A host of water-soluble enzymes are active at membrane surfaces and in association with membranes. Some of these enzymes are involved in signalling and in modification and remodelling of the membranes. A special class of enzymes, the phospholipases, and in particular secretory phospholipase A(2) (sPLA(2)), are only activated at the interface between water and membrane surfaces, where they lead to a break-down of the lipid molecules into lysolipids and free fatty acids. The activation is critically dependent on the physical properties of the lipid-membrane substrate. A topical review is given of our current understanding of the physical mechanisms responsible for activation of sPLA(2) as derived from a range of different experimental and theoretical investigations.

We present a method for constant-pressure and constant-surface tension simulations in dissipative particle dynamics using a Langevin piston approach. We demonstrate that the corresponding equations of motion lead to the relevant ensembles and propose an appropriate scheme of integration. After having identified a suitable set of parameters for the approach, we demonstrate the feasibility of the approach by applying it to two different systems, a simple isotropic fluid and an anisotropic fluid lipid-bilayer membrane in water. Results are presented for, respectively, isothermal bulk compressibility, tracer diffusion coefficient, lipid head-group area, and isothermal area compressibility. We find that our Langevin piston approach leads to improvements over other approaches in terms of faster equilibration and shorter correlation times of various system variables.

With special focus on dissipative particle dynamics simulations of anisotropic and complex soft matter, such as lipid bilayers in water, we have investigated the occurrence of artifacts in the results obtained from dynamical simulations of coarse-grained particle-based models. The particles are modeled by beads that interact via soft repulsive conservative forces (as defined in dissipative particle dynamics simulations), harmonic bond potentials, as well as bending potentials imparting stiffness to the lipid tails. Two different update schemes are investigated: dissipative particle dynamics with a velocity-Verlet-like integration scheme [G. Besold, I. Vattulainen, M. Karttunen, and J. M. Polson, Phys. Rev. E 63, R7611 (2000)] and Lowe-Andersen thermostatting [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] with the standard velocity-Verlet integration algorithm. By varying the integration time step, we examine various physical quantities, in particular pressure profiles and kinetic bead temperatures, for their sensitivity to artifacts caused by the specific combination of integration technique and the thermostat. We then propose a simple fingerprint method that allows monitoring the presence of simulation artifacts.

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Phospholipases are a class of molecular machines that are involved in the active remodelling processes of biological membranes. These lipases are interfacially activated enzymes and in the specific case of phospholipase A(2) (PLA(2)) the enzyme catalyses the hydrolysis of di-acyl phospholipids into products of lysolipids and fatty acids, that dramatically change the physical properties of lipid membrane substrates. Using dissipative particle dynamics simulations on a simple coarse-grained bead-spring model of a fluid lipid bilayer in water, the mechanical and diffusive properties of the bilayer in the pure state and after the action of PLA(2) have been calculated. It is found that, in response to hydrolysis, the lipid membrane becomes mechanically softened and the various in-plane and trans-bilayer diffusional modes become enhanced. The results compare favourably with available experimental data.