linoleate + O2 |
according to their positional specificity of linoleic acid oxygenation plant LOX can be classified into linoleate 9-lipoxygenases and linoleate 13-lipoxygenases. A critical valine is detected at the active site of 9-LOX. In contrast, more bulky phenylalanine or histidine residues are found at this position in 13-LOX. Cloning of LOX-isoform from Momordica charantia and multiple amino acid alignments indicate the existence of a glutamine (Gln599) at the position where 13-LOX usually carry histidine or phenylalanine residues. Analyzing the pH-dependence of the positional specificity of linoleic acid oxygenation it is observed that at pH-values higher than 7.5 this enzyme constitutes a linoleate 13-lipoxygenase whereas at lower pH, (9S)-hydroperoxy-(10E,12Z)-octadecadienoate is the major reaction product. The structural basis for this variable reaction specificity is explored by site directed mutagenesis studies. Site-directed mutagenesis of Gln599 to His (Gln599His) converts the enzyme to a pure 13-lipoxygenase. The reaction specificity of certain LOX-isoforms is not an absolute enzyme property but may be impacted by reaction conditions such as pH of the reaction mixture |
Momordica charantia |
(10E,12Z)-9-hydroperoxy-10,12-octadecadienoate + (9Z,11E)-13-hydroperoxy-9,11-octadecadienoate |
variable positional specificity of linoleic acid oxygenation depending on the pH of the reaction mixture. Below pH 7.5, (10E,12Z)-9-hydroperoxy-10,12-octadecadienoate is the major reaction product whereas at higher pH (9Z,11E)-13-hydroperoxy-9,11-octadecadienoate is dominant. (9Z,11E)-13-hydroperoxy-9,11-octadecadienoate is preferentially formed as S enantiomer (85%) and the S/R-ratio does hardly vary over the entire pH-range. In contrast, the enantiomers composition of (10E,12Z)-9-hydroperoxy-10,12-octadecadienoate is pH-dependent. Below pH 7.5 a strong preponderance of the S enantiomer is observed. At higher pH-values the relative share of (9R,10E,12Z)-9-hydroperoxy-10,12-octadecadienoate becomes increasingly abundant reaching a 1:1 ratio at pH 9 |
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