developed
for proteolysis [9,12,15,18,19]. Most of these studies have focused
on rapid digestion and reduction of sample volume. So far,
there is no study yet on a multienzymatic reaction system and
analysis of posttranslational modification in protein. In the current
study, we showed a simple and rapid analytical method for the
identification of protein sequence by using tandem proteaseimmobilized
microreactors. Proteolysis by the tandem microreactors
showed higher sequence coverage, which is a remarkable
result compared with that of the single microreactor or in-solution
digestion (Table 1). In addition, the tandem microreactor composed
of a protease-immobilized microreactor and a phosphatase-
immobilized microreactor showed the capability to localize
the phosphorylation site(s) in phosphoproteins. The current analytical
method is much simpler than the other conventional methods
[6–8]; for example, the phosphoprotein is just flowed through
the microreactor, and it eliminates purification of digests from the
reaction system without any enrichment strategies. These interesting
features are superior advantages of our approach using the enzyme-
immobilized microreactors over the conventional methods.
In addition, it is known that in-solution digestion by trypsin can induce
artificial modifications such as asparagine deamidation [22]
and N-terminal glutamine cyclization [23,24] on target protein
due to the elevated temperature and alkaline pH buffers used during
digestion. Proteolysis by our protease-immobilized microreactors
was achieved within a short period of time (10 min) at 30 C,
thereby suggesting these artificial modifications as a remote
possibility.