Metabolic origin and significance of 3-methylglutaryl CoA

3-Methylglutaryl (3MG) CoA is not part of any biochemical pathway, yet its byproducts, 3MG carnitine and 3MG acid, are disease biomarkers. Both compounds are excreted in HMG CoA lyase deficiency, while 3MG aciduria occurs in inborn errors of metabolism (IEM) associated with compromised mitochondrial energy metabolism. In one such disorder (i.e., TMEM70 deficiency), 3MG carnitine is also present. Moreover, in a number of chronic and acute maladies, elevated levels of 3MG carnitine are present. The precursor of 3MG CoA is trans-3-methylglutaconyl (3MGC) CoA. When trans-3MGC CoA levels rise, a portion of this metabolite pool is reduced to 3MG CoA, potentially via a side reaction involving glutaryl CoA dehydrogenase (GCDH), which normally catalyzes the oxidative decarboxylation of glutaryl CoA to crotonyl CoA and CO₂. This reaction occurs via a two-step process wherein glutaryl CoA is initially oxidized to glutaconyl CoA, coupled to reduction of the enzyme’s FAD prosthetic group. Enzyme-bound glutaconyl CoA is then decarboxylated to the reaction product, crotonyl CoA. Before GCDH can accept another glutaryl CoA the flavin prosthetic group must be oxidized to FAD by donating electrons to electron transferring flavoprotein (ETF). However, genetic- or disease-induced defects in electron transport chain function can impede this reaction. We propose that trans-3MGC CoA is a substrate for reduced GCDH and, when glutaryl CoA and trans-3MGC CoA are present, GCDH is able to bypass ETF and cycle between oxidized and reduced states, producing crotonyl CoA and CO₂ from glutaryl CoA, and 3MG CoA from trans-3MGC CoA.