A heterodimerizing leucine zipper coiled coil system for examining the specificity of a position interactions: amino acids I, V, L, N, A, and K.

We use a heterodimerizing leucine zipper system to examine the contribution of the interhelical a-a' interaction to dimer stability for six amino acids (A, V, L, I, K, and N). Circular dichroism (CD) spectroscopy monitored the thermal denaturation of 36 heterodimers that generate six homotypic and 30 heterotypic a-a' interactions. Isoleucine (I-I) is the most stable homotypic a-a' interaction, being 9.2 kcal/mol per dimer more stable than the A-A interaction and 4.0 kcal/mol per dimer more stable than either the L-L or V-V interaction, and 7.0 kcal/mol per dimer more stable than the N-N interaction. Only lysine was less stable than alanine. An alanine-based double-mutant thermodynamic cycle calculated coupling energies between the a and a' positions in the heterodimer. The aliphatic amino acids L, V, and I prefer to form homotypic interactions with coupling energies of -0.6 to -0.9 kcal/mol per dimer, but the heterotypic aliphatic interactions have positive coupling energies of <1.0 kcal/mol per dimer. The asparagine homotypic interaction has a coupling energy of -0.5 kcal/mol per dimer, while heterotypic interactions with the aliphatic amino acids produce coupling energies ranging from 2.6 to 4.9 kcal/mol per dimer. The homotypic K-K interaction is 2.9 kcal/mol per dimer less stable than the A-A interaction, but the coupling energy is only 0.3 kcal/mol per dimer. Heterotypic interactions with lysine and either asparagine or aliphatic amino acids produce similar coupling energies ranging from -0.2 to -0.7 kcal/mol per dimer. Thus, of the amino acids that were examined, asparagine contributes the most to dimerization specificity because of the large positive coupling energies in heterotypic interactions with the aliphatic amino acids which results in the N-N homotypic interaction.