Primary structure is the amino-acid sequence joined by peptide bonds. Secondary structures include alpha helices and beta sheets, stabilised by backbone hydrogen bonds. Tertiary structure is a polypeptide’s overall folding, supported by side-chain interactions and sometimes disulfide bridges. Quaternary structure describes multiple polypeptide subunits assembled together. Not every protein has this fourth level.
Shape and chemistry determine binding: enzymes recognise substrates, receptors recognise signals and antibodies recognise antigens. Sequence changes can alter folding, but not every substitution destroys function. Heat or extreme pH can disrupt higher-order structure without breaking every peptide bond. When classifying a feature, distinguish amino-acid order, local folds, one chain’s overall shape and a multi-chain assembly.
Example 1
A protein contains four polypeptide subunits. Which level describes their assembly?
Quaternary structure describes the arrangement of multiple subunits. Each subunit also has its own primary and folded structure.
Example 2
An antibody is heated and no longer binds its antigen. What is supported?
Heating may have altered the binding-site shape. Loss of binding does not by itself show that the amino-acid sequence was destroyed.
Watch out: Effects depend on the position, chemical change and protein context; some substitutions preserve function.