Neurotrophins differentially enhance acetylcholine release, acetylcholine content and choline acetyltransferase activity in basal forebrain neurons

Several lines of evidence indicate that nerve growth factor is important for the development and maintenance of the basal forebrain cholinergic phenotype. In the present study, using rat primary embryonic basal forebrain cultures, we demonstrate the differential regulation of functional cholinergic markers by nerve growth factor treatment (24--96 h). Following a 96-h treatment, nerve growth factor (1--100 ng/mL) increased choline acetyltransferase activity (168--339% of control), acetylcholine content (141--185%), as well as constitutive (148--283%) and K(+)-stimulated (162--399%) acetylcholine release, but increased release was not accompanied by increased high-affinity choline uptake. Enhancement of ACh release was attenuated by vesamicol (1 microM), suggesting a vesicular source, and was abolished under choline-free conditions, emphasizing the importance of extracellular choline as the primary source for acetylcholine synthesized for release. A greater proportion of acetylcholine released from nerve growth factor-treated cultures than from nerve growth factor-naïve cultures was blocked by voltage-gated Ca(2+) channel antagonists, suggesting that nerve growth factor modified this parameter of neurotransmitter release. Cotreatment of NGF (20 ng/mL) with K252a (200 nM) abolished increases in ChAT activity and prevented enhancement of K(+)-stimulated ACh release beyond the level associated with K252a, suggesting the involvement of TrkA receptor signaling. Also, neurotrophin-3, neurotrophin-4 and brain-derived neurotrophic factor (all at 5--200 ng/mL) increased acetylcholine release, although they were not as potent as nerve growth factor and higher concentrations were required. High brain-derived neurotrophic factor concentrations (100 and 200 ng/mL) did, however, increase release to a level similar to nerve growth factor. In summary, long-term exposure (days) of basal forebrain cholinergic neurons to nerve growth factor, and in a less-potent fashion the other neurotrophins, enhanced the release of acetylcholine, which was dependent upon a vesicular pool and the availability of extracellular choline.

Subunit composition of the zinc proteins alpha- and beta-lipovitellin from chicken

Chicken alpha- and beta-lipovitellin are derived from parent vitellogenin proteins and contain four subunits (125, 80, 40, and 30 kDa) and two subunits (125 and 30 kDa), respectively. Metal analyses demonstrate both are zinc proteins containing 2.1 +/- 0.2 mol of zinc/275 kDa per alpha-lipovitellin and 1.4 +/- 0.2 mol of zinc/155 kDa per beta-lipovitellin, respectively. The subunits of beta-lipovitellin, Lv 1 (MW 125 kDa) and Lv 2 (MW 30 kDa), are separated by gel exclusion chromatography in the presence of zwittergent 3-16. Zinc elutes with Lv 1, suggesting that this subunit binds zinc in the absence of Lv 2. The subunits of alpha- and beta-lipovitellin were separated by SDS-PAGE, digested with trypsin, and mapped by reverse-phase HPLC. The peptide maps of the 125-kDa subunits from alpha- and beta-lipovitellin are essentially identical. Similar results are obtained for the 30-kDa subunits of both lipovitellins. The sequences of five and four peptides of the 125-kDa subunit of alpha- and beta-Lv, respectively, and two peptides of the 30-kDa subunit of alpha- and beta-lipovitellin were determined and match those predicted from the gene for vitellogenin II, Vtg II. Comparison of the amino acid composition of the 125- and 30-kDa subunits of alpha- and beta-lipovitellin support the conclusion that they originate from the same gene. The sequences of peptides from the 80- and 40-kDa subunits of alpha-lipovitellin have not been found in the NCBI nonredundant data bank. The 27-amino acid N-terminal sequence of the 40-kDa protein is 56% similar to the last third of the Lv 1-coding region of the Vtg II gene, suggesting it may come from an analogous region of the Vtg I gene. We propose a scheme for the precursor-product relationship of Vtg I.

