Produced by
levansucrase,
levan and
levan oligosaccharides (GFn) have potential applications in food and pharmaceutical industries such as
prebiotics, anti-
tumor and
anti-inflammatory agents. Previous study reported that Bacillus licheniformis RN-01
levansucrase could produce
levan oligosaccharides and long-chain
levan. However, its N251A and N251Y mutants could effectively produce short-chain
oligosaccharides upto GF3, but they could not produce long-chain
levan. We hypothesized that these mutations probably reduced GF3 binding affinity in
levansucrase active site that contains fructosyl-Asp93 intermediate and caused GF3 to be in an unfavorable orientation for transfructosylation; therefore,
levansucrase could not effectively extend GF3 by one fructosyl residue to produce GF4 and subsequently long-chain
levan. However, these mutations probably did not significantly reduce binding affinity or drastically change orientation of GF2; therefore,
levansucrase could still extend GF2 to produce GF3. Using this hypothesis, we employed molecular dynamics to investigate effects of these mutations on GF2/GF3 binding in
levansucrase active site. Our results reasonably support this hypothesis as N251A and N251Y mutations did not significantly reduce GF2 binding affinity, as calculated by MM-GBSA technique and hydrogen bond occupations, or drastically change orientation of GF2 in
levansucrase active site, as measured by distance between atoms necessary for transfructosylation. However, these mutations drastically decreased GF3 binding affinity and caused GF3 to be in an unfavorable orientation for transfructosylation. Furthermore, the free energy decomposition and hydrogen bond occupation results suggest the importance of Arg255 in GF2/GF3 binding in
levansucrase active site. This study provides important and novel insight into the effects of N251A and N251Y mutations on GF2/GF3 binding in
levansucrase active site and how they may disrupt production of long-chain
levan. This knowledge could be beneficial in designing
levansucrase to efficiently produce
levan oligosaccharides with desired length.