MCAT

MCAT (FabD) is a malonyl-CoA:acyl carrier protein transacylase that catalyzes the committed extender-unit step of de novo fatty acid biosynthesis, transferring the malonyl moiety from malonyl-CoA onto the phosphopantetheine thiol of holo-ACP to generate malonyl-ACP and free CoASH (PMID:1314802, PMID:12842120). In bacteria the enzyme is encoded by a single essential gene, originally defined through complementation of a thermosensitive fabD mutant and shown to be indispensable for fatty acid synthesis, with thermolabile or substituted alleles abolishing growth (PMID:1314802, PMID:10464226). Catalysis proceeds through a Ser92 nucleophile, with Arg117 stabilizing the malonyl carboxylate, His201 serving as general base/acid, and a preformed oxyanion hole stabilizing the transition state (PMID:16699188). The enzyme engages its cognate ACP through a defined protein-protein interface in which ACP samples multiple productive conformations (interfacial plasticity) and rigid-body subdomain motions within the transacylase support high catalytic rates and substrate selectivity (PMID:32929027). Substrate specificity toward ACP is relaxed, with activity on both fatty acid synthase and polyketide synthase ACPs, allowing it to provision malonyl-ACP for both pathways (PMID:12173933). In humans, pathogenic variants in the nuclear-encoded mitochondrial MCAT cause a hereditary, LHON-like optic neuropathy (PMID:33918393).

1. 1975 Medium

   Establishing where the transacylase activity is encoded was the first step; mapping fabD to a defined chromosomal locus tied a temperature-sensitive fatty-acid-synthesis defect to a single genetic determinant.

   Evidence Conjugation and P1 transduction mapping with temperature-sensitive growth phenotype in E. coli

   PMID:1090584

   Open questions at the time

   • Did not isolate or sequence the gene product
   • No biochemical demonstration of the encoded enzyme activity
2. 1992 High

   Cloning fabD by complementation and overexpression proved it is the sole gene responsible for malonyl-CoA:ACP transacylase activity and placed it within a fatty acid biosynthesis operon.

   Evidence Genetic complementation, overexpression (~1000-fold activity increase), enzyme assay, and DNA sequencing in E. coli

   PMID:1314802

   Open questions at the time

   • No structural information on the active site
   • Catalytic residues not yet identified
3. 1999 High

   Purification and in vitro reconstitution with cognate holo-ACP, plus a chromosomal temperature-sensitive substitution, confirmed the enzyme's transacylase function and its essentiality in a second organism.

   Evidence His-tagged protein purification, in vitro transacylase assay, and site-directed chromosomal mutant (W258Q) in Pseudomonas aeruginosa

   PMID:10464226

   Open questions at the time

   • Mechanism of the W258Q defect not resolved at residue level
   • Active-site chemistry not addressed
4. 2002 High

   Kinetic profiling against multiple ACPs answered whether the enzyme is dedicated to fatty acid synthesis, showing relaxed ACP specificity that allows it to supply malonyl-ACP to both FAS and polyketide pathways.

   Evidence Purified recombinant Streptomyces glaucescens FabD with in vitro kinetics on FabC, AcpP, and TcmM ACPs and isotope labeling

   PMID:12173933

   Open questions at the time

   • Structural basis of relaxed ACP recognition not defined
   • Does not address specificity in mammalian/mitochondrial systems
5. 2003 Medium

   A continuous coupled assay confirmed the chemical step directly, monitoring CoASH release as malonyl is transferred to the ACP phosphopantetheine thiol.

   Evidence Coupled in vitro enzyme assay linking CoASH product to alpha-ketoglutarate dehydrogenase with fluorometric NAD+ detection

   PMID:12842120

   Open questions at the time

   • No mutagenesis to assign catalytic residues
   • Single-lab assay validation
6. 2006 High

   Crystal structures with malonyl-CoA defined the active-site architecture, assigning Ser92, Arg117, His201, and an oxyanion hole to specific catalytic roles and yielding a mechanistic model.

   Evidence X-ray crystallography of E. coli FabD binary substrate complexes with structure-based active-site analysis

   PMID:16699188

   Open questions at the time

   • Mechanistic roles inferred from structure without mutagenesis in this study
   • ACP-bound state not captured
7. 2020 High

   Trapping the covalent transacylase-ACP complex resolved how the enzyme engages its protein substrate, revealing interfacial plasticity and subdomain motions that sustain high catalytic rates and selectivity.

   Evidence Covalent crosslinking, X-ray structure of the ACP-AT complex, mutagenesis, kinetics, and molecular dynamics in E. coli

   PMID:32929027

   Open questions at the time

   • Generalizability of the interface to mitochondrial MCAT not tested
   • Dynamics under turnover conditions inferred from simulation
8. 2021 Low

   Linking the human enzyme to disease established a physiological consequence of MCAT loss-of-function, identifying variants that cause a LHON-like hereditary optic neuropathy.

   Evidence Sequencing and clinical phenotype correlation in a patient with acute bilateral central visual loss

   PMID:33918393

   Open questions at the time

   • No functional assay demonstrating the variants impair MCAT enzymatic activity
   • Mechanism linking mitochondrial fatty acid synthesis deficit to optic neuropathy not established

• How human mitochondrial MCAT dysfunction mechanistically produces tissue-specific optic nerve degeneration remains unresolved.
• No biochemical characterization of human MCAT catalysis or ACP partner in the timeline
• Connection between mitochondrial malonyl-ACP supply and neuronal vulnerability uncharacterized