A mechanistic model of lecithin:cholesterol acyltransferase activity exploits discoidal HDL composition and structure

Lecithin:cholesterol acyltransferase (LCAT) activity towards discoidal HDL with apoA-I was analyzed in conjunction with re-evaluation of conformational stability of apoA-I (Sparks et al., 1993). The reaction at water-lipid interface involves the formation of acyl-enzyme and cholesterol (Chol) as a nucleophilic agent can compete with water at deacylation step. Raw data on apparent kinetic parameters for LCAT activity toward discoidal HDL with fixed (Sparks et al., 1995) or varying (Sparks et al., 1998) palmitoyloleoylphosphatidylcholine (POPC) content fit the kinetic equation derived. At the increase of Chol content in complexes with fixed POPC, interfacial dissociation constant K(d)(∗) for LCAT penetration decreased and interfacial Michaelis constant K(m)(∗) did not change. Also, differences in stability and unfolding cooperativity between two domains in apoA-I molecule increased. At the increase of surface area of the complexes with varying POPC, K(d)(∗) increased, while K(m)(∗) decreased. For both lipidation states the rate constant of acyl-LCAT formation did not vary and any changes in K(m)(∗) are postulated to originate from the change(s) in association/dissociation rate constants of enzyme-substrate complex. Then, at the increase of POPC, the LCAT-POPC complex becomes more stable. ApoA-I seems to "activate" substrate by increasing the exposure of POPC ester bond to active center of LCAT.