Inhibition of carboxypeptidase A by D-penicillamine: mechanism and implications for drug design

Zinc metalloprotease inhibitors are usually designed to inactivate the enzyme by forming a stable ternary complex with the enzyme and active-site zinc. D-Cysteine inhibits carboxypeptidase, ZnCPD, by forming such a complex, with a K(i) of 2.3 microM. In contrast, the antiarthritis drug D-penicillamine, D-PEN, which differs from D-Cys only by the presence of two methyl groups on the beta-carbon, inhibits ZnCPD by promoting the release of the active-site zinc. We have given the name catalytic chelator to such inhibitors. Inhibition is a two-step process characterized by formation of a complex with the enzyme (K(i(initial)) = 1.2 mM) followed by release of the active-site zinc at rates up to 420-fold faster than the spontaneous release. The initial rate of substrate hydrolysis at completion of the second step also depends on D-PEN concentration, reflecting formation of a thermodynamic equilibrium governed by the stability constants of chelator and apocarboxypeptidase for zinc (K(i(final)) = 0.25 mM). The interaction of D-PEN and D-Cys with the active-site metal has been examined by replacing the active-site zinc by a chromophoric cobalt atom. Both inhibitors perturb the d-d transitions of CoCPD in the 500-600 nm region within milliseconds of mixing but only the CoCPD.D-Cys complex displays a strong S --> Co(II) charge-transfer band at 340 nm indicative of a metal-sulfur bond. While the D-Cys complex is stable, the CoCPD.D-PEN complex breaks down to apoenzyme and Co(D-PEN)(2) with a half-life of 0.5 s. D-PEN is the first drug found to inhibit a metalloprotease by increasing the dissociation rate constant of the active-site metal. The ability of D-PEN to catalyze metal removal from carboxypeptidase A and other zinc proteases suggests a possible mechanism of action in arthritis and Wilson's disease and may also underlie complications associated with its clinical use.

Allosteric inhibitors of inducible nitric oxide synthase dimerization discovered via combinatorial chemistry

Potent and selective inhibitors of inducible nitric oxide synthase (iNOS) (EC ) were identified in an encoded combinatorial chemical library that blocked human iNOS dimerization, and thereby NO production. In a cell-based iNOS assay (A-172 astrocytoma cells) the inhibitors had low-nanomolar IC(50) values and thus were >1,000-fold more potent than the substrate-based direct iNOS inhibitors 1400W and N-methyl-l-arginine. Biochemical studies confirmed that inhibitors caused accumulation of iNOS monomers in mouse macrophage RAW 264.7 cells. High affinity (K(d) approximately 3 nM) of inhibitors for isolated iNOS monomers was confirmed by using a radioligand binding assay. Inhibitors were >1,000-fold selective for iNOS versus endothelial NOS dimerization in a cell-based assay. The crystal structure of inhibitor bound to the monomeric iNOS oxygenase domain revealed inhibitor-heme coordination and substantial perturbation of the substrate binding site and the dimerization interface, indicating that this small molecule acts by allosterically disrupting protein-protein interactions at the dimer interface. These results provide a mechanism-based approach to highly selective iNOS inhibition. Inhibitors were active in vivo, with ED(50) values of <2 mg/kg in a rat model of endotoxin-induced systemic iNOS induction. Thus, this class of dimerization inhibitors has broad therapeutic potential in iNOS-mediated pathologies.

Pharmacological characterization of endogenous acetylcholine release from primary septal cultures

A detailed investigation of endogenous acetylcholine (ACh) release from primary embryonic septal cultures is described in this study. Applications of veratridine (25 microM) or increasing extracellular concentrations of K(+) (6-100 mM) induced robust increases of endogenous ACh release ( approximately 500-15,000 fmol/well/10 min). Release stimulated with K(+) (25 mM) was sustainable and did not differ significantly over 180 min. ACh release was dependent on extracellular choline and decreased proportionally to choline concentrations (0-10 microM). For example, after 30 min of stimulation with K(+) (25 mM), release in the absence of extracellular choline was approximately 25% of that associated with 10 microM choline. The vesicular transport blocker vesamicol (0-5 microM) almost completely prevented stimulated and basal ACh release at the highest concentration evaluated, which suggests a mostly vesicular mode of release in this model. The M(2)-like muscarinic receptor antagonist AF-DX 384 (0-10 microM) enhanced stimulated ACh release ( approximately 150% at the highest concentration evaluated), whereas the nonspecific muscarinic receptor agonist oxotremorine (0-10 microM) decreased stimulated release (approximately 60% at the highest concentration evaluated), suggesting that functional muscarinic autoreceptors exist in primary embryonic septal cultures. Novel findings concerning ACh release from primary embryonic septal cultures are reported herein, and the demonstration of ACh release gives further credit to the use of these cultures for studying cholinergic system functioning and in relation to physiology and pathology.

