Adenylosuccinate lyase

2J91, 2VD6, 4FFX, 4FLC15811564ENSG00000239900ENSMUSG00000022407P30566P54822NM_000026NM_001123378NM_001317923NM_001363840NM_009634NP_000017NP_001116850NP_001304852NP_001350769NP_033764Adenylosuccinate lyase (or adenylosuccinase) is an enzyme that in humans is encoded by the ADSL gene. Adenylosuccinate lyase (or adenylosuccinase) is an enzyme that in humans is encoded by the ADSL gene. Adenylosuccinate lyase converts adenylosuccinate to AMP and fumarate as part of the purine nucleotide cycle. ASL catalyzes two reactions in the purine biosynthetic pathway that makes AMP; ASL cleaves adenylosuccinate into AMP and fumarate, and cleaves SAICAR into AICAR and fumarate. Adenylosuccinate lyase is part of the β-elimination superfamily of enzymes and it proceeds through an E1cb reaction mechanism. The enzyme is a homotetramer with three domains in each monomer and four active sites per homotetramer. Point mutations in adenylosuccinate that cause lowered enzymatic activity cause clinical symptoms that mark the condition adenylosuccinate lyase deficiency. This protein may use the morpheein model of allosteric regulation. Adenylosuccinate lyase (ASL) is an enzyme that catalyzes two reactions in the de novo purine biosynthetic pathway. In both reactions it uses an E1cb elimination reaction mechanism to cleave fumarate off of the substrate. In the first reaction, ASL converts 5-aminoimidazole- (N-succinylocarboxamide) ribotide (SAICAR) to 5-aminoimidazole-4-carboxamide ribotide (AICAR) and fumarate. AICAR proceeds through three more reactions before it becomes adenylosuccinate (also called succinyladenosine monophosphate or SAMP), which ASL then splits into adenosine monophosphate (AMP) and fumarate. ASL is important to cells not only because of its involvement in creating purines needed for cellular replication, but also because it helps regulate metabolic processes by controlling the levels of AMP and fumarate in the cell. Adenylosuccinate lyase belongs to the β-elimination superfamily, and as such its structure is a homotetramer . The monomer of adenylosuccinate lyase has three domains. In Thermotoga maritima, domain 1 contains 7 α-helices in residues 1-93, including the His68 which is highly conserved and was previously thought to be the catalytic acid in the active site. More recent studies have posited that the His171 in domain 2, previously thought to be a catalytic base, may in fact be acting as the catalytic acid, at least in Escherichia coli. Domain 2 is made up of residues 94-341, and contains 5 α-helices and the monomer’s only β-sheet. Domain 3 is made up of 7 α-helices. The core of the tetramer is made up of the four domain 2 copies, and there are two copies each of domains 1 and 3 on each end of the tetramer giving the tetramer D2 dihedral symmetry. The tetramer has four active sites, each where three domains meet. Adenylosuccinate lyase in humans and Bacillus subtilis can be competitively inhibited by the substrate analog adenosine phosphonobutyric acid 2’(3’), 5’-diphosphate (APBADP). APBADP is a competitive inhibitor for both of the reactions catalyzed by adenylosuccinate lyase, and kinetic studies with APBADP show that the substrates for both reactions use the same active site. In the ASL-catalyzed reaction splitting adenylosuccinate into adenosine monophosphate (AMP) and fumarate, the AMP must rotate slightly after the reaction is complete and before fumarate is released in order for both products to fit in the active site. Adenylosuccinate lyase mutants can have considerably reduced activity whether the mutation is in or away from the active site. Disease-causing ASL mutants R396C and R396H are at the entrance to the active site and have lower Vmax than the wild-type ASL, but the mutants K246E and L311V which are away from the active site also cause decreased Vmax. ASL mutant R194C is away from the active site, and though it maintains a Vmax similar to wild-type ASL, it was shown to be the least conformationally stable of the five mutants in vitro and still causes disease.