Enoyl thioester reductases—enzymes of fatty acid synthesis and degradation in mitochondria

• Main Author: Miinalainen, I. (Ilkka)
• Format: Doctoral Thesis
• Language: English
• Published: University of Oulu 2006
• Subjects: 2,4-dienoyl-CoA reductase; 2-enoyl thioester reductase; energy metabolism; knock-out mice; β-oxidation
• Online Access: http://urn.fi/urn:isbn:9514282442; http://nbn-resolving.de/urn:isbn:9514282442

Abstract Fatty acids are one of the most essential categories of biological lipids and their synthesis and degradation are vital for all organisms. Severely compromised phenotypes of yeast mutants and human patients, which have defective components in their degradative or synthetic processes for fatty acid metabolism, have highlighted the importance of these processes for overall metabolism. Most fatty acids are degraded by β-oxidation, which occurs in mitochondria and peroxisomes in mammals, whereas synthesis is catalyzed by cytosolic multifunctional peptides, although a synthesis system involving individual enzymes in mitochondria has been also proposed. In this study a novel mitochondrial 2-enoyl thioester reductase Etr1p from the yeast Candida tropicalis, its homolog Mrf1p from Saccharomyces cerevisiae, and their mammalian ortholog were identified and characterized. Observations indicating that mitochondrial localization as well as enzymatic activity is needed to complement the respiratory-deficient phenotype of the mrf1Δ strain from S. cerevisiae suggests that Etr1p and Mrf1p might act as a part of the mitochondrial fatty acid synthesis machinery, the proper function of which is essential for respiration and the maintenance of mitochondrial morphology in yeast. The mammalian enzyme, denoted Nrbf-1p, showed similar localization, enzymatic activity, and ability to rescue the growth of the mrf1Δ strain suggesting that mammals are also likely to possess the ability and required machinery for mitochondrial fatty acid synthesis. This study further included the characterization of another mitochondrial thioester reductase, 2,4-dienoyl-CoA reductase, which acts as an auxiliary enzyme in the β-oxidation of unsaturated fatty acids. The function of this gene was analyzed by creating a knock-out mouse model. While unstressed mice deficient in 2,4-dienoyl-CoA reductase were asymptomatic, metabolically challenged mice showed symptoms including hypoglycemia, hepatic steatosis, accumulation of acylcarnitines, and severe intolerance to acute cold exposure. Although the oxidation of saturated fatty acids proceeds normally, the phenotype was in many ways similar to mouse models of the disrupted classical β-oxidation pathway, except that an altered ketogenic response was not observed. This mouse model shows that a proper oxidative metabolism for unsaturated fatty acids is important for balanced fatty acid and energy metabolism.