Characterization of Lipoxygenases from Cyanothece sp.
von Julia Newie
Datum der mündl. Prüfung:2016-01-01
Betreuer:Prof. Dr. Ivo Feussner
Gutachter:Prof. Dr. Ivo Feussner
Gutachter:Prof. Dr. Kai Tittmann
EnglischLipoxygenases (LOXs) are non-heme iron- or manganese-containing dioxygenases that catalyze the regio- and stereospecific peroxidation of polyunsaturated fatty acids containing at least one cis,cis-1,4-pentadiene system. In previous studies, two iron-containing LOX isozymes, CspLOX1 and CspLOX2, have been identified in Cyanothece sp. and their crystal structures have been solved. In this study, both enzymes were characterized in detail to elucidate the relationship between protein structure and function. Most interesting were thereby the N-terminal α-helical extension of CspLOX1 which is not present in other LOXs and the unusual oxygenation specificity of CspLOX2. The here described liposome binding studies with CspLOX1 revealed that the α-helical extension is crucial for binding of CspLOX1 to vesicles. This domain might thus present a new membrane targeting domain that could be involved in direct substrate acquisition from membranes. Furthermore, the structural factors of CspLOX2 that control the oxygen insertion at the middle position of the pentadiene system were analyzed. Substitution of amino acids in the core of the active site by smaller residues shifted the major products of linoleic acid from the bis-allylic 11R- to either 9R-, 9S-, 13R- or 13S-hydroperoxy octadecadienoic acid in correlation with the position of the substituted amino acid. Furthermore, in a gain of function approach, the active site of CspLOX1 was constricted by two point mutations to mimic the CspLOX2 active site. This in turn induced 11-HPODE formation in CspLOX1. Together with computational data, these results suggest that steric shielding plays an important role in the formation of the bis-allylic product. Steric shielding is probably one of the most important factors to explain the oxygenation specificity also for other LOXs.
Keywords: lipoxygenase; enzyme kinetics; metalloenzyme; protein structure; membrane binding assay; Cyanobacteria; lipid oxidation