Amyloid beta-peptide inhibits high-affinity choline uptake and acetylcholine release in rat hippocampal slices

The characteristic pathological features of the postmortem brain of Alzheimer's disease (AD) patients include, among other features, the presence of neuritic plaques composed of amyloid beta-peptide (A beta) and the loss of basal forebrain cholinergic neurons, which innervate the hippocampus and the cortex. Studies of the pathological changes that characterize AD and several other lines of evidence indicate that A beta accumulation in vivo may initiate and/or contribute to the process of neurodegeneration and thereby the development of AD. However, the mechanisms by which A beta peptide influences/causes degeneration of the basal forebrain cholinergic neurons and/or the cognitive impairment characteristic of AD remain obscure. Using in vitro slice preparations, we have recently reported that A beta-related peptides, under acute conditions, potently inhibit K+-evoked endogenous acetylcholine (ACh) release from hippocampus and cortex but not from striatum. In the present study, we have further characterized A beta-mediated inhibition of ACh release and also measured the effects of these peptides on choline acetyltransferase (ChAT) activity and high-affinity choline uptake (HACU) in hippocampal, cortical, and striatal regions of the rat brain. A beta(1-40) (10(-8) M) potently inhibited veratridine-evoked endogenous ACh release from rat hippocampal slices and also decreased the K+-evoked release potentiated by the nitric oxide-generating agent, sodium nitroprusside (SNP). It is interesting that the endogenous cyclic GMP level induced by SNP was found to be unaltered in the presence of A beta(1-40). The activity of the enzyme ChAT was not altered by A beta peptides in hippocampus, cortex, or striatum. HACU was reduced significantly by various A beta peptides (10(-14) to 10(-6) M) in hippocampal and cortical synaptosomes. However, the uptake of choline by striatal synaptosomes was altered only at high concentration of A beta (10(-6) M). Taken together, these results indicate that A beta peptides, under acute conditions, can decrease endogenous ACh release and the uptake of choline but exhibit no effect on ChAT activity. In addition, the evidence that A beta peptides target primarily the hippocampus and cortex provides a potential mechanistic framework suggesting that the preferential vulnerability of basal forebrain cholinergic neurons and their projections in AD could relate, at least in part, to their sensitivity to A beta peptides.

Beta-amyloid peptides as direct cholinergic neuromodulators: a missing link?

Beta-Amyloid peptide (Abeta) is found in diffuse and focal deposits throughout the brain from Alzheimer's disease (AD) patients. Another feature of AD is the widespread degeneration and dysfunction of the basal-forebrain cholinergic system. Until now, it has been unclear how these features of AD might be related. Recent reports, however, suggest that Abeta can potently inhibit various cholinergic neurotransmitter functions independently of apparent neurotoxicity. This capacity of Abeta might contribute to the vulnerability of selected cholinergic neuronal populations in AD. Moreover, the high potency (picomolar to nanomolar concentrations) of these effects and the secretion of Abeta by brain cells indicate that Abeta-induced cholinergic hypoactivity might have physiological in addition to pathological significance.

Site-directed mutagenesis of the active site glutamate in human matrilysin: investigation of its role in catalysis

Glu-198 of human matrilysin is a conserved residue in the matrix metalloproteinases and is considered to play an important role in catalysis by acting as a general base catalyst toward the zinc-bound water molecule, on the basis of mechanistic proposals for other zinc proteases. In the present study, Glu-198 is mutated into Asp, Cys, Gln, and Ala, and the zinc binding properties, kinetic parameters, and pH dependence of each mutant are determined in order to examine the role of Glu-198 in catalysis. The mutations chosen either modify (C and D) or eliminate (A and Q) the general base properties of residue-198. All the mutants bind 2 mol of zinc per mol of enzyme, indicating that Glu-198 is not crucial to the binding of the catalytic zinc to the enzyme. The value of kcat/Km for the E198D mutant is only 4-fold lower than that of wild-type enzyme at the pH optimum of 7.5, while that for the E198C mutant is decreased by 160-fold. The E198Q and E198A enzymes containing the mutations that have eliminated the nucleophilic and acid/base properties of the residue are still active, having lower kcat/Km values of 590- and 1900-fold, respectively. The decrease in activity of all the mutants is essentially due to a decrease in kcat. The kcat/Km values of the mutants as a function of pH display broad bell-shaped curves that are similar to the wild-type enzyme. The acidic pKa value is not greatly affected by the change in the chemical properties of residue-198. The similarity in the pH profiles for the mutant and wild-type enzymes indicates that the ionization of Glu-198 is not responsible for the acidic pKa. Ionization of the zinc-bound water may be responsible for this pKa since the three His ligands and the scaffolding of the matrilysin catalytic zinc site are different from that observed in carboxypeptidase A and would predict a lower pKa for the metal-bound water. If the zinc-bound water is the nucleophile in the reaction, the role of Glu-198 in catalysis may be to stabilize the transition state or act as a general acid catalyst after the rate-determining step.

