Functional analysis and enzymatic modification by site-directed mutagenesis of an omega-6 fatty acid desaturas from Arctic Chlamydomonas sp.

Abstract

Arctic Chlamydomonas sp. is a dominant microalgal strain in cold or frozen freshwater in the Arctic region. The full-length open reading frame of the omega-6 fatty acid desaturase gene (AChFAD) was obtained from the transcriptomic database of Arctic Chlamydomonas sp. from the KOPRI Culture Collection of Polar Microorganisms (KCCPM). Amino acid sequence analysis indicated the presence of three conserved histidine-rich segments as unique characteristics of omega-6 FADs, and three transmembrane regions transported to plastidic membranes by chloroplast transit peptides in the N-terminal region. Arctic Chlamydmonas sp. omega-6 fatty acid desaturase with 48.2 kDa showed enzymatic activity enhancing the concentration of linoleic fatty acid in the E. coli expression system. The AChFAD6 desaturase activity was examined by expressing wild-type and V254A mutant (Mut-AChFAD6) heterologous recombinant proteins. Quantitative gas chromatography indicated that the concentration of linoleic acids in AChFAD6-transformed cells increased more than three-fold (6.73 ± 0.13 mg g-1 dry cell weight (DCW)) compared with cells transform- ed with vector alone. In contrast, transformation with Mut-AChFAD6 increased the concentration of oleic acid to 9.23 ± 0.18 mg g-1 DCW, indicating a change in enzymatic activity to mimic that of stearoyl-CoA desaturase (SCD). These results demonstrate that AChFAD6 of Arctic Chlamydomonas sp. increases membrane fluidity by enhancing desaturating C18 fatty acids and facilitates production of large quantities of linoleic fatty acids in prokaryotic expression systems. Therefore, genetic modification on the membrane-spanning regions of fatty acid desaturases might increase the yield of target fatty acids and modulate enzymatic activities for industrial application.