Characterization of functional residues in the interfacial recognition domain of lecithin cholesterol acyltransferase (LCAT)

Lecithin cholesterol acyltransferase (LCAT) is an interfacial enzyme active on both high-density (HDL) and low-density lipoproteins (LDL), Threading a lignments of LCAT with lipases suggest that residues 50-74 form an interfac ial recognition site and this hypothesis was tested by site-directed mutage nesis, The (Delta 56-68) deletion mutant had no activity on any substrate, Substitution of W61 with F, Y, L or G suggested that an aromatic residue is required for full enzymatic activity. The activity of the W61F and W61Y mu tants was retained on HDL but decreased on LDL, possibly owing to impaired accessibility to the LDL lipid substrate. The decreased activity of the sin gle R52A and K53A mutants on HDL and LDL and the severer effect of the doub le mutation suggested that these conserved residues contribute to the foldi ng of the LCAT lid. The membrane-destabilizing properties of the LCAT 56-68 helical segment were demonstrated using the corresponding synthetic peptid e. An M65N-N66M substitution decreased both the fusogenic properties of the peptide and the activity of the mutant enzyme on all substrates. These res ults suggest that the putative interfacial recognition domain of LCAT plays an important role in regulating the interaction of the enzyme with its org anized lipoprotein substrates.