Nanoscopic bubbles with plastic membrane and built-in
enzymes for multistep one-pot reactions.
Living cells are highly complex synthetic machines: Numerous multistep
reactions run simultaneously side by side and with unbelievable
efficiency and specificity. For these mainly enzymatic reactions to work
so well collectively, nature makes use of a variety of concepts. One of
the most important of these is division into compartments. Enzymes are
not only separated spatially, but also positioned in specific locations
within the cell. Researchers from the Netherlands, led by Jan C. M. van
Hest and Alan E. Rowan, have now developed an approach to copy this idea,
as they report in the journal Angewandte Chemie. They constructed
nanoreactors by controlled positioning of two different enzymes in the
central water reservoir or the plastic membrane of synthetic nanoscopic
bubbles. In combination with a third enzyme in the surrounding solution,
this system has made it possible to run three different enzymatic
reactions simultaneously, without interference, in a �one-pot� reaction.
To mimic a cellular environment, the scientists produced nanoscopic
bubbles surrounded by a membrane made of a special plastic. The
plastic is a block copolymer that is analogous to a lipid, the natural
building block of cell membranes, in its structure, with a
water-friendly �head� and a water-repellent �tail�. In analogy to
liposomes, which are made from lipids, these bubbles are called
polymersomes. Thanks to nearly limitless possibilities in the
production of these plastic membranes, the spectrum of properties
displayed by polymersomes can be precisely tailored.
The researchers produced their polymersomes such that they let small
molecules pass through while forming a barrier to larger ones. This
allows enzymes to be trapped inside the polymersomes (in the water
reservoir) while the smaller substrate or product molecules pass
through unhindered.
To demonstrate the potential of their �nanoreactors�, the researchers
bound the enzyme horseradish peroxidase into the membrane itself.
Within the water reservoir, they trapped the enzyme glucose oxidase.
The surrounding solution contained the enzyme lipase B. Glucose
molecules with four acetyl groups attached were added as the substrate.
In the first step, the lipase B split off the acetyl groups. The
resulting glucose could cross the membrane, where it encountered the
glucose oxidase and was oxidized by it. This reaction formed hydrogen
peroxide, which is just what the horseradish peroxidase was waiting
for in order to convert the sample substrate ABTS
(2,2�-azinobis(3-ethylbenzthiazoline-6-sulfonic acid)) - also contained
in the solution - into its radical cation.
