Novel catalytic mechanism of nucleophilic substitution by asparagine residue involving cyanoalanine intermediate revealed by mass spectrometric monitoring of an enzyme reaction.

l-2-Haloacid dehalogenase from Pseudomonas sp. YL catalyzes the hydrolytic dehalogenation, in which Asp(10) acts as a nucleophile to attack the alpha-carbon of l-2-haloalkanoates to form an ester intermediate, which is subsequently hydrolyzed to produce d-2-hydroxyalkanoates. Surprisingly, replacement of the catalytic residue, Asp(10), by Asn did not result in total inactivation of the enzyme (Kurihara, T., Liu, J.-Q., Nardi-Dei, V., Koshikawa, H., Esaki, N., and Soda, K. (1995) J. Biochem. 117, 1317-1322). In this study, we monitored the D10N mutant enzyme reaction by ion-spray mass spectrometry, and found that the enzyme shows a unique structural change when it was incubated with the substrate, l-2-chloropropionate. LC/MS and tandem MS/MS analyses revealed that Asn(10) attacks the substrate to form an imidate, and a proton and d-lactic acid are eliminated to produce a nitrile (beta-cyanoalanine residue), followed by hydrolysis to reproduce Asn(10). This is the first report of the function of Asn to catalyze nucleophilic substitution through its conversion to beta-cyanoalanine residue as an intermediate structure. Also, these results demonstrate that mass spectrometry is remarkably useful in monitoring enzyme reactions.

The process of amyloid-like fibril formation by methionine aminopeptidase from a hyperthermophile, Pyrococcus furiosus.

Amyloid is associated with serious diseases including Alzheimer's disease and senile-systemic amyloidosis due to misfolded proteins. In the course of study of the denaturation process of methionine aminopeptidase (MAP) from the hyperthermophile P. furiosus, we found that MAP forms amyloid-like fibrils, and we then investigated the mechanism of amyloid fibril formation. The kinetic experiments on denaturation monitored by CD at 222 nm indicated that MAP in the presence of 3.37 M GuHCl at pH 3.31 changed to a conformation containing a considerable content of beta-sheet structure after the destruction of the alpha-helical structure. MAP in this beta-rich conformation was highly associated, and its stability was remarkably high: the midpoint of the GuHCl denaturation curve was 4.82 M at pH 3.0, and a thermal transition was not observed up to 125 degrees C by calorimetry. The amyloid-like fibril formation of MAP was confirmed by Congo red staining with a typical peak at 542 nm in the difference spectrum, showing a cross-beta X-ray diffraction pattern with a clear sharp reflection at 4.7 A and a characteristic unbranched fibrillar appearance with a length of about 1000 A and a diameter of about 70 A in the electron micrographs. Present results indicate that the amyloid-like form of MAP appears just after the protein is almost completely denatured, and even highly stable proteins can also form amyloid-like conformation under conditions where the denatured state of the protein is abundantly populated.

DL-2-Haloacid dehalogenase from Pseudomonas sp. 113 is a new class of dehalogenase catalyzing hydrolytic dehalogenation not involving enzyme-substrate ester intermediate.

DL-2-Haloacid dehalogenase from Pseudomonas sp. 113 (DL-DEX 113) catalyzes the hydrolytic dehalogenation of D- and L-2-haloalkanoic acids, producing the corresponding L- and D-2-hydroxyalkanoic acids, respectively. Every halidohydrolase studied so far (L-2-haloacid dehalogenase, haloalkane dehalogenase, and 4-chlorobenzoyl-CoA dehalogenase) has an active site carboxylate group that attacks the substrate carbon atom bound to the halogen atom, leading to the formation of an ester intermediate. This is subsequently hydrolyzed, resulting in the incorporation of an oxygen atom of the solvent water molecule into the carboxylate group of the enzyme. In the present study, we analyzed the reaction mechanism of DL-DEX 113. When a single turnover reaction of DL-DEX 113 was carried out with a large excess of the enzyme in H(2)(18)O with a 10 times smaller amount of the substrate, either D- or L-2-chloropropionate, the major product was found to be (18)O-labeled lactate by ionspray mass spectrometry. After a multiple turnover reaction in H(2)(18)O, the enzyme was digested with trypsin or lysyl endopeptidase, and the molecular masses of the peptide fragments were measured with an ionspray mass spectrometer. No peptide fragments contained (18)O. These results indicate that the H(2)(18)O of the solvent directly attacks the alpha-carbon of 2-haloalkanoic acid to displace the halogen atom. This is the first example of an enzymatic hydrolytic dehalogenation that proceeds without producing an ester intermediate.

