Discovery of GBT440, an Orally Bioavailable R-State Stabilizer of Sickle Cell Hemoglobin.

We report the discovery of a new potent allosteric effector of sickle cell hemoglobin, GBT440 (), that increases the affinity of hemoglobin for oxygen and consequently inhibits its polymerization when subjected to hypoxic conditions. Unlike earlier allosteric activators that bind covalently to hemoglobin in a 2:1 stoichiometry, binds with a 1:1 stoichiometry. Compound is orally bioavailable and partitions highly and favorably into the red blood cell with a RBC/plasma ratio of ∼150.

GBT440 increases haemoglobin oxygen affinity, reduces sickling and prolongs RBC half-life in a murine model of sickle cell disease.

A major driver of the pathophysiology of sickle cell disease (SCD) is polymerization of deoxygenated haemoglobin S (HbS), which leads to sickling and destruction of red blood cells (RBCs) and end-organ damage. Pharmacologically increasing the proportion of oxygenated HbS in RBCs may inhibit polymerization, prevent sickling and provide long term disease modification. We report that GBT440, a small molecule which binds to the N-terminal α chain of Hb, increases HbS affinity for oxygen, delays in vitro HbS polymerization and prevents sickling of RBCs.

Transition state model and mechanism of nucleophilic aromatic substitution reactions catalyzed by human glutathione S-transferase M1a-1a.

An active site His107 residue distinguishes human glutathione S-transferase hGSTM1-1 from other mammalian Mu-class GSTs. The crystal structure of hGSTM1a-1a with bound glutathione (GSH) was solved to 1.9 A resolution, and site-directed mutagenesis supports the conclusion that a proton transfer occurs in which bound water at the catalytic site acts as a primary proton acceptor from the GSH thiol group to transfer the proton to His107.

COHbC and COHbS crystallize in the R2 quaternary state at neutral pH in the presence of PEG 4000.

Human hemoglobin binds oxygen cooperatively and functions as a tetramer composed of two identical alphabeta heterodimers. While human hemoglobin is the best characterized allosteric protein, the quaternary R (oxygenated or liganded) to T (deoxygenated) structural transition remains controversial. The R2 state has been postulated to represent either an intermediate or final quaternary state elicited by ligand binding.

Structure of mutant human carbonmonoxyhemoglobin C (betaE6K) at 2.0 A resolution.

Previous studies have demonstrated that in vitro crystallization of R-state liganded hemoglobin C (HbC), a naturally occurring mutant human hemoglobin (betaE6K), in high-phosphate buffer solutions provides a potential model system for the intracellular crystallization of HbC associated with chronic hemolytic anemia in CC disease. The first high-resolution crystal structure of liganded HbC is reported here. HbC was crystallized from high phosphate and the structure of the carbonmonoxy-liganded R-state form was refined at 2.

The enhanced affinity for thiolate anion and activation of enzyme-bound glutathione is governed by an arginine residue of human Mu class glutathione S-transferases.

A series of chimeric human Mu class glutathione S-transferases were designed to determine mechanisms by which they activate enzyme-bound glutathione (GSH) for reaction with electrophilic substrates. In view of evidence that the His(107) residue of hGSTM1a-1a is important for catalysis (Patskovsky, Y. V.

An asparagine-phenylalanine substitution accounts for catalytic differences between hGSTM3-3 and other human class mu glutathione S-transferases.

The hGSTM3 subunit, which is preferentially expressed in germ-line cells, has the greatest sequence divergence among the human mu class glutathione S-transferases. To determine a structural basis for the catalytic differences between hGSTM3-3 and other mu class enzymes, chimeric proteins were designed by modular interchange of the divergent C-terminal domains of hGSTM3 and hGSTM5 subunits. Replacement of 24 residues of the C-terminal segment of either subunit produced chimeric enzymes with catalytic properties that reflected those of the wild-type enzyme from which the C-terminus had been derived.

Functions of His107 in the catalytic mechanism of human glutathione S-transferase hGSTM1a-1a.

Domain interchange analyses and site-directed mutagenesis indicate that the His107 residue of the human subunit hGSTM1 has a pronounced influence on catalysis of nucleophilic aromatic substitution reactions, and a H107S substitution accounts for the marked differences in the properties of the homologous hGSTM1-1 (His107) and hGSTM4-4 (Ser107) glutathione S-transferases. Reciprocal replacement of His107 and Ser107 in chimeric enzymes results in reciprocal conversion of catalytic properties. With 1-chloro-2, 4-dinitrobenzene as a substrate, the His107 residue primarily influences the pH dependence of catalysis by lowering the apparent pKa of kcat/Km from 7.

Expression, crystallization and preliminary X-ray analysis of ligand-free human glutathione S-transferase M2-2.

Human glutathione-S-transferase M2-2 (hGSTM2-2) was expressed in Escherichia coli and purified by GSH-affinity chromatography. The recombinant enzyme and the protein isolated from human tissue were indistinguishable based on physicochemical, enzymatic and immunological criteria. The catalytically active dimeric hGSTM2-2 was crystallized without GSH or other active-site ligands in two crystal forms.

Rationale for reclassification of a distinctive subdivision of mammalian class Mu glutathione S-transferases that are primarily expressed in testis.

A rat testicular Mu-class glutathione S-transferase (GST) resolved by reversed-phase high performance liquid chromatography cross-reacted with peptide sequence-specific antisera raised against the human hGSTM3 subunit. Electrospray ionization mass spectrometry indicated that this rat GST subunit (designated rGSTM5 in this report) has a significantly greater molecular mass (26,541 Da) than the other rat GST subunits. The mouse homologue (mGSTM5 subunit) was also identified and characterized by high performance liquid chromatography and electrospray ionization mass spectrometry.