PDZ/PDZ interaction between PSD-95 and nNOS neuronal proteins: A thermodynamic analysis of the PSD95-PDZ2/nNOS-PDZ interaction.

N-Methyl-D-aspartate (NMDA) receptors are key components in synaptic communication and are highly relevant in central nervous disorders, where they trigger excessive calcium entry into the neuronal cells causing harmful overproduction of nitric oxide by the neuronal nitric oxide synthase (nNOS) protein. Remarkably, NMDA receptor activation is aided by a second protein, postsynaptic density of 95 kDa (PSD95), forming the ternary protein complex NMDA/PSD95/nNOS. To minimize the potential side effects derived from blocking this ternary complex or either of its protein components, a promising approach points to the disruption of the PSD-95/nNOS interaction which is mediated by a PDZ/PDZ domain complex. Since the rational development of molecules targeting such protein-protein interaction relies on energetic and structural information herein, we include a thermodynamic and structural analysis of the PSD95-PDZ2/nNOS-PDZ. Two energetically relevant events are structurally linked to a "two-faced" or two areas of recognition between both domains. First, the assembly of a four-stranded antiparallel β-sheet between the β hairpins of nNOS and of PSD95-PDZ2, mainly enthalpic in nature, contributes 80% to the affinity. Second, binding is entropically reinforced by the hydrophobic interaction between side chains of the same nNOS β-hairpin with the side chains of α2-helix at the binding site of PSD95-PDZ2, contributing the remaining 20% of the total affinity. These results suggest strategies for the future rational design of molecules able to disrupt this complex and constitute the first exhaustive thermodynamic analysis of a PDZ/PDZ interaction.