Sirtuin 4 is a lipoamidase regulating pyruvate dehydrogenase complex activity.

Area of Science:

• Biochemistry
• Molecular Biology
• Metabolic Regulation

Background:

• Sirtuins (SIRTs) are NAD+-dependent enzymes involved in various cellular processes, including aging and metabolism.
• Mitochondrial sirtuins SIRT4 and SIRT5 exhibit limited deacetylase activity, with SIRT4's catalytic function remaining largely uncharacterized.
• The pyruvate dehydrogenase complex (PDH) is a key metabolic enzyme complex regulating the conversion of pyruvate to acetyl-CoA.

Purpose of the Study:

• To elucidate the catalytic activity and function of mitochondrial Sirtuin 4 (SIRT4).
• To investigate SIRT4's role in the regulation of the pyruvate dehydrogenase complex (PDH).
• To characterize SIRT4 as a potential therapeutic target for metabolic disorders.

Main Methods:

• Enzymatic assays to determine SIRT4's catalytic activity on various substrates, including lipoyl-lysine and acetyl-lysine.
• Biochemical analyses of PDH complex components, specifically dihydrolipoyllysine acetyltransferase (DLAT).
• Cellular and in vivo studies in mouse liver to assess SIRT4's physiological impact on PDH activity and lipoylation.

Main Results:

• SIRT4 functions as a lipoamidase, efficiently hydrolyzing lipoamide cofactors from lysine residues.
• SIRT4's lipoamidase activity is significantly higher than its deacetylase activity.
• SIRT4 directly targets DLAT, a component of the PDH complex, leading to reduced PDH activity.
• SIRT4-mediated inhibition of PDH activity was observed in cellular models and in mouse liver.
• Glutamine-induced metabolic flux activates SIRT4's lipoamidase function, inhibiting PDH.

Conclusions:

• SIRT4 is a mitochondrial lipoamidase that regulates PDH activity by cleaving lipoamide cofactors from DLAT.
• This lipoamidase activity represents the primary catalytic function of SIRT4, distinct from its known deacetylase function.
• SIRT4 acts as a crucial regulator of cellular metabolism, particularly in response to metabolic cues like glutamine stimulation.