Structure and dynamics of the metal site of cadmium-substituted carboxypeptidase A in solution and crystalline states and under steady-state peptide hydrolysis

PAC spectra (perturbed angular correlation of gamma-rays) of cadmium-substituted carboxypeptidase A (CPD) show that the enzyme in solution imposes a flexible, pH- and chloride-dependent coordination structure on the metal site, in contrast to what is found in the crystalline state. A much more restricted coordination geometry occurs for the steady-state peptide intermediates of Bz-Gly-l-Phe and Bz-Gly-Gly-l-Phe in solution, suggesting that substrate binding locks the structure in a rigid conformation. The results further indicate that the peptide intermediate has a six-coordinated metal coordination geometry with an OH- ligand at the solvent site and a carbonyl oxygen at an additional ligand site. In marked contrast, conformational rigidity is not induced by the inhibitor/poor substrate Gly-L-Tyr nor by the products of high turnover substrates, Bz-Gly, Bz-Gly-Gly, and L-Phe. These results are consistent with an intact scissile peptide bond in the enzyme-substrate complex of Bz-Gly-L-Phe and Bz-Gly-Gly-L-Phe. A single nuclear quadrupole interaction (NQI) is observed for the crystalline state of the enzyme between pH 5.7 and pH 9.4. This NQI agrees with calculations based on the metal coordination geometry for cadmium in crystalline CPD derived from X-ray diffraction studies. A single broad distribution of NQIs is observed for CPD in sucrose solutions and 0.1 M NaCl at pH values below 6.5. This NQI (NQI-1') has parameters very close to those for the crystalline state. The enzyme metal site, characterized by this NQI, is converted into two new enzyme metal sites over the pH range of 6.5-8.3. The metal coordination sphere of one of these has a NQI (NQI-1) with parameters similar to those at lower pH values (NQI-1') while the other NQI (NQI-2) is characterized by markedly different NQI parameters. Angular overlap model (AOM) calculations indicate that the coordination sites giving NQI-1' and NQI-1 both have a metal-bound water molecule while the coordination site giving NQI-2 has a metal-bound hydroxide ion. PAC results at pH 8.3-10.5 indicate that in this pH range the two metal coordination geometries related to NQI-1 and NQI-2 occur in a pH independent ratio of 2:1, with the one with the water ligand being the most abundant species. The observed pH-independent equilibrium between the two different metal coordination geometries for cadmium can be explained by an equilibrium between tautomeric forms of a hydrogen bond between the Glu-270 carboxyl group and the metal-bound water (Glu-270 COO-...(HOH)M <==> Glu-270 COOH...(OH-)M) being slow on the time scale of a PAC experiment, i.e., slower than 0.5 micros. We finally suggest that NQI-1' observed at low pH reflects an enzyme species containing a metal-coordinated water molecule and the protonated carboxyl group of Glu-270.

Discrete determinants in transfer RNA for editing and aminoacylation

During translation errors of aminoacylation are corrected in editing reactions which ensure that an amino acid is stably attached to its corresponding transfer RNA (tRNA). Previous studies have not shown whether the tRNA nucleotides needed for effecting translational editing are the same as or distinct from those required for aminoacylation, but several considerations have suggested that they are the same. Here, designed tRNAs that are highly active for aminoacylation but are not active in translational editing are presented. The editing reaction can be controlled by manipulation of nucleotides at the corner of the L-shaped tRNA. In contrast, these manipulations do not affect aminoacylation. These results demonstrate the segregation of nucleotide determinants for the editing and aminoacylation functions of tRNA.

