Crystal Structure of CYP106A2 in Substrate-Free and Substrate-Bound Form

CYP106A2 from Bacillus megaterium ATCC 13368 is known as a bacterial steroid hydroxylase that is also capable of hydroxylating a variety of terpenoids. To analyze the substrate specificity of this enzyme further, different resin acids of the abietane and pimarane types were tested with regard to binding and conversion. Product formation could be shown for all tested substrates. Spectroscopic studies revealed type I binding spectra for isopimaric acid, but dehydroabietic acid did not induce a high-spin shift of the enzyme. Interestingly, binding of abietic acid resulted in a type II difference spectrum typical for nitrogenous inhibitors. Co-crystallization of CYP106A2 with abietic acid and structure determination revealed bending of the heme cofactor when abietic acid was bound in the active site. Quantum chemical calculations strongly suggest that this heme distortion is the cause of the unusual spectroscopic characteristics.