ACAD9, a complex I assembly factor with a moonlighting function in fatty acid oxidation deficiencies.

Area of Science:

• Mitochondrial metabolism
• Enzyme function
• Protein biogenesis

Background:

• Oxidative phosphorylation and fatty acid oxidation are key mitochondrial pathways.
• Acyl-CoA dehydrogenase 9 (ACAD9) was implicated in complex I biogenesis, but its enzymatic role was unclear.
• ACAD9's function in fatty acid oxidation and complex I assembly requires further investigation.

Purpose of the Study:

• To investigate the enzymatic activity of ACAD9 in fatty acid oxidation.
• To determine if ACAD9's catalytic activity is essential for mitochondrial complex I biogenesis.
• To elucidate the role of ACAD9 in acylcarnitine metabolism and its diagnostic significance.

Main Methods:

• In vivo enzyme activity assays.
• Fibroblast knockdown experiments (ACAD9 and VLCAD deficient cells).
• Acylcarnitine profiling following fatty acid loading.
• Analysis of complex I assembly intermediates.

Main Results:

• ACAD9 demonstrates in vivo enzymatic activity, producing specific acylcarnitines (C14:1-carnitine from oleate, C12-carnitine from palmitate).
• These findings explain the origin of obscure acylcarnitines used in diagnosing very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency.
• Catalytically inactive ACAD9 partially or fully rescued complex I biogenesis in ACAD9-deficient cells, indicating its structural role.

Conclusions:

• ACAD9 possesses enzymatic activity in vivo, contributing to specific fatty acid oxidation pathways.
• The enzymatic function of ACAD9 may be a vestigial trait from a gene duplication event.
• ACAD9 protein is crucial for mitochondrial complex I assembly, independent of its catalytic activity.