The loading module of rifamycin synthetase is an adenylation-thiolation didomain with substrate tolerance for substituted benzoates.

The rifamycin synthetase is primed with a 3-amino-5-hydroxybenzoate starter unit by a loading module that contains domains homologous to the adenylation and thiolation domains of nonribosomal peptide synthetases. Adenylation and thiolation activities of the loading module were reconstituted in vitro and shown to be independent of coenzyme A, countering literature proposals that the loading module is a coenzyme A ligase. Kinetic parameters for covalent arylation of the loading module were measured directly for the unnatural substrates benzoate and 3-hydroxybenzoate. This analysis was extended through competition experiments to determine the relative rates of incorporation of a series of substituted benzoates. Our results show that the loading module can accept a variety of substituted benzoates, although it exhibits a preference for the 3-, 5-, and 3,5-disubstituted benzoates that most closely resemble its biological substrate. The considerable substrate tolerance of the loading module of rifamycin synthetase suggests that the module has potential as a tool for generating substituted derivatives of natural products.

Biosynthesis of complex polyketides in a metabolically engineered strain of E. coli.

The macrocyclic core of the antibiotic erythromycin, 6-deoxyerythronolide B (6dEB), is a complex natural product synthesized by the soil bacterium Saccharopolyspora erythraea through the action of a multifunctional polyketide synthase (PKS). The engineering potential of modular PKSs is hampered by the limited capabilities for molecular biological manipulation of organisms (principally actinomycetes) in which complex polyketides have thus far been produced. To address this problem, a derivative of Escherichia coli has been genetically engineered. The resulting cellular catalyst converts exogenous propionate into 6dEB with a specific productivity that compares well with a high-producing mutant of S. erythraea that has been incrementally enhanced over decades for the industrial production of erythromycin.

Chemical rescue of phosphoryl transfer in a cavity mutant: a cautionary tale for site-directed mutagenesis.

We have explored the ability of a nucleoside diphosphate kinase (NDPK) mutant in which the nucleophilic histidine has been replaced by glycine (H122G) to transfer phosphate from ATP to alcohols of varying pK(a), size, shape, and polarity. This cavity mutant does indeed act as a primitive alcohol kinase. The rate of its phosphoryl transfer to alcohols varies considerably, with values spanning a DeltaDeltaG(double dagger) range of 4 kcal/mol, whereas the alcohols have very similar intrinsic reactivities. Analysis of these results suggests that the ability to carry out phosphoryl transfer within the cavity is not a simple function of being small enough to enter the cavity, but rather is a complex function of steric, solvation, entropic, van der Waals packing, and electrostatic properties of the alcohol. In addition, large differences are observed between the reactivities of alcohols within the nucleophile cavity of H122G and the reactivities of the same alcohols within the nucleophile cavity of H122A, a mutant NDPK that differs from H122G by a single methyl group within the cavity. The crystal structures of the two cavity mutants are very similar to one another and to wild-type NDPK, providing no evidence for a structurally perturbed active site. The differences in reactivity between the two mutant proteins illustrate a fundamental limitation of energetic analysis from site-directed mutagenesis: although removal of a side chain is generally considered to be a conservative change, the energetic effects of any given mutation are inextricably linked to the molecular properties of the created cavity and the surrounding protein environment.

Nucleophilic activation by positioning in phosphoryl transfer catalyzed by nucleoside diphosphate kinase.

The nonenzymatic reaction of ATP with a nucleophile to generate ADP and a phosphorylated product proceeds via a dissociative transition state with little bond formation to the nucleophile. Consideration of the dissociative nature of the nonenzymatic transition state leads to the following question: To what extent can the nucleophile be activated in enzymatic phosphoryl transfer? We have addressed this question for the NDP kinase reaction. A mutant form of the enzyme lacking the nucleophilic histidine (H122G) can be chemically rescued for ATP attack by imidazole or other exogenous small nucleophiles. The ATP reaction is 50-fold faster with the wild-type enzyme, which has an imidazole nucleophile positioned for reaction by a covalent bond, than with H122G, which employs a noncovalently bound imidazole nucleophile [(kcat/KM)ATP]. Further, a 4-fold advantage for imidazole positioned in the nucleophile binding pocket created by the mutation is suggested from comparison of the reaction of H122G and ATP with an imidazole versus a water nucleophile, after correction for the intrinsic reactivities of imidazole and water toward ATP in solution. X-ray structural analysis shows no detectable rearrangement of the residues surrounding His 122 upon mutation to Gly 122. The overall rate effect of approximately 10(2)-fold for the covalent imidazole nucleophile relative to water is therefore attributed to positioning of the nucleophile with respect to the reactive phosphoryl group. This is underscored by the more deleterious effect of replacing ATP with AlphaTauPgammaS in the wild-type reaction than in the imidazole-rescued mutant reaction, as follows. For the wild-type, AlphaTauPgammaS presumably disrupts positioning between nucleophile and substrate, resulting in a large thio effect of 300-fold, whereas precise alignment is already disrupted in the mutant because there is no covalent bond to the nucleophile, resulting in a smaller thio effect of 10-fold. In summary, the results suggest a catalytic role for activation of the nucleophile by positioning in phosphoryl transfer catalyzed by NDP kinase.

