Javed A. Khan

Molecular basis for the inhibition of human NMPRTase, a novel target for anticancer agents

Nicotinamide phosphoribosyltransferase (NMPRTase) has a crucial role in the salvage pathway of NAD+ biosynthesis, and a potent inhibitor of NMPRTase, FK866, can reduce cellular NAD+ levels and induce apoptosis in tumors. We have determined the crystal structures at up to 2.1-Å resolution of human and murine NMPRTase, alone and in complex with the reaction product nicotinamide mononucleotide or the inhibitor FK866. The structures suggest that Asp219 is a determinant of substrate specificity of NMPRTase, which is confirmed by our mutagenesis studies. FK866 is bound in a tunnel at the interface of the NMPRTase dimer, and mutations in this binding site can abolish the inhibition by FK866. Contrary to current knowledge, the structures show that FK866 should compete directly with the nicotinamide substrate. Our structural and biochemical studies provide a starting point for the development of new anticancer agents.

Nicotinamide adenine dinucleotide metabolism as an attractive target for drug discovery

Nicotinamide adenine dinucleotide (NAD+) has crucial roles in many cellular processes, both as a coenzyme for redox reactions and as a substrate to donate ADP-ribose units. Enzymes involved in NAD+ metabolism are attractive targets for drug discovery against a variety of human diseases, including cancer, multiple sclerosis, neurodegeneration and Huntington’s disease. A small-molecule inhibitor of nicotinamide phosphoribosyltransferase, an enzyme in the salvage pathway of NAD+ biosynthesis, is presently in clinical trials against cancer. An analog of a kynurenine pathway intermediate is efficacious against multiple sclerosis in an animal model. Indoleamine 2,3-dioxygenase plays an important role in immune evasion by cancer cells and other disease processes. Inhibitors against kynurenine 3-hydroxylase can reduce the production of neurotoxic metabolites while increasing the production of neuroprotective compounds. This review summarizes the existing knowledge on NAD+ metabolic enzymes, with emphasis on their relevance for drug discovery.

Conserved Surface Features Form the Double-stranded RNA Binding Site of Non-structural Protein 1 (NS1) from Influenza A and B Viruses

Influenza A viruses cause a highly contagious respiratory disease in humans and are responsible for periodic widespread epidemics with high mortality rates. The influenza A virus NS1 protein (NS1A) plays a key role in countering host antiviral defense and in virulence. The 73-residue N-terminal domain of NS1A (NS1A-(1–73)) forms a symmetric homodimer with a unique six-helical chain fold. It binds canonical A-form double-stranded RNA (dsRNA). Mutational inactivation of this dsRNA binding activity of NS1A highly attenuates virus replication. Here, we have characterized the unique structural features of the dsRNA binding surface of NS1A-(1–73) using NMR methods and describe the 2.1-Å x-ray crystal structure of the corresponding dsRNA binding domain from human influenza B virus NS1B-(15–93). These results identify conserved dsRNA binding surfaces on both NS1A-(1–73) and NS1B-(15–93) that are very different from those indicated in earlier “working models” of the complex between dsRNA and NS1A-(1–73). The combined NMR and crystallographic data reveal highly conserved surface tracks of basic and hydrophilic residues that interact with dsRNA. These tracks are structurally complementary to the polyphosphate backbone conformation of A-form dsRNA and run at an ∼45° angle relative to the axes of helices α2/α2′. At the center of this dsRNA binding epitope, and common to NS1 proteins from influenza A and B viruses, is a deep pocket that includes both hydrophilic and hydrophobic amino acids. This pocket provides a target on the surface of the NS1 protein that is potentially suitable for the development of antiviral drugs targeting both influenza A and B viruses.

Crystal structure of equine apolactoferrin at 303 K providing further evidence of closed conformations of N and C lobes

Lactoferrin is an iron-binding protein. In the iron-bound state, the two domains of each lobe are invariably closed over an Fe(3+) ion. On the other hand, the structures of iron-free forms of lactoferrins from various species show different domain orientations. In order to determine the effects of external conditions such as pH, temperature and the presence of other additive agents on the crystal packing and consequently the influence of crystal packing forces on the final conformations of the two lobes of apolactoferrin, the structure of equine apolactoferrin has been determined at 303 K. The equine apolactoferrin was crystallized at 303 K using a microdialysis setup in which the concentration of protein was kept at 70 mg ml(-1) in 0.025 M Tris-HCl pH 8.0 with a reservoir containing 19% ethanol in the same buffer. The structure has been determined by molecular replacement using equine diferric lactoferrin as a model and was refined to an R factor of 0.23. The value of the overall B factor in the present structure is 81.3 A(2). The overall structure of the protein is similar to its earlier structure based on crystals grown at 277 K as well as that of diferric equine lactoferrin. The N and C lobes have been found to be slightly differently oriented (4.9 and 7.1 degrees, respectively) compared with the structures of equine diferric lactoferrin and apolactoferrin analyzed at 277 K, but the domain orientations in the two structures are identical as they remain closed over the empty iron-binding cleft. Overall, the structures of equine diferric lactoferrin, equine apolactoferrin at 277 K and the present structure of equine apolactoferrin at 303 K display identical domain orientations, suggesting that variation in the temperature of crystal growth, data collection and the processes of iron binding and iron release do not influence the arrangements of domains in equine lactoferrin.