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Figure 7.
FIGURE 7. Proposed catalytic mechanism of AlgE4. A and B,
the alginate polymer enters the catalytic site. B and C, the
carboxylate moiety of the mannuronic acid in subsite +1 is
protonated, enabling it to form a hydrogen bond with Asp^152
(and/or 178), which stabilizes the substrate-enzyme complex. C,
upon deprotonation of Tyr^149 (via Arg^195) the alkoxide ion
group extracts H-5 from the re-face of the mannuronic acid in
subsite +1. C and D, a double bond is formed, which makes the
conformation of the +1 mannuronic acid partially planar. D, the
protonated His^154 performs a nucleophilic attack on the C-5
atom of the +1 sugar from the si-face with the concomitant flip
of the +1 sugar ring into the ^1C[4] chair conformation of
guluronic acid. D and E, the carboxylic acid moiety on sugar +1
is deprotonated. E and F, the epimerized sugar leaves the active
site and His^154 is protonated again. F, the epimerase is ready
to perform a new reaction.
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