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Crystal Structure of 5'-Deoxy-5'-Methylthioadenosine Phosphorylase in Complex with a Transition State AnalogueCrystal Structure of 5'-Deoxy-5'-Methylthioadenosine Phosphorylase in Complex with a Transition State Analogue
Structural highlights
DiseaseMTAP_HUMAN Defects in MTAP are the cause of diaphyseal medullary stenosis with malignant fibrous histiocytoma (DMSMFH) [MIM:112250. An autosomal dominant bone dysplasia characterized by pathologic fractures due to abnormal cortical growth and diaphyseal medullary stenosis. The fractures heal poorly, and there is progressive bowing of the lower extremities. Some patients show a limb-girdle myopathy, with muscle weakness and atrophy. Approximately 35% of affected individuals develop an aggressive form of bone sarcoma consistent with malignant fibrous histiocytoma or osteosarcoma. Note=DMSMFH causing mutations found in MTAP exon 9 result in exon skipping and dysregulated alternative splicing of all MTAP isoforms (PubMed:22464254).[1] Note=Loss of MTAP activity may play a role in human cancer. MTAP loss has been reported in a number of cancers, including osteosarcoma, malignant melanoma and gastric cancer.[HAMAP-Rule:MF_03155] FunctionMTAP_HUMAN Catalyzes the reversible phosphorylation of S-methyl-5'-thioadenosine (MTA) to adenine and 5-methylthioribose-1-phosphate. Involved in the breakdown of MTA, a major by-product of polyamine biosynthesis. Responsible for the first step in the methionine salvage pathway after MTA has been generated from S-adenosylmethionine. Has broad substrate specificity with 6-aminopurine nucleosides as preferred substrates.[2] Evolutionary Conservation Check, as determined by ConSurfDB. You may read the explanation of the method and the full data available from ConSurf. Publication Abstract from PubMedMethythioadenosine phosphorylase (MTAP) functions solely in the polyamine pathway of mammals to remove the methylthioadenosine (MTA) product from both spermidine synthase (2.5.1.16) and spermine synthase (2.5.1.22). Inhibition of polyamine synthesis is a validated anticancer target. We designed and synthesized chemically stable analogues for the proposed transition state of human MTAP on the basis of the known ribooxacarbenium character at all reported N-ribosyltransferase transition states [Schramm, V. L. (2003) Acc. Chem. Res. 36, 588-596]. Methylthio-immucillin-A (MT-ImmA) is an iminoribitol tight-binding transition state analogue inhibitor with an equilibrium dissociation constant of 1.0 nM. The immucillins resemble the ribooxacarbenium ion transition states of N-ribosyltransferases and are tightly bound as the N4' cations. An ion pair formed between the iminoribitol cation and phosphate anion mimics the ribooxacarbenium cation-phosphate anion pair formed at the transition state and is confirmed in the crystal structure. The X-ray crystal structure of human MTAP with bound MT-Imm-A also reveals that the 5'-methylthio group lies in a flexible hydrophobic pocket. Substitution of the 5'-methylthio group with a 5'-phenylthio group gives an equilibrium binding constant of 1.0 nM. Methylthio-DADMe-immucillin-A is a pyrrolidine analogue of the transition state with a methylene bridge between the 9-deazaadenine group and the pyrrolidine ribooxacarbenium mimic. It is a slow-onset inhibitor with a dissociation constant of 86 pM. Improved binding energy with DADMe-immucillin-A suggests that the transition state is more closely matched by increasing the distance between leaving group and ribooxacarbenium mimics, consistent with a more dissociative transition state. Increasing the hydrophobic volume near the 5'-position at the catalytic site with 5'-phenylthio-DADMe-immucillin-A gave a dissociation constant of 172 pM, slightly weaker than the 5'-methylthio group. p-Cl-phenylthio-DADMe-immucillin-A binds with a dissociation constant of 10 pM (K(m)/K(i) value of 500000), the tightest binding inhibitor reported for MTAP. These slow-onset, tight-binding transition state analogue inhibitors are the most powerful reported for MTAP and have sufficient affinity to be useful in inhibiting the polyamine pathway. Picomolar transition state analogue inhibitors of human 5'-methylthioadenosine phosphorylase and X-ray structure with MT-immucillin-A.,Singh V, Shi W, Evans GB, Tyler PC, Furneaux RH, Almo SC, Schramm VL Biochemistry. 2004 Jan 13;43(1):9-18. PMID:14705926[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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