3. Redistribution effect versus competitive effect of a lipid-inducing domain peptide

Similar to the enzyme, again we will not consider the substrate dilution due to the protein insertion in the bilayer. That is because the number of molecules binding to the lipid phase is much smaller than the number of the phospholipid substrate molecules, and besides, the domain-inducing peptide either would not penetrate the lipid phase or its interface concentration can be considered negligible.

As seen before, depending on the enzymatic model, substrate redistribution such as the transitions from homogeneous distribution to nonhomogeneous

Effects of Vesicular Membranes Reordering on the Activity of Lipid Metabolizing Enzymes DOI: http://dx.doi.org/10.5772/intechopen.85972

distribution could change the enzyme activity. However, another effect must be considered when there are domains that have been induced by soluble peptides (e.g., basic peptides such as pentalysine), which interact directly with the phospholipid substrate of the membrane (i.e., acidic phospholipid such as PIP2). In this case, such interaction could be enough to consider a competitive effect over the enzyme activity; i.e., in the lipid-water interface, the domain-inducing peptide would compete with the enzyme for the substrate, because there would be less free substrate to bind to the enzyme. Then, we have proposed that the superposition of both redistribution and competitive effects may explain some results in the literature that appear as contradictory. [19]

#### 3.1 The kinetic effects of peptide induction of phospholipid domains

In order to calculate the effects on the PLC-β activity (a lipolytic enzyme) on PIP2 (lipid substrate) due to pentalysine-induced domain formation, it has been assumed that the stoichiometry of binding is one lipid substrate per one domaininducing peptide [19]. In case of larger stoichiometry for the phospholipid binding to the peptide (as Kim et al. describes [20]), this would imply that the competitive effect from peptides tends to decrease the enzyme activity more dramatically at low substrate molar fractions in the 1:1 stoichiometric case. Then, to estimate the amount of substrate bound to all the domain-inducing peptides in any lipid phase, it was assumed that the domain-inducing peptide near the surface of the lipid phase is in equilibrium with the phospholipid substrate, obeying a Langmuir isotherm, and this peptide concentration was determined by the electrochemical equilibrium in according with a Boltzmann-like relationship, which included the membrane potential in the lipid phase and the peptide concentration in the bulk solution [21]. Moreover, knowing the substrate binding to the domain-inducing peptide, free substrate can be calculated, and then the molar fraction of free substrate can be taken into account into the deduced previous kinetic models (Eqs. 3 and 9).

As a result, if there is competition effect due to peptide binding substrate, in case of the surface-binding model, the enzymatic activity will always diminish because the substrate reordering has no effect in the enzyme activity. Instead, in case of the phospholipid-binding model with peptide-induced breakage of substrate homogeneity the enzyme activity may either increase or decrease depending on the difference between the competitive effect (diminishing the enzyme activity) and the substrate distribution effect (increasing the enzyme activity) [19]. A theoretical estimation of PLCβ, acting on PIP2 as substrate, and having enriched substrate domain induced by pentalysine, has been shown in Figure 4 of Salinas et al. 2005 [19]. A maximum for an enriched domain, with acute declination for others, are shown.