Metal and pH dependence of heptapeptide catalysis by human matrilysin

Human matrilysin devoid of its propeptide is expressed in Escherichia coli and purified to homogeneity by heparin chromatography after refolding of the guanidine hydrochloride solubilized protein. Matrilysin autolytically removes its N-terminal tripeptide Met-Tyr-Ser during the refolding process. The enzyme contains 1.91 +/- 0.08 zinc atoms/mol of protein and retains full activity when stored several months at 4 degrees C. It hydrolyzes the fluorescent substrate Dns-PLALWAR at the Ala-Leu bond with a kcat of 3.1 s-1 and K(m) of 1.8 x 10(-5) M at pH 7.5, 37 degrees C, values closely similar to those for the matrilysin produced by activation of the Chinese hamster ovary and E. coli-expressed promatrilysin. The properties of this form of matrilysin demonstrate that the propeptide is not essential for proper folding or stability of the enzyme but likely determines the N-terminal amino acid of the mature enzyme. The pH dependence of kcat/K(m) for Dns-PLALWAR shows that matrilysin has a broad pH optimum (5.0-9.0) and the pKa values obtained are 4.3 and 9.6 at 25 degrees C. The activity is inhibited by several metal binding agents including 1, 10-phenanthroline, OP, but not by the nonchelating isomer, 1,7-phenanthroline. OP inhibits instantaneously by likely forming a transient ternary enzyme.metal.chelator complex. The zinc atom is then removed from the protein in a time-dependent manner. In agreement with the kinetic studies, dialysis in the presence of OP and CaCl2 removes only the catalytic zinc atom. The monozinc enzyme can be reactivated to 90%, 56%, 27%, and 17% of the native activity by addition of zinc, manganese, nickel, and cobalt, respectively. Cadmium, on the other hand, forms an inactive Cd/Zn hybrid. The differences in the chelator accessibility properties of the two zinc sites can thus be exploited to yield metallohybrids of matrilysin.

D-Phe complexes of zinc and cobalt carboxypeptidase A

The binding of D-phenylalanine, D-Phe, to both zinc and cobalt carboxypeptidase A, ZnCPD and CoCPD, has been investigated by a combination of kinetic and spectroscopic techniques. Kinetic studies of the ZnCPD catalyzed hydrolysis of dansyl-Gly-Ala-L-Phe indicate that D-Phe inhibition occurs through a two-site sequential competitive inhibition mode with Ki values of 45 microM and 11.6 mM at pH 8.4, 1 M NaCl, 25 degrees C. Spectral titration of CoCPD under the same conditions indicates a very strong binding mode of D-Phe (KD < 100 microM) that only slightly perturbs the visible cobalt electronic transitions. However, the conversion of CoCPD.D-Phe into a CoCPD.D-Phe2 (KD, 1.13 mM) is accompanied by a very strong spectral perturbation resulting in a complex that is characterized by Amax values of 506 nm (epsilon = 27 M-1 cm-1) and 605 nm (epsilon = 17 M-1 cm-1) and a shoulder at 530 nm (epsilon = 23 M-1 cm-1). The spectral properties of this ternary complex differ markedly from that of the CoCPD.L-Phe.N3-ternary complex. X-ray absorption fine structure, XAFS, studies indicate that these differences are likely due to a more regular tetrahedral coordination sphere for the ternary azide complexes compared to an octahedral coordination geometry for the Zn and CoCPD.D-Phe2 complexes.

A mechanism for reducing entropic cost of induced fit in protein--RNA recognition

Induced fit has been postulated to be an important component of ligand interactions with proteins, including protein-DNA interactions. We imagined that the entropic cost of induced fit might be highly dependent on the local protein sequence context around critical contact residues. To investigate this question, we analyzed the basis for active or inactive phenotypes found in a library of combinatorial sequence variants of a surface-located helix-loop peptide which is essential for the anticodon-binding activity of a class I tRNA synthetase. Molecular dynamics simulations of the domain encompassing the helix-loop peptide of the active variants consistently demonstrated fixation of the local motion of five critical (for function) residues which are highly mobile in inactive variants. Additional experiments with other rationally chosen mutants extended the correlation between phenotype and motion of these vital residues. We propose that the need for fixation of local motion is an important constraint on sequences of surface peptides which form parts of RNA-binding sites. The fixation of motion of critical residues in the unbound protein can significantly reduce the entropic cost of complex formation by induced fit.

