The structural basis for specificity in lipoxygenase catalysis

Expression and characterization of manganese lipoxygenase of the rice blast fungus reveals prominent sequential lipoxygenation of α-linolenic acid

Magnaporthe oryzae causes rice blast disease and has become a model organism of fungal infections. M. oryzae can oxygenate fatty acids by 7,8-linoleate diol synthase, 10R-dioxygenase-epoxy alcohol synthase, and by a putative manganese lipoxygenase (Mo-MnLOX). The latter two are transcribed during infection. The open reading frame of Mo-MnLOX was deduced from genome and cDNA analysis. Recombinant Mo-MnLOX was expressed in Pichia pastoris and purified to homogeneity. The enzyme contained protein-bound Mn and oxidized 18:2n-6 and 18:3n-3 to 9S-, 11-, and 13R-hydroperoxy metabolites by suprafacial hydrogen abstraction and oxygenation. The 11-hydroperoxides were subject to β-fragmentation with formation of 9S- and 13R-hydroperoxy fatty acids. Oxygen consumption indicated apparent kcat values of 2.8 s(-1) (18:2n-6) and 3.9 s(-1) (18:3n-3), and UV analysis yielded apparent Km values of 8 and 12 μM, respectively, for biosynthesis of cis-trans conjugated metabolites. 9S-Hydroperoxy-10E,12Z,15Z-octadecatrienoic acid was rapidly further oxidized to a triene, 9S,16S-dihydroperoxy-10E,12Z,14E-octadecatrienoic acid. In conclusion, we have expressed, purified and characterized a new MnLOX from M. oryzae. The pathogen likely secretes Mo-MnLOX and phospholipases to generate oxylipins and to oxidize lipid membranes of rice cells and the cuticle.

Manganese lipoxygenase of F. oxysporum and the structural basis for biosynthesis of distinct 11-hydroperoxy stereoisomers

The biosynthesis of jasmonates in plants is initiated by 13S-lipoxygenase (LOX), but details of jasmonate biosynthesis by fungi, including Fusarium oxysporum, are unknown. The genome of F. oxysporum codes for linoleate 13S-LOX (FoxLOX) and for F. oxysporum manganese LOX (Fo-MnLOX), an uncharacterized homolog of 13R-MnLOX of Gaeumannomyces graminis. We expressed Fo-MnLOX and compared its properties to Cg-MnLOX from Colletotrichum gloeosporioides. Electron paramagnetic resonance and metal analysis showed that Fo-MnLOX contained catalytic Mn. Fo-MnLOX oxidized 18:2n-6 mainly to 11R-hydroperoxyoctadecadienoic acid (HPODE), 13S-HPODE, and 9(S/R)-HPODE, whereas Cg-MnLOX produced 9S-, 11S-, and 13R-HPODE with high stereoselectivity. The 11-hydroperoxides did not undergo the rapid β-fragmentation earlier observed with 13R-MnLOX. Oxidation of [11S-(2)H]18:2n-6 by Cg-MnLOX was accompanied by loss of deuterium and a large kinetic isotope effect (>30). The Fo-MnLOX-catalyzed oxidation occurred with retention of the (2)H-label. Fo-MnLOX also oxidized 1-lineoyl-2-hydroxy-glycero-3-phosphatidylcholine. The predicted active site of all MnLOXs contains Phe except for Ser(348) in this position of Fo-MnLOX. The Ser348Phe mutant of Fo-MnLOX oxidized 18:2n-6 to the same major products as Cg-MnLOX. Our results suggest that Fo-MnLOX, with support of Ser(348), binds 18:2n-6 so that the proR rather than the proS hydrogen at C-11 interacts with the metal center, but retains the suprafacial oxygenation mechanism observed in other MnLOXs.