The kinetic parameters of the branching enzyme of Rhodothermus obamensis (RoBE) were established after in vitro incubation with different branched or unbranched ¦Á-glucans of controlled structure.
A minimal chain length of ten glucosyl units was required for the donor substrate to be recognized by RoBE that essentially produces branches of DP 3-8. We show that RoBE preferentially creates new branches by intermolecular mechanism. Branched glucans define better substrates for the enzyme leading to the formation of hyper-branched particles of 30-70 nm in diameter (dextrins). Interestingly, RoBE catalyzes an additional ¦Á-4-glucanotransferase activity not described so far for a member of the GH13 family.
RoBE is able to transfer ¦Á(1 ¡ú 4)-linked-glucan in C4 position (instead of C6 position for the branching activity) of a glucan to create new ¦Á(1 ¡ú 4) linkages yielding to the elongation of linear chains subsequently used for further branching. This result is a novel case for the thin border that exists between enzymes of the GH13 family.
This work reveals the original catalytic properties of the thermostable branching enzyme of R. obamensis. It defines new approach to produce highly branched ¦Á-glucan particles in vitro.