X-ray absorption fine structure as a monitor of zinc coordination sites during oogenesis of Xenopus laevis

The x-ray absorption fine structure (XAFS) zinc K-edge steps for intact stages I,II and V,VI Xenopus laevis oocytes demonstrate that the zinc concentration is about 3 and 1 mM, respectively. However, the chi(k) function for the early stage oocytes differs markedly from that for the late one. Analysis of the XAFS data for stage I,II oocytes indicates that zinc is bound to 2.0 +/- 0.5 sulfur atoms at an average coordination distance of 2.29 +/- 0.02 angstroms and 2.0 +/- 0.5 nitrogen or oxygen (N/O) atoms at 2.02 +/- 0.02 angstroms. In marked contrast, in stage V,VI oocytes, zinc is bound to 4.1 +/- 0.4 N/O atoms at an average distance of 1.98 +/- 0.01 angstroms. Our previous studies demonstrated that 90% of the zinc in stage VI oocytes is sequestered within yolk platelets, associated with a single molecule, lipovitellin, the proteolytically processed product of vitellogenin. XAFS analysis of yolk platelets, lipovitellin, and vitellogenin demonstrates that zinc is bound to 4.0 +/- 0.5 N/O ligands at an average distance of 1.98 +/- 0.01 angstroms in each case, identical to that of stage V,VI oocytes. The higher shell contributions in the Fourier transforms indicate that two of the N/O zinc ligands are His in both stage V,VI and I,II oocytes. The results show that in stage I,II oocytes, there is a high concentration of a zinc protein whose zinc coordination site likely is composed of (His)2(Cys)2, such as, e.g., TFIIIA. As the oocytes develop, the predominant zinc species becomes one that exhibits the (His)2(N/0)2 zinc site found in lipovitellin. Hence, the ligands to the zinc atoms in intact oocytes and the changes that take place as a function of oogenesis and after their fertilization, during embryogenesis, now can be examined and explored.

Single sequence of a helix-loop peptide confers functional anticodon recognition on two tRNA synthetases

The specific aminoacylation of RNA oligonucleotides whose sequences are based on the acceptor stems of tRNAs can be viewed as an operational RNA code for amino acids that may be related to the development of the genetic code. Many synthetases also have direct interactions with tRNA anticodon triplets and, in some cases, these interactions are thought to be essential for aminoacylation specificity. In these instances, an unresolved question is whether interactions with parts of the tRNA outside of the anticodon are sufficient for decoding genetic information. Escherichia coli isoleucyl- and methionyl-tRNA synthetases are closely related enzymes that interact with their respective anticodons. We used binary combinatorial mutagenesis of a 10 amino acid anticodon binding peptide in these two enzymes to identify composite sequences that would confer function to both enzymes despite their recognizing different anticodons. A single peptide was found that confers function to both enzymes in vivo and in vitro. Thus, even in enzymes where anticodon interactions are normally important for distinguishing one tRNA from another, these interactions can be 'neutralized' without losing specificity of amino-acylation. We suggest that acceptor helix interactions may play a role in providing the needed specificity.

Structure of binary and ternary complexes of zinc and cobalt carboxypeptidase A as determined by X-ray absorption fine structure

Carboxypeptidase A, ZnCPD, is typical of a wide range of exo- and endo-metalloproteases that have three protein ligands and a water molecule bound to a catalytic zinc atom and a glutamate residue in the active site that likely acts in conjunction with the Zn-bound water to bring about catalysis. Such enzymes generally have bell-shaped pH-activity profiles (EH2 in equilibrium with EH in equilibrium with E) where the concentration of the catalytic species EH is regulated by pKEH2 and pKEH. The present X-ray absorption fine structure, XAFS, study has determined the structure and pH behavior of the binary and ternary product complexes in order to examine the role of the Zn-bound water in catalysis. Increasing the pH from 7 to 10 of the ZnCPD.L-Phe complex results in the same type of progressive spectral changes in the near-edge XAFS spectrum as is seen for ZnCPD, but the changes are complete by more than 1 pH value below that observed for ZnCPD. The results are in agreement with kinetic studies that show E binds the protonated form of L-Phe more tightly than EH, thus in effect decreasing the value of pKEH. The XAFS results show the average interatomic distance. R. for the zinc ligands of the EH.L-Phe complex decreases by 0.02 A upon formation of the E.L-Phe complex, essentially identical to that obtained for the EH and E forms of the native enzyme. Addition of azide to ZnCPD.L-Phe at pH 7 markedly changes the zinc coordination sphere from 4 N/O atoms at 2.021 +/- O.06 A and 1.4 +/- 0.5 N/O atoms at 2.54 +/- 0.5 A to 3.9 N/O atoms at 1.995 +/- 0.006 A. The decrease in R of about 0.03 A in both the Zn- and CoCPD.L-Phe.N3-complexes is likely due to the ligand exchange from a neutral water to an anion. The XAFS spectra of the ternary complex is pH independent from 7 to 9, in agreement with the ionization of the water being the source of the spectral changes in the free enzyme and its binary L-Phe complex. The enzyme.azide.L-Phe complex is likely bound in a manner analogous to that expected for a post-transition state in a biproduct complex for peptide hydrolysis--that is, the carboxylate anion of the peptide bound to the Zn and the protonated form of L-Phe H-bonded to the catalytic Glu-270 carboxylate. The XAFS results on the "spectroscopically silent" ZnCPD are compared to nuclear magnetic resonance and electron absorption studies on CoCPD.

