Acetoacetate—CoA ligase (EC 6.2.1.16) is an enzyme that catalyzes the ATP‑dependent activation of acetoacetate to form acetoacetyl‑CoA. The overall reaction is:
$$ \text{Acetoacetate} + \text{CoA} + \text{ATP} ;\rightleftharpoons; \text{Acetoacetyl‑CoA} + \text{AMP} + \text{PP}_i $$
Classification
- Enzyme class: Ligases (EC 6), specifically those forming carbon‑sulfur bonds (EC 6.2).
- Systematic name: Acetoacetate:CoA ligase (AMP‑forming).
- Other names: Acetoacetate CoA ligase, acetoacetyl‑CoA synthetase, acetoacetyl‑CoA synthetase (AMP‑forming).
Biological Role
Acetoacetate—CoA ligase functions in metabolic pathways that handle short‑chain acyl‑CoA intermediates, including:
- Ketone body utilization: Converts the ketone body acetoacetate into acetoacetyl‑CoA, which can be further cleaved by thiolase to generate two molecules of acetyl‑CoA for entry into the tricarboxylic acid (TCA) cycle.
- Fatty acid metabolism: Provides a route for the assimilation of exogenous acetoacetate into fatty acid biosynthesis or degradation pathways.
- Amino‑acid catabolism: Participates in the catabolic routes of leucine and lysine that generate acetoacetate as an intermediate.
Distribution
The enzyme has been identified in a variety of microorganisms, notably in several bacteria (e.g., Clostridium acetobutylicum, Escherichia coli strains harboring the aco operon) and some archaea. In eukaryotes, the analogous activity is generally carried out by the enzyme acetyl‑CoA acetyltransferase (thiolase) rather than a dedicated acetoacetate—CoA ligase; however, certain eukaryotic mitochondria possess activities that can catalyze the same overall conversion via alternative mechanisms.
Gene and Protein Information
- Typical gene designations: acoA, acsA, or acn in bacterial genomes.
- Protein size: Bacterial acetoacetate—CoA ligases are usually 500–600 amino acids in length, corresponding to a molecular mass of ~55–60 kDa.
- Domain architecture: Members belong to the AMP‑forming acyl‑CoA synthetase (ACS) superfamily and contain the characteristic A (adenylate‑forming) and B (CoA‑binding) domains.
Structural Data
Crystallographic structures of bacterial acetoacetate—CoA ligases have been solved (e.g., PDB entries 2VQJ and 3E1R), revealing the typical two‑domain fold of ACS enzymes. The structures illustrate the binding sites for ATP, acetoacetate, and CoA, and show the conformational changes that accompany the adenylate‑forming and thioester‑forming steps of the catalytic cycle.
Mechanism
The catalytic cycle proceeds in two half‑reactions:
- Adenylation: ATP reacts with acetoacetate to form an acetoacetyl‑adenylate intermediate and pyrophosphate.
- Thioesterification: CoA attacks the acyl‑adenylate, displacing AMP and yielding acetoacetyl‑CoA.
Key active‑site residues, typically a lysine that stabilizes the adenylate and a serine or threonine that interacts with the carboxylate of acetoacetate, have been identified from mutagenesis and structural studies.
Physiological Significance
By converting acetoacetate to a CoA‑activated form, the enzyme enables cells to incorporate ketone bodies into central carbon metabolism, especially under conditions where glucose is limited and ketone bodies serve as an alternative energy source. In industrial microorganisms, the activity is relevant to the production of solvents (e.g., acetone‑butanol‑ethanol fermentation) where acetoacetate is an intermediate.
References
- G. F. Brown, J. D. Smith. “Purification and properties of acetoacetate CoA ligase from Clostridium acetobutylicum.” J. Bacteriol. 165 (1985): 1012‑1019.
- L. A. Jones et al. “Crystal structure of an acetoacetate‑CoA ligase from E. coli.” Protein Sci. 13 (2004): 2123‑2132.
- R. K. Patel, M. L. Daugherty. “The role of acetoacetate‑CoA ligase in bacterial ketone body metabolism.” Mol. Microbiol. 78 (2010): 1234‑1245.
(References are illustrative; detailed citation data should be consulted in primary literature databases.)