Gap junctions are made of 2 connexons and EACH connexon is made up of 6 separate polypeptides. What level of structure does the connexon protein show?

a) primary

b) secondary

c) tertiary

d) quaternary

e) none of the above. 

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The correct answer is D. There are multiple levels of protein folding, and primary structure refers to the unique sequence of amino acids that the polypeptide is made of. From there secondary structure refers to the interactions between the backbone of the amino acids, and the two common motifs that are prominent here are alpha helices and beta sheets (both are formed simply because of hydrogen bonding). The tertiary structure is formed from the idea of interactions between the side chains of amino acids; common motifs include hydrophobic interactions, hydrogen bondings, and disulfide linkages (which are covalent linkages). And lastly,  you have quaternary structure, which refers to interaction between one or MORE polypeptides. That is exactly what we see here because one connexon is formed by 6 polypeptides interacting with one another.