Matrilysin: expression, purification, and characterization

The expression vector pGEX-2T under the control of the IPTG-inducible tac promotor is effective for the production of a fusion protein of glutathione transferase (GST, 26 kDa) and promatrilysin (28 kDa) separated from the C-terminus of GST by a thrombin cleavage site. Zwittergen (palmityl sulfobetaine), 2%, solubilizes the fusion protein that is found associated with inclusion bodies. The solubilized fusion protein is purified by affinity chromatography on GSH agarose. Promatrilysin is obtained by thrombin cleavage either on the column or after GSH elution of the fusion protein. Mono S chromatography of the recovered protein yields homogeneous promatrilysin. The zinc content of promatrilysin and its activated enzyme product is slightly greater than 2 mol of zinc per mole of protein. The results indicate that the matrix metalloproteinases (MMPs) contain two metal-binding sites at which zinc is firmly bound and possibly a third site at which it is weakly bound. Primary sequence alignments for all the MMPs have a sequence homologous to the zinc-binding site of astacin, HExxHxxGxxH, suggesting one of the zinc sites is a catalytic one, in agreement with the known inhibition of these enzymes by chelators. However, the other zinc-binding site(s) likely reflect the different ways that astacin and the MMP subfamilies are stabilized, i.e., disulfides in astacin and metal ions in the MMPs.

Switching recognition of two tRNA synthetases with an amino acid swap in a designed peptide

The genetic code is based on specific interactions between transfer RNA (tRNA) synthetases and their cognate tRNAs. The anticodons for methionine and isoleucine tRNAs differ by a single nucleotide, and changing this nucleotide in an isoleucine tRNA is sufficient to change aminoacylation specificity to methionine. Results of combinatorial mutagenesis of an anticodon-binding-helix loop peptide were used to design a hybrid sequence composed of amino acid residues from methionyl- and isoleucyl-tRNA synthetases. When the hybrid sequence was transplanted into isoleucyl-tRNA synthetase, active enzyme was generated in vivo and in vitro. The transplanted peptide did not confer function to methionyl-tRNA synthetase, but the substitution of a single amino acid within the transplanted peptide conferred methionylation and prevented isoleucylation. Thus, the swap of a single amino acid in the transplanted peptide switches specificity between anticodons that differ by one nucleotide.

Protein thermal denaturation, side-chain models, and evolution: amino acid substitutions at a conserved helix-helix interface

Random mutant libraries with substitutions at the interface between the N- and C-terminal helices of Saccharomyces cerevisiae iso-1-cytochrome c were screened. All residue combinations that have been identified in naturally occurring cytochrome c sequences are found in the libraries. Mutants with these combinations are biologically functional. Enthalpies, heat capacities, and midpoint temperatures of denaturation are used to determine the entropy and Gibbs free energy of denaturation (delta GD) for the ferri form of the wild-type protein and 13 interface variants. Changes in delta GD cannot be allocated solely to enthalpic or entropic effects, but there is no evidence of enthalpy-entropy compensation. The lack of additivity of delta GD values for single versus multiple amino acid substitutions indicates that the helices interact thermodynamically. Changes in delta GD are not in accord with helix propensities, indicating that interactions between the helices and the rest of the protein outweigh helix propensity. Comparison of delta GD values for the interface variants and nearly 90 non-cytochrome c variants to side-chain model data leads to several conclusions. First, hydrocarbon side chains react to burial-like transfer from water to cyclohexane, but even weakly polar side chains respond differently. Second, despite octanol being a poor model for protein interiors, octanol-to-water transfer free energies are useful stability predictors for changing large hydrocarbon side chains to smaller ones. Third, unlike cyclohexane and octanol, the Dayhoff mutation matrix predicts stability changes for a variety of substitutions, even at interacting sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Zinc metallochemistry in biochemistry

The chemically stable but stereochemically flexible, non-toxic nature of zinc combined with its amphoteric properties has permitted it to orchestrate a number of zinc-binding motifs critical to life processes. For zinc enzymes, catalytic, cocatalytic, and structural zinc sites exist. DNA-binding proteins have zinc fingers, twists, and clusters exist.

