Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that catalyze the oxidative cleavage of β(1-4) glycosidic bonds and have attracted great attention due to importance in increasing the efficiency in degradation of recalcitrant polymeric substrates, in synergism with the action of hydrolytic enzymes, as an accessory function. However, LPMOs act via oxidative cleavage rather than hydrolysis. In industrial applications, LPMOs of fungal origin are the most frequent, while other taxonomic groups have been described as possible alternative sources of these enzymes. In the present study, we aimed to identify and characterize in silico a LPMO of cyanobacterial origin with putative functions in chitin depolymerization. The search for sequence similarity and conservation of domains with other characterized LPMOs identified a 289 AA protein from the cyanobacterium Mastigocoleus testarum of the Nostocales Order, being a probable LPMO of the CAZy-AA10 class. This protein is referred to as MtLPMO10. Phylogenetic analysis revealed that MtLPMO10 is homologous to the Tma12 protein from the fern Tectaria macrodonta, with 52.11% identity, which was the first LPMO characterized as originating from the plant kingdom. The occurrence of shared structural and functional patterns with other AA10 class LPMOs, as well as the existence of variations in these established patterns, contributed to the understanding of the biological functions of this class of enzymes. The predicted protein tertiary arrangement by the AlphaFold server pointed out structural features common to LPMOs, especially a histidine brace composed of His31 and His132 and an immunoglobulin-like domain consisting of antiparallel beta strands. Molecular dynamics simulation (MD) allowed evaluating the enzyme-substrate affinity, using an initial pose based on data obtained from the literature. There was stability of the MtLPMO10-chitoheptaose complex during 100ns of MD, while the MtLPMO10-celloheptaose complex broke apart in 30ns of MD. Also, there was a shorter Cu(I)-H4 distance in the first complex compared to the Cu(I)-H1 distance (averages 6.0 ± 0.7 Å and 7.9 ± 0.7 Å, respectively), suggesting a C4-type regioselectivity, as defined for Tma12. This study highlights the existence of lytic polysaccharide monooxygenases in cyanobacteria and paves the way for further investigations related to this enigmatic class of enzymes and their potential use in biotechnological applications.
|
