Both histidine residues of the conserved HHXXXDG motif are essential for wax ester synthase/acyl-CoA:diacylglycerol acyltransferase catalysis

Bacterial acyltransferases of the wax ester synthase/diacylglycerol acyltransferase (WS/DGAT) family possess a highly conserved HHXXXDG motif. In this study, we describe the first experimental evidence that this motif is part of the active site of WS/DGAT from the Acinetobacter baylyi strain ADP 1 and that it is crucial for enzymatic activity. The second histidine residue of this motif (H-133) turned out to be essential for the catalytic activity. In addition, the replacement of the first histidine (His(132)) also led to explicitly decreased activity. A complete loss of activity was only observed upon substitution of both histidine residues by leucine, revealing that both are necessary for maximal activity. In contrast, the replacement of Asp(137) and Gly(138) against alanine had only little effect on enzyme activity, thus demonstrating that they are not essential for WS/DGAT catalysis although belonging to the highly conserved motif. One peculiarity of WS/DGAT enzymes is their little substrate specificity regarding hydrophobic compounds. In this study, we demonstrated the inability of WS/DGAT to accept polar compounds as substrates.