Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 210–222 of the course textbook. Image credits are on slide 14.
All amino acids share an amine group, a carboxyl group and a hydrogen on one carbon; they differ in their R-group. R-groups are non-polar (hydrophobic) or hydrophilic; hydrophilic ones are polar or charged, either acidic (negative) or basic (positive). Twenty R-groups in any order give an almost endless variety of proteins, because the R-groups decide how a chain folds and what it binds.
The primary structure is the sequence of amino acids, set by the gene. The sequence fixes where every R-group sits, so it determines the three-dimensional shape. That is why proteins have precise, repeatable structures, and why small unfolded proteins can refold correctly on their own.
Parts of the chain coil into an alpha helix or lie side by side as a beta-pleated sheet. Both are stabilized by hydrogen bonds at regular positions between the C=O of one amino acid and the N-H of another in the backbone, not between R-groups.
Tertiary structure is held by bonds between R-groups: hydrogen bonds between polar groups, ionic bonds between charged groups, disulfide bonds between pairs of cysteines, and hydrophobic interactions among non-polar groups. Amine and carboxyl groups gain or lose H+, so their charge, and the ionic bonds, depend on pH.
In soluble globular proteins, non-polar amino acids cluster in the core and polar ones face the water. Integral membrane proteins have non-polar regions where they cross the lipid bilayer and polar regions where they meet water; channel proteins also have polar amino acids lining the pore.
Some proteins have two or more polypeptides. Insulin (two chains joined by disulfide bonds) and collagen (three chains in a triple helix) are non-conjugated. Haemoglobin has four chains, each with a haem prosthetic group containing iron, so it is a conjugated protein.
Globular proteins such as insulin are compact and soluble; insulin’s precise shape lets it bind its receptor as it travels in the blood. Fibrous proteins such as collagen are long and insoluble; collagen’s repeating sequence, with glycine at every third position, lets three chains pack into a strong rope, giving tendons and skin tensile strength.
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A printable worksheet covering all seven statements, from quick recall to data questions and a longer answer. It has 40 marks and takes about 40 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Pipe cleaners, coloured beads for polar, non-polar and charged R-groups
Safety: No practical hazards. Take care with sharp pipe cleaner ends.
Carry this out with your teacher's supervision and follow your school's lab rules.
Anfinsen's work suggested that sequence determines shape. In 2020 AlphaFold predicted many protein structures with high accuracy.
An AI system can predict a structure without giving a human explanation. Consider whether that counts as knowing why.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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