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Biology SL/HL: Describe the 4 structures of proteins


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Biology SL/HL: Describe the 4 structures of proteins

There are four main layers to protein structure that you need to be able to describe:

Primary - this refers to the amino acid sequence of a protein. The amino acids are held together by peptide bonds and bound together they form the polypeptide chain.

Secondary - these are highly regular sub-structures such as alpha helices and beta pleated sheets. Parts of the polypeptide chain move to form these structures which are held together by the formation of hydrogen bonds.

Tertiary - the three dimensional structure of a protein as it folds up. This is driven by hydrophobic interactions and involves specific tertiary interactions such as the formation of disulphide bonds or salt bridges between parts of the polypeptide chain.

Quaternary - multiple polypeptide chains joined together to form a larger protein molecule. An example of this is haemoglobin molecules which carry oxygen around the body in the blood. There may also be some non-protein synthetic groups, such as in haemoglobin. The synthetic group in haemoglobin is a molecule of iron.

Revision tip!

If you have difficulty remembering which of these is which, it's easy to think of it as working up from small to big.

1 = Primary = 1 chain

2 = Secondary = 1 chain folded a bit (alpha helices & beta pleated sheets - two of these structures in SECONDARY)

3 = Tertiary = 1 chain's final THREE dimensional structure

4 = Quaternary = the biggest, so multiple chains and remember haemoglobin as your example!

We hope this helps you! If you have any questions, post them below or for any other questions check out the Biology section and our other forums.

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