Ras-catalyzed hydrolysis of GTP: a new perspective from model studies.

Despite the biological and medical importance of signal transduction via Ras proteins and despite considerable kinetic and structural studies of wild-type and mutant Ras proteins, the mechanism of Ras-catalyzed GTP hydrolysis remains controversial. We take a different approach to this problem: the uncatalyzed hydrolysis of GTP is analyzed, and the understanding derived is applied to the Ras-catalyzed reaction. Evaluation of previous mechanistic proposals from this chemical perspective suggests that proton abstraction from the attacking water by a general base and stabilization of charge development on the gamma-phosphoryl oxygen atoms would not be catalytic. Rather, this analysis focuses attention on the GDP leaving group, including the beta-gamma bridge oxygen of GTP, the atom that undergoes the largest change in charge in going from the ground state to the transition state. This leads to a new catalytic proposal in which a hydrogen bond from the backbone amide of Gly-13 to this bridge oxygen is strengthened in the transition state relative to the ground state, within an active site that provides a template complementary to the transition state. Strengthened transition state interactions of the active site lysine, Lys-16, with the beta-nonbridging phosphoryl oxygens and a network of interactions that positions the nucleophilic water molecule and gamma-phosphoryl group with respect to one another may also contribute to catalysis. It is speculated that a significant fraction of the GAP-activated GTPase activity of Ras arises from an additional interaction of the beta-gamma bridge oxygen with an Arg side chain that is provided in trans by GAP. The conclusions for Ras and related G proteins are expected to apply more widely to other enzymes that catalyze phosphoryl (-PO(3)2-) transfer, including kinases and phosphatases.

Mapping the transition state for ATP hydrolysis: implications for enzymatic catalysis.

BACKGROUND: Phosphoryl transfer, typically involving high energy phosphate donors such as ATP, is the most common class of biological reactions. Despite this, the transition state for phosphoryl transfer from ATP in solution has not been systematically investigated. Characterization of the transition state for the uncatalyzed hydrolysis of ATP would provide a starting point for dissection of enzyme-catalyzed reactions. RESULTS: We examined phosphoryl transfer from ATP, GTP and pyrophosphate to a series of alcohols; these reactions are analogous to the phosphorylation of sugars and other biological alcohols and to the hydrolysis of ATP. The Brønsted beta(nucleophile) value of 0.07 is small, indicating that there is little bond formation between the incoming nucleophile and the electrophilic phosphoryl group in the transition state. Coordination of Mg2+ has no measurable effect on this value. The Brønsted beta(leaving group) value of -1.1 for phosphoryl transfer to water from a series of phosphoanhydrides is large and negative, suggesting that the bond between phosphorous and the leaving group oxygen is largely broken in the transition state. CONCLUSIONS: Uncatalyzed hydrolysis of ATP in solution occurs via a dissociative, metaphosphate-like transition state, with little bond formation between nucleophile and ATP and substantial cleavage of the bond between the gamma-phosphoryl moiety and the ADP leaving group. Bound Mg2+ does not perturb the dissociative nature of the transition state, contrary to proposals that enzyme-bound metal ions alter this structure. The simplest expectation for phosphoryl transfer at the active site of enzymes thus entails a dissociative transition state. These results provide a basis for analyzing catalytic mechanisms for phosphoryl transfer.

Alteration of axial coordination by protein engineering in myoglobin. Bisimidazole ligation in the His64-->Val/Val68-->His double mutant.

Pig and human myoglobin have been engineered to reverse the positions of the distal histidine and valine (i.e. His64(E7)-->Val and Val68(E11)-->His). Spectroscopic and ligand binding properties have been measured for human and pig H64V/V68H myoglobin, and the structure of the pig H64V/V68H double mutant has been determined to 2.07-A resolution by x-ray crystallography. The crystal structure shows that the N epsilon of His68 is located 2.3 A away from the heme iron, resulting in the formation of a hexacoordinate species. The imidazole plane of His68 is tilted relative to the heme normal; moreover it is not parallel to that of His93, in agreement with our previous proposal (Qin, J., La Mar, G. N., Dou, Y., Admiraal, S. J., and Ikeda-Saito, M. (1994) J. Biol. Chem. 269, 1083-1090). At cryogenic temperatures, the heme iron is in a low spin state, which exhibits a highly anisotropic EPR spectrum (g1 = 3.34, g2 = 2.0, and g3 < 1), quite different from that of the imidazole complex of metmyoglobin. The mean iron-nitrogen distance is 2.01 A for the low spin ferric state as determined by x-ray spectroscopy. The ferrous form of H64V/V68H myoglobin shows an optical spectrum that is similar to that of b-type cytochromes and consistent with the hexacoordinate bisimidazole hemin structure determined by the x-ray crystallography. The double mutation lowers the ferric/ferrous couple midpoint potential from +54 mV of the wild-type protein to -128 mV. Ferrous H64V/V68H myoglobin binds CO and NO to form stable complexes, but its reaction with O2 results in a rapid autooxidation to the ferric species. All of these results demonstrate that the three-dimensional positions of His64 and Val68 in the wild-type myoglobin are as important as the chemical nature of the side chains in facilitating reversible O2 binding and inhibiting autooxidation.

