Molecular dynamics provides insight into how N251A and N251Y mutations in the active site of Bacillus licheniformis RN-01 levansucrase disrupt production of long-chain levan

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.