The adsorption to and hydrolysis of 1,3-didecanoyl glycerol monolayers by pancreatic lipase. Effects of substrate packing density

Exploring the influence of phospholipid monolayer conformation and environmental conditions on the interfacial binding of Gibberella Zeae lipase

The involvement of different parameters on Gibberella zeae lipase (GZEL) membrane binding were characterized by using monomolecular film technology and circular dichroism spectroscopy. Among four kinds of phospholipid monolayers, 1,2‑dimyristoyl‑sn‑glycero‑3‑phosphoethanolamine have the highest maximum insertion pressure (MIP) value. Comparing the GZEL adsorption to phosphatidylcholine monolayers with different acyl chains in sn-1 and sn-2 positions, the higher MIP values were found for 1,2‑dilauroyl‑sn‑glycero‑3‑phosphocholine. Significantly improvement between 1,2‑dioleoyl‑sn‑glycero‑3‑phosphocholine and 1,2‑distearoyl‑sn‑glycero‑3‑phosphocholine suggested that the presence of fatty acid unsaturation may affect protein adsorption by changing the chemical structure in each phospholipid. The MIP value was shown higher (48.6 mN m^-1) at pH 5 and pH 6 (47.5 ± 1.9 mN m^-1) but decreased significantly (34.2 mN m^-1) at pH 9. This may indicate that the proportion of helices in the protein decreases with the alteration of the catalytic center, thus affecting the binding of the protein to its substrate. The MIP values obviously decreased with increasing salt ion concentration, suggesting that excessive salt ion concentration may destabilize the secondary and tertiary structures of the protein, thereby affecting the characteristics of its adsorption at the interfaces. Present studies improve our understanding on the protein-membrane interaction of this enzyme.

Function of C-terminal peptides on enzymatic and interfacial adsorption properties of lipase from Gibberella zeae

BACKGROUND

The crystal structure of lipase from Gibberella zeae (GZEL) indicates that its C-terminal extension is composed of a loop and a α-helix. This structure is unique, possibly providing novel evidence on lipase mechanisms.

METHODS

Two C-terminally truncated mutants (GZEL-Δ(α-helix) and GZEL-Δ(α-helix+loop)) were constructed. The role of these secondary structure segments on enzymatic activities and interfacial binding properties of GZEL was investigated by using conventional pH-stat method and monomolecular film techniques. In addition, inactive variants (Ser144Ala) of wild-type GZEL and two truncated mutants were constructed and produced specifically for interfacial binding experiments.

RESULTS

Compared to the wild-type GZEL, lipase and phospholipase activities were significantly decreased in the two mutants. Deletion of the α-helix had great influence on the lipase activity of GZEL, resulting in residual 7.3% activity; the additional deletion of the loop led to 8.1% lipase activity. As for the phospholipase function, residual activities of 63.0% and 35.4% were maintained for GZEL-Δ(α-helix) and GZEL-Δ(α-helix+loop), respectively. Findings obtained with monomolecular film experiments further indicated that the reduction in phospholipase activity occurred with the anionic phospholipid as substrate, but was not seen with zwitterionic phospholipid. Results of the maximum insertion pressure, synergy factor and binding kinetic parameters documented that the α-helix structure of GZEL strongly influence the binding and insertion of enzyme to the phospholipid monolayer. Moreover, the interfacial binding function of α-helix was partly conformed by connecting to the C-terminal of Aspergillus oryzae lipase.

GENERAL SIGNIFICANCE

Our results provide important information on the understanding of the structure-function relationship of GZEL.

Surface fluorescence resonance energy transfer studies on interfacial adsorption of Thermomyces (humicola) lanuginosa lipase, using monomolecular films of cis-parinaric acid

The fluorescence resonance energy transfer (FRET) technique was adapted to study the process whereby lipase is adsorbed to monomolecular lipid films spread at the air-water interface. When cis-parinaric acid (cis-PnA) was spread over an aqueous subphase before the injection of sodium taurodeoxycholate (NaTDC) and Thermomyces lanuginosa lipase (TLL), no FRET was observed. Under these conditions, no adsorption of TLL was detected using an ELISA. In contrast, FRET occurred when cis-PnA was spread over an aqueous subphase containing NaTDC and TLL. The FRET signals observed were attributed to the interactions between the adsorbed TLL and the cis-PnA monomolecular films. Comparisons between the fluorescence emission spectra corresponding to the bulk phase and the aspirated film, in the presence and absence of TLL, showed that cis-PnA was undetectable in the bulk phase. We concluded that the FRET originated from the interface and not from the bulk phase. Using surface FRET, we estimated that the surface excess of the catalytically inactive mutant, TLL(S146A), was 1.6 higher than that present in the wild-type TLL. This finding is in agreement with independent measurements of the surface excess of TLL and TLL(S146A) on monomolecular films of cis-PnA.

Effect of monolayer lipid structure and composition on the lipoprotein lipase-catalyzed hydrolysis of triacylglycerol

The effect of lipid composition and structure on the lipoprotein lipase-catalyzed hydrolysis of triacylglycerols was determined in a monolayer system consisting of purified bovine milk lipoprotein lipase and fatty acid-free albumin. In a monolayer of dioleoylphosphatidylcholine containing 1-6 mol% of either tri[14C]oleoylglycerol or tri[14C] palmitoylglycerol , lipoprotein lipase catalyzed the hydrolysis of the unsaturated triacylglycerol at a higher rate than the saturated lipid and in either the presence or absence of apolipoprotein C-II, the activator protein for the enzyme. For example, with 3 mol% triacylglycerol and in the presence of apolipoprotein C-II, the rate of the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol was 27 mumol oleic acid produced/h per mg enzyme vs. 12 mumol for tri[14C] palmitoylglycerol . The effect of phospholipid fatty acyl chain length and unsaturation/saturation, polar head group and surface density on the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol was determined. The rate of enzyme hydrolysis of triacylglycerol was similar whether the phospholipid was a diester or diether lipid or the polar head group was ethanolamine or choline. In general, phospholipids with shorter and unsaturated fatty acyl chains gave higher rates of lipoprotein lipase hydrolysis of triacylglycerol than the corresponding longer and saturated lipids. However, with all phospholipids tested, the rate of enzyme hydrolysis decreased with increasing surface density. Lipoprotein lipase showed no activity toward triacylglycerol in a monolayer of sphingomyelin; addition of dioleoylphosphatidylcholine to the monolayer enhanced the rate of enzyme catalysis. Cholesterol (50 mol%) in a dipalmitoylphosphatidylcholine monolayer increased the rate of the lipoprotein lipase-catalyzed hydrolysis of tri[14C]oleoylglycerol, whereas cholesterol decreased the rate in a dioleoylphosphatidylcholine monolayer. The effect of phospholipid structure and surface density on lipoprotein lipase activity could not be accounted for by the amount of apolipoprotein C-II which was present at the interface. Based on these findings and other reports in the literature, we suggest that the catalytic activity of lipoprotein lipase toward tri[14C] oleylglycerol in various monolayers is dependent on the conformation or appropriate physical state of the triacylglycerol substrate at the lipid interface.