The inhibitory properties and primary structure of a novel serine proteinase inhibitor from the fruiting body of the basidiomycete, Lentinus edodes.

A novel proteinase inhibitor, Lentinus proteinase inhibitor, has been purified from the fruiting bodies of the edible mushroom, Lentinus edodes, by buffer extraction and affinity chromatography on immobilized anhydrotrypsin. The protein simultaneously inhibits bovine beta-trypsin and alpha-chymotrypsin at independent sites, with apparent dissociation constants of 3.5 x 10(-10) M and 4 x 10(-8) M, respectively. The purified protein is eluted as two well-separated peaks on reversed-phase HPLC, one of which is inhibitory-active and the other inactive, and they are interconvertible under folding/unfolding conditions. Among the mammalian and microbial serine proteinases examined, including human enzymes of blood coagulation and fibrinolysis, activated factor XI was inhibited by the Lentinus proteinase inhibitor. Chemical modification studies suggest involvement of one or more arginine residues in the inhibition of trypsin. The complete primary structure composed of 142 amino acids with an acetylated N-terminus was determined by protein analysis. The theoretical molecular mass (15999.2) from the sequence is close to the experimental value of 15999.61 +/- 0.61 determined by mass spectrometry. Although there are no apparently homologous proteinase inhibitors in the protein database, there is a rather striking similarity to the propeptide segment of a microbial serine proteinase, as well as to the N-terminal region of the mature enzyme.

Characterization and primary structure of a fatty acid-binding protein and its isoforms from the liver of the Amphibia, Rana catesbeiana.

Fatty acid-binding protein (FABP) was purified from the liver of the Amphibia, Rana catesbeiana, by gel filtration and ion-exchange chromatography. The complete primary structure of the frog liver FABP was determined by protein analysis. Two isoforms, I and II, were separated by reversed phase HPLC, and found to differ by 10 atomic mass units as measured by ion-spray ionization mass spectrometry. A detailed analysis of enzymatic peptides revealed a single Pro (isoform I)/Ser (isoform II) replacement at position 16. It seems remarkable that a rather neutral point mutation results in the nearly complete separation of the two isoforms by reversed phase chromatography. Homology modeling suggests the location of this site on the first helix of the helix-turn-helix domain and the presence of a single thiol group of cysteine-91 at the inside of the ligand-binding cavity. Binding studies using a natural fluorescent fatty acid, cis-parinaric acid, showed a lower Kd value for the serine form and large enhancement of fluorescence intensity upon glutathione-thiolation at cysteine-91. Examination of phylogenetic relationships identified the frog liver protein as a mammalian liver type FABP, and suggested a change in the vertebrate liver FABP gene expression at the bony fish/cartilagenous fish boundary.

The unusually slow unfolding rate causes the high stability of pyrrolidone carboxyl peptidase from a hyperthermophile, Pyrococcus furiosus: equilibrium and kinetic studies of guanidine hydrochloride-induced unfolding and refolding.

To elucidate the energetic features of the anomalously high-level stabilization of a hyperthermophile pyrrolidone carboxyl peptidase (PfPCP) from a hyperthermophilic archaeon, Pyrococcus furiosus, equilibrium and kinetic studies of the guanidine hydrochloride (GuHCl)-induced unfolding and refolding were carried out with CD measurements at 220 nm in comparison with those from the mesophile homologue (BaPCP) from Bacillus amyloliquefaciens. The mutant protein of PfPCP substituted with Ser at both Cys142 and Cys188 (PfC142/188S) was used. The GuHCl unfolding for PfC142/188S and BaPCP was reversible. It was difficult to obtain the equilibrated unfolding curve of the hyperthermophile proteins at temperatures below 50 degreesC and pH 7, because of the remarkably slow rate of the unfolding. The unfolding for PfC142/188S attained equilibrium after 7 days at 60 degreesC, resulting in the coincidence between the unfolding and refolding curves. The Gibbs energy change of unfolding, DeltaGH2O (56.6 kJ/mol), for PfC142/188S at 60 degreesC and pH 7 was dramatically higher than that (7.6 kJ/mol) for BaPCP at 40 degreesC and pH 7. The unfolding and refolding kinetics for PfC142/188S and BaPCP at both 25 and 60 degreesC at pH 7 were approximated as a single exponential. The rate constant in water (kuH2O) of the unfolding reaction for PfC142/188S (1.6 x 10(-)15 s-1) at 25 degreesC and pH 7 was drastically reduced by 7 orders of magnitude compared to that (1.5 x 10(-)8 s-1) for BaPCP, whereas the refolding rates (krH2O) in water for PfC142/188S (9.3 x 10(-)2 s-1) and BaPCP (3.6 x 10(-)1 s-1) at 25 degreesC and pH 7 were similar. These results indicate that the greater stability of the hyperthermophile PCP was characterized by the drastically slow unfolding rate.

