ATP citrate lyase is the primary enzyme responsible for the synthesis of cytosolicacetyl-CoA in many tissues. The enzyme is a tetramer of apparently identical subunits. In animals, the product, acetyl-CoA, is used in several important biosynthetic pathways, including lipogenesis and cholesterogenesis.[5] It is activated by insulin.[6]
In plants, ATP citrate lyase generates acetyl-CoA for cytosolically-synthesized metabolites; Acetyl-CoA is not transported across subcellular membranes of plants. Such metabolites include: elongated fatty acids (used in seed oils, membrane phospholipids, the ceramide moieties of sphingolipids, cuticle, cutin, and suberin); flavonoids; malonic acid; acetylated phenolics, alkaloids, isoprenoids, anthocyanins, and sugars; and, mevalonate-derived isoprenoids (e.g., sesquiterpenes, sterols, brassinosteroids); malonyl and acyl-derivatives (d-amino acids, malonylated flavonoids, acylated, prenylated and malonated proteins).[4] De novo fatty acid biosynthesis in plants occurs in plastids; thus, ATP citrate lyase is not relevant to this pathway.
Animal ACL enzymes are homomeric; a fusion of the ACLA and ACLB genes probably occurred early in the evolutionary history of this kingdom.[4]
The mammalian ATP citrate lyase has a N-terminal citrate-binding domain that adopts a Rossmann fold, followed by a CoA binding domain and CoA-ligase domain and finally a C-terminal citrate synthase domain. The cleft between the CoA binding and citrate synthase domains forms the active site of the enzyme, where both citrate and acetyl-coenzyme A bind.
In 2010, a structure of truncated human ATP citrate lyase was determined using X-ray diffraction to a resolution of 2.10 Å.[3] In 2019, a full length structure of human ACLY in complex with the substrates coenzyme A, citrate and Mg.ADP was determined by X-ray crystallography to a resolution of 3.2 Å.[1] Moreover, in 2019 a full length structure of ACLY in complex with an inhibitor was determined by cryo-EM methods to a resolution of 3.7 Å.[10] Additional structures of heteromeric ACLY-A/B from the green sulfur bacteriaChlorobium limicola and the archaeonMethanosaeta concilii show that the architecture of ACLY is evolutionarily conserved.[1] Full length ACLY structures showed that the tetrameric protein oligomerizes via its C-terminal domain. The C-terminal domain had not been observed in the previously determined truncated crystal structures. The C-terminal region of ACLY assembles in a tetrameric module that is structurally similar to citryl-CoA lyase (CCL) found in deep branching bacteria.[1][11] This CCL module catalyses the cleavage of the citryl-CoA intermediate into the products acetyl-CoA and oxaloacetate.
In 2019, cryo-EM structures of human ACLY, alone or bound to substrates or products were reported as well.[12][13] ACLY forms a homotetramer with a rigid citrate synthase homology (CSH) module, flanked by four flexible acetyl-CoA synthetase homology (ASH) domains; CoA is bound at the CSH–ASH interface in mutually exclusive productive or unproductive conformations. The structure of a catalytic mutant of ACLY in the presence of ATP, citrate and CoA substrates reveals a CoA and phosphor-citrate intermediate in the N-terminal domain. Cryo-EM structures of products bound ACLY and substrates bound ACLY were also determined at 3.0 Å and 3.1 Å. An EM structure of mutant E599Q in complex with CoA and phospho-citrate intermediate was determined at resolution of 2.9 Å. Comparison between these structures of apo-ACLY and ligands bound ACLY demonstrated conformational changes on ASH domain (N-terminal domain) when different ligands bind.
^Aoshima M, Ishii M, Igarashi Y (May 2004). "A novel enzyme, citryl-CoA lyase, catalysing the second step of the citrate cleavage reaction in Hydrogenobacter thermophilus TK-6". Molecular Microbiology. 52 (3): 763–70. doi:10.1111/j.1365-2958.2004.04010.x. PMID15101982. S2CID32105039.
Lovell SC, Davis IW, Arendall WB, de Bakker PI, Word JM, Prisant MG, et al. (February 2003). "Structure validation by Calpha geometry: phi,psi and Cbeta deviation". Proteins. 50 (3): 437–50. doi:10.1002/prot.10286. PMID12557186. S2CID8358424.
Lill U, Schreil A, Eggerer H (July 1982). "Isolation of enzymically active fragments formed by limited proteolysis of ATP citrate lyase". European Journal of Biochemistry. 125 (3): 645–50. doi:10.1111/j.1432-1033.1982.tb06731.x. PMID6749502.
Srere PA, Lipmann F (1953). "An enzymatic reaction between citrate, adenosine triphosphate and coenzyme A". Journal of the American Chemical Society. 75 (19): 4874. doi:10.1021/ja01115a547.
