  Soluble alpha-amylases play an important role in the catabolism of
polysaccharides. In this work, we show that the enzyme can interact with the
lipid membrane and further alter its mechanical properties. Vesicle fluctuation
spectroscopy is used for quantitative measurement of the membrane bending
rigidity of phosphatidylcholines lipid vesicles from the shape fluctuation
based on the whole contour of Giant Unilamellar Vesicles (GUVs). The bending
rigidity of the lipid vesicles of
1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine in water increases
significantly with the presence of 0.14 micromolar alpha-amylase in the
exterior solution. However, as the concentration increases above 1 micromolar,
the bending rigidity decreases but remains higher than estimated without the
protein. Contact between the alpha-amylase in the outer solution and the outer
leaflet leads to spontaneous membrane curvature and the corresponding
morphological changes of the GUVs. The presence of outbuds directly
demonstrates that AA has a preferable interaction with the membrane, giving a
positive spontaneous curvature of $C_0 \leq 0.05 \ \mu \rm{m}^{-1}$ at $18 \
\mu$M $(\approx$ 1 mg/ml) of AA concentration. Above 1 mg/ml of AA
concentration the shape of GUVs collapse completely suggesting a highly
convoluted state.
