chain (R)

The side chain determines the amino acid’s properties – hydrophobic, polar, acidic, or basic.

Peptide Bond Formation

When two amino acids link, a peptide bond forms between the carboxyl carbon of one and the amino nitrogen of the next, releasing a water molecule (a condensation reaction):

\$\ce{H2N-CHR-COOH + H2N-CHR'-COOH -> H2N-CHR-CO-NH-CHR'-COOH + H2O}\$

The resulting chain is called a polypeptide.

Protein Structure Levels

  1. Primary structure – the linear sequence of amino acids.

  2. Secondary structure – local folding into α‑helices or β‑sheets stabilized by hydrogen bonds.

  3. Tertiary structure – the overall 3D shape formed by interactions among side chains.

  4. Quaternary structure – multiple polypeptide chains assembling into a functional unit.

Drawing a Protein – Step‑by‑Step

1️⃣ Write the sequence of amino acids using their one‑letter codes (e.g., M for methionine).

2️⃣ Connect the α‑carbons with a straight line to represent the backbone.

3️⃣ Attach side chains to each α‑carbon – use different shapes or colours for each R group.

4️⃣ Show peptide bonds as short lines between the carbonyl carbon and the nitrogen.

5️⃣ Indicate secondary structure by shading α‑helices (spiral) or β‑sheets (zig‑zag).

6️⃣ Label key functional sites (e.g., active site, binding pocket) if relevant.

One‑Letter CodeFull NameSide Chain (R)
AAlanine–CH₃
CCysteine–CH₂–SH
DAspartic acid–CH₂–COOH

Exam Tips for IGCSE Chemistry 0620

  • Use the one‑letter code for quick sequence notation.

  • Show the peptide bond clearly when drawing structures.

  • Remember that α‑helices twist right‑handed; β‑sheets run in opposite directions.

  • When asked to describe a protein, mention all four structural levels.

  • Use colour or shading to highlight active sites or binding pockets.

  • Practice drawing hydrogen bonds (dashed lines) to show secondary structure.

Good luck – you’ve got this! 🚀