The big idea: A protein's shape is written in its sequence. The order of amino acids decides where each R-group ends up, and so how the chain folds.
The points to remember
- The primary structure is the sequence of amino acids, set by the gene.
- In water, the chain folds so hydrophobic R-groups end up inside and hydrophilic ones outside.
- R-groups that meet form hydrogen bonds, ionic bonds and disulfide bonds that hold the fold.
- So the sequence, and the precise position of each amino acid, decides the 3-D shape: the conformation.
Remember it as: Sequence → R-group positions → bonds → shape.
Real example: human insulin made by bacteria: Bacteria given the human insulin gene make a chain with exactly the human sequence. It folds into the same shape as insulin made in the pancreas, so it works in people with diabetes.
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The big idea: Even very large proteins fold the same way every time, because each amino acid sits in the same place in every copy.
The points to remember
- The same sequence always folds into the same shape: protein structures are precise, predictable and repeatable.
- That is why every copy of an enzyme has the same active site.
- Change one amino acid and its R-group changes, so the bonds and the fold can change.
- A changed fold can mean a protein that works less well, or not at all.
Remember it as: Same sequence, same shape; new sequence, maybe a new shape.
Same sequence
- Same R-groups in the same positions
- Same bonds form
- Same 3-D shape, same function
One amino acid changed
- A different R-group at one position
- Some bonds change
- The shape and function may change
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How it is asked: A four-mark Explain linking the sequence to the tertiary structure, and questions on what a single change in sequence does.
After synthesis, globular proteins fold into their final tertiary structure. Explain the relationship between the sequence of amino acids and the tertiary structure.
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And a two-mark Outline on a change in sequence:
Outline how a change to one amino acid in a protein's sequence can change its function.
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