Crystal Structure of LCAT Bound to a Small Molecule Allosteric Activator Reveals its Active Conformation

Abstract

There is an inverse relationship between risk for coronary heart disease (CHD) and the level of high density lipoprotein cholesterol (HDL‐C). As CHD is one of the leading causes of death worldwide, there is great interest in mechanisms to raise HDL‐C for treatment of CHD. Lecithin:cholesterol acyltransferase (LCAT) catalyzes the transfer of an acyl chain from phosphatidylcholine to cholesterol to create a cholesteryl ester (CE) in the process of reverse cholesterol transport. The formation of the more hydrophobic CE leads to the maturation of HDL from discoidal to spherical and promotes the clearance of cholesterol from arterial plaques. Mutations in LCAT are known to cause two genetic diseases: fish eye disease (FED) and familial LCAT deficiency (FLD). Individuals with either FED or FLD have low HDL‐C and corneal opacities, while FLD patients ultimately develop renal failure. Recombinant LCAT and LCAT‐activating compounds are being investigated as treatments for both CHD and FLD. However, only recently has any structural information on LCAT been available to guide development of these therapies. Based on recent crystal structures and hydrogen deuterium exchange data, it is now apparent that LCAT has a dynamic lid that is able to protect the LCAT active site from solvent. This lid likely plays a critical role in activation of LCAT by apolipoprotein A‐I, the major protein component of HDLs. We report on a new structure of LCAT bound to an LCAT‐activating compound as well as an acyl intermediate state mimic, isopropyl dodecylfluorophosphonate. The combination of these two ligands promotes a lid‐open conformation. We confirmed that the compound binds to an allosteric site via site directed mutants that both eliminate binding and increase the thermal stability of LCAT. These studies pave the way for the design of rational drugs or improved biotherapeutics that can be used to treat CHD and FLD and augment reverse cholesterol transport. The new structure also allows for a better understanding of the acyl binding site of LCAT and its catalytic mechanism.Support or Funding InformationThis work was supported by US National Institutes of Health (NIH) grants HL122416 and Postdoctoral Fellowship F32HL131288 (K.A.M.).