Describe the four levels of protein structure. why dont all proteins have a fourth level of structure?

So protein structure is when amino acids are joined by the formation of covalent peptide bonds at carboxyl terminus at polypeptide chain
1st: just a sequence order of amino acids
2nd: folding of amino acids into alpha helix’s and beta pleated sheats due to hydrogen bonds between polypeptide backbone atoms
3rd: overall three d shape of protein formed by non covalent interactions between r group atoms, minimizes free energy
4th: noncovalent interactions btwn multiple polypeptide units.

Not all proteins have a fourth level Bc not every protein is made up of more than one polypeptide to be functional.

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