Α-methylacyl-coa Racemase. an Enzyme at Crossroads in Lipid Metabolism

α-Methylacyl-CoA racemase (Amacr) is an enzyme at the merging point of two important pathways of lipid metabolism: elimination of methyl-branched fatty acids and synthesis of bile acids. Amacr is regarded as obligatory for these processes. Patients with Amacr-deficiency suffer from adult onset sensory motor neuropathy and/or severe neonatal cholestasis with coagulopathy and fat-soluble vitamin malabsorption. Amacr is also linked to cancer and so far has been proposed as a new marker for diagnosis of at least prostate and colon cancers. Common sources of phytol derived branchedchain fatty acids for man are ruminant fats, meat and dairy products. The bile acid synthesis is the main pathway for cholesterol catabolism. Amacr is considered to be a member of family III of the CoA transferases (L-carnitine dehydratase bile acid inducible protein F (CaiB-BaiF) family) and localized to two subcellular compartments, mitochondria and peroxisomes. In this work the mouse gene encoding Amacr was characterized, the gene was inactivated and mutational and structural studies were used to determine the loop and the active site structure of the enzyme. It was shown that mouse Amacr which locates both to mitochondria and peroxisomes, is an identical product of a single gene, which is located at chromosome 15, region 15B1. Neither alternative replication, splicing, or any post-translational modifications of the enzyme occur. The mouse model for Amacr-deficiency indicated a role of Amacr in detoxification of methylbranched fatty acids, and suggested that a diet free from these phytol metabolites may function as a treatment for the deficiency. Furthermore, major changes were observed in the bile acid pool of the knock-out mice compared to wild type mice. However, the study suggests that there is an Amacrindependent pathway for synthesis of bile acids albeit of low capacity, which provides a way for Amacr-deficient individuals to survive. The mutational and structural studies confirmed Amacr as a member of family III of the CoA transferases. Furthermore, according to comparisons of the structural data of Amacr and other members of the family (FRC, YfdW), the superfamily can be divided into two subgroups, racemases and transferases. Proteins in the subfamilies share the CoA-binding mode, but the substrate specificities as well as the catalysed reaction differ greatly.