The analysis of AcmB crystal structure reveals that its structure is similar to that reported previously for KstD1 from R. erythropolis, with the exception of a 40-amino acid long membrane-associated domain that is localized in close vicinity of the active site previously referred to as the 'loop'. Helix 8 and 9 of this domain exhibit amphipathic character enabling its anchoring to the membrane. Theoretical prediction indicates that such anchoring positions the enzyme active site oriented toward the membrane surface, which may facilitate the formation of the enzyme-substrate complex even with highly hydrophobic steroids. This domain also narrows the entrance to the active site and strongly interacts with the extended alkyl sidechain in cholest-4-en-3-one, a native substrate of AcmB . Prediction of Gbinding and the energy of interaction supports this hypothesis, especially compared to the results calculated for KstD1, which lacks preference for binding cholest-4-en-3-one over steroids with degraded C17 substituent. The presence of a loop within the active centre of KstD1 that may affect the enzyme-catalysed reactions was previously reported for several homologous models by Luo et al . The long loops (which could also form elements of secondary structure) may also negatively affect the enzyme activity by hindering substrate access to the active site .