Long-chain-fatty-acid--CoA ligase ACSBG2
cytoplasm
The second member of the human and murine bubblegum family is a testis- and brainstem-specific acyl-CoA synthetase.
Acyl-CoA synthetases that activate fatty acids to their CoA derivatives play a central role in fatty acid metabolism. ACSBG1, an acyl-CoA synthetase originally identified in the fruit fly mutant bubblegum, was hypothesized to contribute to the biochemical pathology of X-linked adrenoleukodystrophy. We looked for homologous proteins and identified ACSBG2 in humans, mice, and rats. Human ACSBG1 and ACSBG2 amino acid sequences are 50% identical. ACSBG2 expression was confined to the testis and brainstem. Immunohistochemistry and in situ hybridization studies further localized ACSBG2 expression to testicular Sertoli cells and large motoneurons in the medulla oblongata and cervical spinal cord. Full-length cDNA encoding human and mouse ACSBG2 was cloned. In transfected COS-1 cells, both human and murine ACSBG2 were detected as 75- to 80-kDa proteins by Western blot. Cells overexpressing ACSBG2 had increased ability to activate oleic acid (C18:1omega9) and linoleic acid (C18:2omega6) but not other fatty acid substrates tested. Within a highly conserved motif known to be important for catalysis, human ACSBG2 contains a histidine residue where all other known acyl-CoA synthetases, including mouse and rat ACSBG2, contain an arginine. This substitution resulted in a shift of the human ACSBG2 pH optimum to a more acidic pH. Mutation of this histidine to arginine improved catalytic function at neutral pH by shifting the pH profile without affecting substrate specificity. Although the role of ACSBG2 in testicular and neuronal lipid metabolism remains unclear, the limited tissue expression pattern and limited substrate specificity rule out a likely role for this enzyme in X-linked adrenoleukodystrophy pathology.