Crystal structure of methionine aminopeptidase from hyperthermophile, Pyrococcus furiosus.

The structure of methionine aminopeptidase from hyperthermophile Pyrococcus furiosus (PfMAP) with an optimal growth temperature of 100 degreesC was determined by the multiple isomorphous replacement method and refined in three different crystal forms, one monoclinic and two hexagonal, at resolutions of 2.8, 2.9, and 3.5 A. The resolution of the monoclinic crystal form was extended to 1.75 A by water-mediated transformation to a low-humidity form, and the obtained diffraction data used for high-resolution structure refinement. This is the first description of a eukaryotic type methionine aminopeptidase structure. The PfMAP molecule is composed of two domains, a catalytic domain and an insertion domain, connected via two antiparallel beta-strands. The catalytic domain, which possesses an internal 2-fold symmetry and contains two cobalt ions in the active site, resembles the structure of a prokaryotic type MAP from Escherichia coli (EcMAP), while the structure of the insertion domain containing three helices has a novel fold and accounts for a major difference between the eukaryotic and prokaryotic types of methionine aminopeptidase. Analysis of the PfMAP structure in comparison with EcMAP and other mesophile proteins reveals several factors which may contribute to the hyperthermostability of PfMAP: (1) a significantly high number of hydrogen bonds and ion-pairs between side-chains of oppositely charged residues involved in the stabilization of helices; (2) an increased number of hydrogen bonds between the positively charged side-chain and neutral oxygen; (3) a larger number of buried water molecules involved in crosslinking the backbone atoms of sequentially separate segments; (4) stabilization of two antiparallel beta-strands connecting the two domains of the molecule by proline residues; (5) shortening of N and C-terminal tails and stabilization of the loop c3E by deletion of three residues.

Reaction mechanism of fluoroacetate dehalogenase from Moraxella sp. B.

Fluoroacetate dehalogenase (EC 3.8.1.3) catalyzes the dehalogenation of fluoroacetate and other haloacetates. The amino acid sequence of fluoroacetate dehalogenase from Moraxella sp. B is similar to that of haloalkane dehalogenase (EC 3.8.1.5) from Xanthobacter autotrophicus GJ10 in the regions around Asp-105 and His-272, which correspond to the active site nucleophile Asp-124 and the base catalyst His-289 of the haloalkane dehalogenase, respectively (Krooshof, G. H., Kwant, E. M., Damborský, J., Koca, J., and Janssen, D. B. (1997) Biochemistry 36, 9571-9580). After multiple turnovers of the fluoroacetate dehalogenase reaction in H218O, the enzyme was digested with trypsin, and the molecular masses of the peptide fragments formed were measured by ion-spray mass spectrometry. Two 18O atoms were shown to be incorporated into the octapeptide, Phe-99-Arg-106. Tandem mass spectrometric analysis of this peptide revealed that Asp-105 was labeled with two 18O atoms. These results indicate that Asp-105 acts as a nucleophile to attack the alpha-carbon of the substrate, leading to the formation of an ester intermediate, which is subsequently hydrolyzed by the nucleophilic attack of a water molecule on the carbonyl carbon atom. A His-272 --> Asn mutant (H272N) showed no activity with either fluoroacetate or chloroacetate. However, ion-spray mass spectrometry revealed that the H272N mutant enzyme was covalently alkylated with the substrate. The reaction of the H272N mutant enzyme with [14C]chloroacetate also showed the incorporation of radioactivity into the enzyme. These results suggest that His-272 probably acts as a base catalyst for the hydrolysis of the covalent ester intermediate.

Pyrrolidone carboxyl peptidase from the hyperthermophilic Archaeon Pyrococcus furiosus: cloning and overexpression in Escherichia coli of the gene, and its application to protein sequence analysis.

