RESUMO
Adenosine Kinase (ADK) regulates the cellular levels of adenosine (ADO) by fine-tuning its metabolic clearance. The transfer of γ-phosphate from ATP to ADO by ADK involves regulation by the substrates and products, as well as by Mg2+ and inorganic phosphate. Here we present new crystal structures of mouse ADK (mADK) binary (mADK:ADO; 1.2 Å) and ternary (mADK:ADO:ADP; 1.8 Å) complexes. In accordance with the structural demonstration of ADO occupancy of the ATP binding site, kinetic studies confirmed a competitive model of auto-inhibition of ADK by ADO. In the ternary complex, a K+ ion is hexacoordinated between loops adjacent to the ATP binding site, where Asp310 connects the K+ coordination sphere to the ATP binding site through an anion hole structure. Nuclear Magnetic Resonance 2D 15N-1H HSQC experiments revealed that the binding of K+ perturbs Asp310 and residues of adjacent helices 14 and 15, engaging a transition to a catalytically productive structure. Consistent with the structural data, the mutants D310A and D310P are catalytically deficient and loose responsiveness to K+. Saturation Transfer Difference spectra of ATPγS provided evidence for an unfavorable interaction of the mADK D310P mutant for ATP. Reductions in K+ concentration diminish, whereas increases enhance the in vitro activity of mADK (maximum of 2.5-fold; apparent Kd = 10.4 mM). Mechanistically, K+ increases the catalytic turnover (Kcat) but does not affect the affinity of mADK for ADO or ATP. Depletion of intracellular K+ inhibited, while its restoration was accompanied by a full recovery of cellular ADK activity. Together, this novel dataset reveals the molecular basis of the allosteric activation of ADK by K+ and highlights the role of ADK in connecting depletion of intracellular K+ to the regulation of purine metabolism.
Assuntos
Adenosina Quinase/metabolismo , Redes e Vias Metabólicas , Potássio/metabolismo , Purinas/metabolismo , Adenosina Quinase/química , Adenosina Quinase/genética , Aminoácidos , Sítios de Ligação , Ativação Enzimática , Cinética , Imageamento por Ressonância Magnética , Conformação Molecular , Mutação , Fosforilação , Ligação Proteica , Purinas/química , Relação Estrutura-AtividadeRESUMO
In adult schistosomes, the enzyme adenosine kinase (AK) is responsible for the incorporation of some adenosine analogues, such as 2-fluoroadenosine and tubercidin, into the nucleotide pool, but not others. In the present study, the structures of four complexes of Schistosoma mansoni AK bound to adenosine and adenosine analogues are reported which shed light on this observation. Two differences in the adenosine-binding site in comparison with the human counterpart (I38Q and T36A) are responsible for their differential specificities towards adenosine analogues, in which the Schistosoma enzyme does not tolerate bulky substituents at the N7 base position. This aids in explaining experimental data which were reported in the literature more than two decades ago. Furthermore, there appears to be considerable plasticity within the substrate-binding sites that affects the side-chain conformation of Ile38 and causes a previously unobserved flexibility within the loop comprising residues 286-299. These results reveal that the latter can be sterically occluded in the absence of ATP. Overall, these results contribute to the body of knowledge concerning the enzymes of the purine salvage pathway in this important human parasite.
Assuntos
Adenosina Quinase/química , Adenosina/química , Schistosoma mansoni/enzimologia , Adenosina/análogos & derivados , Adenosina/metabolismo , Adenosina Quinase/genética , Adenosina Quinase/metabolismo , Animais , Cristalização , Cristalografia por Raios X , Humanos , Schistosoma mansoni/genética , Schistosoma mansoni/metabolismo , Alinhamento de Sequência , Especificidade por Substrato/genéticaRESUMO
The structural requirements of pyrrolo[2,3-d]pyrimidine nucleoside (PPN) analogues as adenosine kinase (AK) inhibitors were in silico studied by using CoMSIA method. All models were trained with 32 compounds, after which they were evaluated for predictive ability with additional 5 compounds. Quantitative information on structure-activity trends is provided for further rational development and direction of selective synthesis. The best CoMSIA model included hydrophobic, H-bond donor and H-bond acceptor fields and had a good predictive quality according to internal validation criteria. In addition, this model predicted adequately the compounds contained in the test set. The analysis of the model gives a comprehensive qualitative and quantitative description of the molecular features at C4 and C5 positions of the pyrrolo[2,3-d]pyrimidine scaffold and C5-position of the beta-d-ribofuranose of PPN analogues, relevant for a high AK inhibitory activity.