Which of the following statements is true regarding proteins?
|A||All proteins exhibit quaternary folding.|
|B||The primary structure of a protein refers simply to the linear order of amino acids it contains.|
|C||Not all proteins contain amino acids, only the ones that have extensive three-dimensional folding.|
|D||All proteins are composed of one polypeptide chain.|
Protein primary structure refers only to the linear order of amino acids. Secondary and tertiary folding are characteristics of all proteins, and involve interactions between various amino acids and ultimately cause specific three-dimensional folding. Both A and D are incorrect for related reasons; some proteins consist of more than one polypeptide chain, and it is only these proteins that exhibit quaternary folding (the association of multiple polypeptides). All proteins contain amino acids, by definition, so C cannot be correct.