Zinc content of promatrilysin, matrilysin and the stromelysin catalytic domain

Promatrilysin expressed in Escherichia coli and Chinese hamster ovary cells contains 2.36 +/- 0.19 and 2.13 +/- 0.39 moles of zinc per mole of protein, respectively, while the activated enzyme contains 2.22 +/- 0.21. The catalytic domain of stromelysin-1 expressed in E. coli contains 2.22 +/- 0.11. Thus these matrix metalloproteinases contain two metal binding sites at which zinc is bound firmly and possibly a third site at which it is bound weakly. Promatrilysin and matrilysin do not contain significant amounts of Fe, Cu, Mn, or Ni. All known matrix metalloproteinases have a sequence homologous to the zinc binding site of astacin, HExxHxxGxxH, suggesting that one of the zinc sites is catalytic in agreement with the known inhibition of these enzymes by chelators.

XAFS studies of carboxypeptidase A: detection of a structural alteration in the zinc coordination sphere coupled to the catalytically important alkaline pKa

X-ray absorption fine structure (XAFS) spectra of carboxypeptidase A (ZnCPD) show progressive spectral changes particularly in the near edge region when the pH is changed from neutral to alkaline values. Both least square fitting and radial distribution function (RDF) data analysis yield two distributions of atoms in the first coordination shell of ZnCPD at all pH values and at both 150 and 297 K. Direct comparison of the first and higher coordination shells of ZnCPD reveals structural differences between pH 7.0 and pH 9.9. At pH 7.0, the zinc ion has four ligand atoms (N or O) at an average distance of 2.024 +/- 0.006 A, and a smaller distribution of 1.3 atoms (N or O) at 2.54 +/- 0.05 A from the zinc ion (from the least square fitting analysis). At pH 9.9, the larger distribution contains four atoms at a 0.022 A shorter distance (2.002 A) from the zinc, while the smaller distribution contains 0.7 atoms at 2.52 +/- 0.06 A. The smaller distribution can be attributed mainly to the contribution of the epsilon 2-oxygen of Glu 72 and to the atoms farther away for which the contribution cannot be fully separated from the first shell peak. The structural changes of ZnCPD at intermediate pHs are consistent with the changes observed at pH 7.0 and pH 9.9. The XAFS Debye-Waller factor shows an increased structural disorder for the four atom distribution at the alkaline pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Probing weakly polar interactions in cytochrome c

Theoretical, statistical, and model studies suggest that proteins are stabilized by weakly polar attractions between sulfur atoms and properly oriented aromatic rings. The two sulfur-containing amino acids, methionine and cysteine, occur frequently among functional alleles in random mutant libraries of Saccharomyces cerevisiae iso-1-cytochrome c genes at positions that form a weakly polar aromatic-aromatic interaction, the wild-type protein. To determine if a weakly polar sulfur-aromatic interaction replaced the aromatic-aromatic interaction, the structure and stability of two variants were examined. Phenylalanine 10, which interacts with tyrosine 97, was replaced by methionine and cysteine. The cysteine was modified to form the methionine and cysteine analog, S-methyl cysteine (CysSMe). Proton NMR studies indicate that changing Phe 10 to Met or CysSMe affects only local structure and that the structures of sulfur-containing variants are nearly identical. Analysis of chemical shifts and nuclear Overhauser effect data indicates that both sulfur-containing side chains are in position to form a weakly polar interaction with Tyr 97. The F10M and F10CSMe variants are 2-3 kcal mol-1 less stable than iso-1-cytochrome c at 300 K. Comparison of the stabilities of the F10M and F10CSMe variants allows evaluation of the potential weakly polar interaction between the additional sulfur atom of F10CSMe and the aromatic moiety of Tyr 97. The F10CSMe;C102T variant is 0.7 +/- 0.3 kcal mol-1 more stable than the F10M;C102T protein. The increased stability is explained by the difference in hydrophobicity of the sulfur-containing side chains. We conclude that any weakly polar interaction between the additional sulfur and the aromatic ring is too weak to detect or is masked by destabilizing contributions to the free energy of denaturation.