Metabolic biology of 3-methylglutaconic acid-uria: a new perspective

Abstract

Over the past 25 years a growing number of distinct syndromes/mutations associated with compromised mitochondrial function have been identified that share a common feature: urinary excretion of 3-methylglutaconic acid (3MGA). In the leucine degradation pathway, carboxylation of 3-methylcrotonyl CoA leads to formation of 3-methylglutaconyl CoA while 3-methylglutaconyl CoA hydratase converts this metabolite to 3-hydroxy-3-methylglutaryl CoA (HMG CoA). In "primary" 3MGA-uria, mutations in the hydratase are directly responsible for the accumulation of 3MGA. On the other hand, in all "secondary" 3MGA-urias, no defect in leucine catabolism exists and the metabolic origin of 3MGA is unknown. Herein, a path to 3MGA from mitochondrial acetyl CoA is proposed. The pathway is initiated when syndrome-associated mutations/DNA deletions result in decreased Krebs cycle flux. When this occurs, acetoacetyl CoA thiolase condenses two acetyl CoA into acetoacetyl CoA plus CoASH. Subsequently, HMG CoA synthase 2 converts acetoacetyl CoA and acetyl CoA to HMG CoA. Under syndrome-specific metabolic conditions, 3-methylglutaconyl CoA hydratase converts HMG CoA into 3-methylglutaconyl CoA in a reverse reaction of the leucine degradation pathway. This metabolite fails to proceed further up the leucine degradation pathway owing to the kinetic properties of 3-methylcrotonyl CoA carboxylase. Instead, hydrolysis of the CoA moiety of 3-methylglutaconyl CoA generates 3MGA, which appears in urine. If experimentally confirmed, this pathway provides an explanation for the occurrence of 3MGA in multiple disorders associated with compromised mitochondrial function.

Figure 1. Leucine degradation pathway in mitochondria

Leucine is initially transaminated to α-ketoisocaproic acid by branched chain amino acid aminotransferase, with α-ketoglutarate as the amino group acceptor. Branched chain α-keto acid dehydrogenase generates isovaleryl CoA and acyl CoA dehydrogenase converts isovaleryl CoA to 3-methylcrotonyl CoA. 3-methylcrotonyl CoA carboxylase, 3-methylglutaconyl CoA hydratase, and HMG CoA lyase complete the metabolic path to acetyl CoA and acetoacetate.

Figure 2. Proposed metabolic path to 3MGA from acetyl CoA in a metabolically compromised mitochondria

The four enzymes reactions are (1) T2 Thiolase; (2) HMG CoA synthase 2; (3) 3-Methylglutaconyl CoA hydratase and (4) Thioester hydrolase. 3MG CoA, 3-methylglutaconyl CoA.

Figure 3. Acetoacetyl CoA metabolism in metabolically compromised muscle/heart mitochondria

Under conditions where acetyl CoA entry to the Krebs cycle is impeded, acetoacetyl CoA is generated by T2 thiolase, which is converted to HMG CoA by HMG CoA synthase 2 (HMGCS2). Together with HMG CoA Lyase and SCOT, a “futile cycle” is created that regenerates acetoacetyl CoA. An alternative metabolic fate of HMG CoA, that escapes this cycle, is conversion to 3-methylglutaconyl CoA (3MG CoA).