Characterization of a Novel Alkalophilic Lipase From Aneurinibacillus thermoaerophilus: Lid Heterogeneity and Assignment to Family I.5.

Recent investigations of Aneurinibacillus thermoaerophilus strains have allowed identification of a unique solvent tolerant lipase, distinct from known lipases. This work reports the expression and purification of this lipase (LipAT) and the first characterization of its structure and temperature and pH-dependent behaviour. LipAT has a secondary structural content compatible with the canonical lipase α/β hydrolase fold, and is dimeric at neutral pH. The protein was folded from pH 5 to 10, and association into folded aggregates at pH 7 and 8 likely protected its secondary structures from thermal unfolding. The enzyme was active from 25 to 65 °C under neutral pH, but its maximal activity was detected at pH 10 and 45 °C. The ability of LipAT to recover from high temperature was investigated. Heating at 70 °C and pH 10 followed by cooling prevented the restoration of activity, while similar treatments performed at pH 8 (where folded aggregates may form) allowed recovery of 50% of the initial activity. In silico analyses revealed a high conservation (85% or more) for the main lipase signature sequences in LipAT despite an overall low residue identity (60% identity compared to family I.5 lipases). In contrast, the active site lid region in LipAT is very distinct showing only 25% amino acid sequence identity to other homologous lipases in this region. Comparison of lids among lipases from the I.5 family members and LipAT reveals that this region should be a primary target for elucidation, optimisation and prediction of structure-function relationships in lipases.