Dynamics of Aromatic Side Chains in the Active Site of FKBP12

FKBP12, a small human enzyme, aids protein folding by catalyzing cis–trans isomerization of peptidyl-prolyl bonds, and is involved in cell signaling pathways, calcium regulation, and the immune response. The underlying molecular mechanisms are not fully understood, but it is well-known that aromatic residues in the active site and neighboring loops are important for substrate binding and catalysis. Here we report micro- to millisecond exchange dynamics of aromatic side chains in the active site region of ligand-free FKBP12, involving a minor state population of 0.5% and an exchange rate of 3600 s–1, similar to previous results for the backbone and methyl-bearing side chains. The exchange process involves tautomerization of H87. In the major state H87 is highly flexible and occupies the common HNe2 tautomer, while in the minor state it occupies the rare HNδ1 tautomer, which typically requires stabilization by specific interactions, such as hydrogen bonds. This finding suggests that the exchange process is ...