Appearance of the Ser‐His‐Asp catalytic triad at the putative active site domain of the acyl coenzyme A:cholesterol acyltransferase (ACAT) enzymes

Abstract

To understand the molecular mechanism of cholesterol esterification reaction we sought to determine the active site residues of the ACAT enzymes using chemical modification and site directed mutagenesis approaches. The ACAT enzymes are 56% similar after N‐terminal 100 amino acids (AA). The sequence similarity within ACAT1 AA 386 to 462 and ACAT2 AA 364 to 440 is 86% and this region contains two conserved motifs FYXDWWN and HEY. We suspect this region has the putative active site domain of the enzymes due its high sequence conservation from yeast to humans. As these enzymes have an intrinsic thioesterase activity, we hypothesized that by analogy with the thioesterase domain of fatty acid synthase, the active site of ACAT enzymes may comprise a catalytic triad of S‐H‐D residues. Our results show that in ACAT1, S456, H460 and D400 are essential for activity of the enzyme. In contrast, in ACAT2, only H438 was necessary for enzymatic activity. The A2S434A mutant retained about 20% of the wild type activity whereas A2D378 mutants were not expressed after mutagenesis. Based on these data, we propose the active site may be different among the two ACAT enzymes. In ACAT1, a S‐H‐D catalytic triad serves as the active site whereas in ACAT2 there may be a H‐D catalytic diad in which the esterification reaction occurs in a substrate assisted manner with the 3β OH group of cholesterol acting as the nucleophile. (NIH:HL‐49373)