Probing protein-cofactor interactions in the terminal oxidases by second derivative spectroscopy: study of bacterial enzymes with cofactor substitutions and heme A model compounds.

Second derivative absorption spectra are reported for the aa3-cytochrome c oxidase from bovine cardiac mitochondria, the aa3-600 ubiquinol oxidase from Bacillus subtilis, the ba3-cytochrome c oxidase from Thermus thermophilis, and the aco-cytochrome c oxidase from Bacillus YN-2000. Together these enzymes provide a range of cofactor combinations that allow us to unequivocally identify the origin of the 450-nm absorption band of the terminal oxidases as the 6-coordinate low-spin heme, cytochrome a. The spectrum of the aco-cytochrome c oxidase further establishes that the split Soret band of cytochrome a, with features at 443 and 450 nm, is common to all forms of the enzyme containing ferrocytochrome a and does not depend on ligand occupancy at the other heme cofactor as previously suggested. To test the universality of this Soret band splitting for 6-coordinate low-spin heme A systems, we have reconstituted purified heme A with the apo forms of the heme binding proteins, hemopexin, histidine-proline-rich glycoprotein and the H64V/V68H double mutant of human myoglobin. All 3 proteins bound the heme A as a (bis)histidine complex, as judged by optical and resonance Raman spectroscopy. In the ferroheme A forms, none of these proteins displayed evidence of Soret band splitting. Heme A-(bis)imidazole in aqueous detergent solution likewise failed to display Soret band splitting. When the cyanide-inhibited mixed-valence form of the bovine enzyme was partially denatured by chemical or thermal means, the split Soret transition of cytochrome a collapsed into a single band at 443 nm.(ABSTRACT TRUNCATED AT 250 WORDS)

1H NMR study of the solution molecular and electronic structure of engineered distal myoglobin His64(E7) Val/Val68(E11) His double mutant. Coordination of His64(E11) at the sixth position in both low-spin and high-spin states.

A genetically engineered human myoglobin (Mb) in which the distal His, His64(E7), and the distal Val, Val68(E11), are replaced by Val and His, respectively, has been expressed in Escherichia coli, for the purpose of assessing the potential role of a E11 residue in providing a hydrogen bond donor to the coordinated ligand. Molecular modeling indicates that such an interaction is possible. The 1H NMR spectrum of the ferric form of the double mutant Mb exhibits large hyperfine shifts and strong paramagnetic relaxation for which the temperature dependence of the hyperfine shifts reveals a thermal equilibrium between a low-spin and high-spin state (70, 30% at 25 degrees C, respectively). Standard sequence specific two-dimensional (2D) NMR assignments of the E and F helical backbones allow the identification of the peptide protons for the proximal His93(F8) and substituted distal His68(E11). Steady-state nuclear Overhauser effect from these peptide protons locate strongly hyperfine shifted His93(F8) and His68(E11) side chain protons which dictate that both the imidazole rings are coordinated to the iron. 2D bond correlation and one-dimensional and 2D dipolar correlation experiments locate and assign the resonances for the heme. The pattern of the heme contact shifts in both the low-spin and high-spin state, together with the nature of the temperature dependence of the His93(F8) and His68(E11) resonances, establish that the two His are ligated in the high-spin as well as low-spin forms. The pattern of heme methyl hyperfine shifts in the low-spin state, and the smaller hyperfine shifts for His68(E11) as compared to His93(F8) in the high-spin state, indicate that the axial bond to the distal His68(E11) is weakened or strained as compared with that for the proximal His93(F8) in both spin states. This weak ligation originates from a tilted iron-His68 bond, the only conformation in which His68 can place its imidazole group sufficiently close to bind to the heme iron in the conventional Mb folding. Not only do these results support the belief that distal His is indispensable for the control of the ligand binding in Mb and hemoglobin, but also reveal the significance of the evolution that the stereochemical disposition of both His64 and Val68 are unique and non-exchangeable for interacting with the bound ligand.