The combined organic extracts were dried (MgSO4) and concentrated. north (5) and south (6) conformers, as well as a flexible analogue (7) built with a cyclopentane ring. The seven-membered 1,3-diazepinone ring in all the three focuses on was built from the related benzoyl-protected carbocyclic bis-allyl ureas by ring-closing metathesis. The results demonstrate CDAs binding preference for any south LB-100 sugars pucker in agreement with the high-resolution crystal constructions of additional CDA inhibitors bound in the active site. Intro Cytidine deaminase (CDA; EC 3.5.4.5) is a zinc-dependent enzyme that catalyzes the deamination of cytidine to uridine via the formation of an unstable, hydrated transition-state (ts) intermediate (Number ?(Figure11).(1) The spontaneous deamination of cytidine is very slow and proceeds with a rate constant around 10?10 s?1; however, the enzyme is able to accelerate the pace of deamination by an impressive 12?14 orders of magnitude.(2) This incredible enhancement reflects an extraordinary affinity for the hydrated intermediate (defined by two main twist furanose puckering domains centered around a 3T2 (north-type) and a 2T3 (south-type) conformation. Despite the small difference in energy between these two conformations, an growing picture from recent observations is that the majority of nucleoside(tide) target enzymes, whether anabolic or catabolic, appear to possess stringent conformational requirements for substrate binding, receiving the furanose ring only in a particular, well-define shape.(10) Because the value of and the related glycosyl torsion angle are interdependent, with the second option responding inside a concerted manner to minimize steric clashes, the value of is also of essential importance for the two connecting pieces to provide ideal recognition. In the specific case of CDA, the crystal constructions of the enzyme bound to zebularine hydrate (2), THU (3), and diazepinone riboside (4) display the conformation of the sugars ring in the south hemisphere, close to a 2-endo conformation. In particular, the 1.48 ? resolution structure by Kumakasa et al.(5) shows the ribose ring of THU clearly inside a south conformation with the 3-OH hydrogen bonded to Mouse Monoclonal to Rabbit IgG (kappa L chain) the side chains of highly conserved Asn54 and Glu56. Using our interactive tool, PROSIT,(11) which calculates the pseudorotational guidelines of nucleosides and nucleotides bound to proteins, we determined a value of = 158.55 (2-endo) for THU which is clearly in the south hemisphere. Other guidelines calculated included the maximum puckering amplitude (maximum = 36.68) and the glycosyl torsion angle of the tetrahydrouracil ring, which is in the anti range ( = ?136.51). Similarly, the conformational guidelines for the bound diazepinone riboside (4)(7) determined by PROSIT matched very closely those of THU (= 158.18, maximum = 32.10, and = ?149.66). The LB-100 unique preference of CDA for the south (2-endo) conformation stands in razor-sharp contrast with the related purine deaminase, adenosine deaminase (ADA), which prefers to bind both substrate and inhibitors in the antipodal north (3-endo) conformation.(12) Despite the similarity of the mechanisms of deamination, which in both enzymes proceed via analogous tetrahedral intermediates, the secondary and tertiary structural motifs of the two enzymes are completely unrelated and lack any evolutionary homology.(13) With the aim of studying the impact the sugar conformation has on CDA, and how it might impinge within the additional essential interactions with important residues in the active site that are conjugated -system,(21) the isolation of a cyclic product proved that a orbital component and the adjacent C?N glycosyl relationship through the anomeric effect. Thus, substitute of the ribose or 2-deoxyribose ring of adenosine and cytidine with carbocyclic pseudosugars removes any communication between the sugars and the nucleobase and the rates of deamination decrease.(27) The LB-100 diazepinone-based inhibitors (4?7), which lack any possibility of interacting with CDA via zinc coordination and work independently of hydration, are therefore ideal candidates to study the effect of sugars (or pseudosugar) ring puckering within the mechanism of CDA. Because all the crystal constructions of bound inhibitors to CDA have shown the conformation of the sugars ring to be in the south hemisphere,3c,3d,5,7 close to a 2-endo conformation, we hypothesized the conformationally south-locked diazepinone nucleoside 6 would be the most potent inhibitor in comparison with the antipodal.
The combined organic extracts were dried (MgSO4) and concentrated
