Orthologous lipoxygenases of Pleurotus spp. – A comparison of substrate specificity and sequence homology

A selection of Pleurotus spp. was screened for intracellular lipoxygenase activity, and strains with distinguished activities were chosen for a screening on the molecular level. Lipoxygenase genes from five different Pleurotus spp. were amplified from the corresponding cDNA, functionally expressed using a cold shock expression system in Escherichia coli BL21DE3 Star cells and characterized for specific activity and reaction optima. All lipoxygenase sequences coded for proteins of 643 amino acids, sharing similarities >95% among each other and to a previously characterized LOXPsa1 from Pleurotus sapidus. Lipoxygenase activities were quantified using linoleic acid as substrate and reached similar values ranging from 95 U/mg to 118 U/mg, with Km values between 58 and 106 μM. Optimum reaction conditions were pH 7 and 30–35 ̊C. However, the uncommon trait of accepting (+)-valencene, a sesquiterpene hydrocarbon substrate, differed strongly between two clusters of highly homologous sequences. No exchange of amino acids adjacent to the active site was found. Cloning and expression of a truncated LOXPsa1 sequence missing 144 amino acid residues of the N-terminal barrel domain yielded soluble protein but no measurable activity.