A gene for a pyrrolidone carboxyl peptidase (Pcp: EC 3.4.19.3, pyroglutamyl peptidase), which removes amino-terminal pyroglutamyl residues from peptides and proteins, has been cloned from the hyperthermophilic Archaeon Pyrococcus furiosus using its cosmid protein library, sequenced, and expressed in Escherichia coli. The DNA sequence encodes a protein containing 208 amino acid residues with methionine at the N-terminus. Analysis of the recombinant protein expressed in E. coli, including amino acid sequence analysis from the N-terminus by automated Edman degradation and ionspray mass spectrometric analysis of the peptides generated by enzymatic digestions with lysylendopeptidase and Staphylococcus aureus V8 protease, showed its primary structure to be completely identical with that deduced from its cDNA sequence. Comparison of the amino acid sequence of P. furiosus Pcp (P.f.Pcp) with those of bacterial Pcps revealed that a high degree of sequence identity (more than 40%) and conservation of the amino acid residues comprising the catalytic triad, Cys142, His166, and Glu79. On the other hand, a unique short stretch sequence (positions around 175-185) that is absent in bacterial Pcps was found in P.f.Pcp. A similar stretch has also been reported recently in the amino acid sequence of Pcp from the hyperthermophilic Archaeon Thermococcus litoralis [Littlechild et al., in abstracts of the "International Congress on Exthermophiles '98" p. 58 (1998)]. To elucidate their contribution to the hyperthermostability of these enzymes, further structural studies are required.

Identification of the catalytic triad residues of porcine liver acylamino acid-releasing enzyme.

Acylamino acid-releasing enzyme (AARE) [EC 3.4.19.1] is a tetrameric serine protease, which belongs to the oligopeptidase family and specifically removes acetyl amino acids from N-terminally acetylated peptides. By using diisopropyl fluorophosphate, we previously identified one of the residues comprising the catalytic triad of this enzyme as Ser587 [Miyagi, M. et al. (1995) J. Biochem. 118, 771-779]. To elucidate the other two residues forming the catalytic triad of this new serine-type protease, wild-type and four mutant AAREs, in which each candidate residue of the catalytic triad deduced from sequence alignment with other oligopeptidases was substituted by site-directed mutagenesis, were expressed in Escherichia coli as fusion proteins with short peptide chains at both N- and C-termini of a subunit of porcine liver enzyme. All of the recombinant AAREs were estimated to have similar conformational and quaternary structures to the native porcine liver enzyme from their CD spectra and behavior on gel-filtration, but the mutants in which Ala587, Asn675, or Tyr707 was substituted for Ser587, Asp675, or His707, respectively, did not show detectable hydrolytic activity toward acetyl-L-methionyl L-alanine. These facts suggest that Ser587, Asp675, and His707 are essential residues for the AARE activity and comprise the catalytic triad of the enzyme in this order. Thus, AARE has been shown to have a protease-like domain in its C-terminal region, as do other proteins classified as members of the oligopeptidase family.

A novel derivatization method with 5-bromonicotinic acid N-hydroxysuccinimide for determination of the amino acid sequences of peptides.

We have developed a novel method that effectively identifies the N-terminal product ions produced in the tandem mass spectrometry (MS/MS) analysis of peptides done in conjunction with the specific derivatization of the N-terminal amino group using 5-bromonicotinic acid N-hydroxysuccinimide ester (BrNA-NHS). Electrospray ionization with low-energy collision-induced dissociation (CID) MS/MS clearly differentiated the N-terminal product ions labeled with the 5-bromonicotinyl group from other ions, on the basis of the appearance of CID peaks with a doublet pattern characteristically separated by 2 mass units produced by the equal natural abundances of 79Br and 81Br. The tracing of a series of these bromine-containing product ions allows the easy amino acid sequencing of peptides. Using Gln-Arg-Leu-Gln-Ser-Asn-Gln-Leu-Lys as the test peptide, we found that within 30 minutes at pH 6.5 and 37 degrees C its alpha-amino group was completely acylated with BrNA-NHS (peptide: BrNA-NHS = 1:40; mol/mol). The epsilon-amino group of the C-terminal lysine residue was less likely to be acylated under these conditions, being only partly modified (about 20%). This suggests the possibility of keeping the epsilon-amino group free from acylation. The method was successfully applied to the determination of the amino acid sequences of peptides from porcine kidney aminoacylase I produced by digestion with lysyl endopeptidase and with Staphylococus aureus V8 protease.

Electrostatic stabilization in methionine aminopeptidase from hyperthermophile Pyrococcus furiosus.

The thermostability of methionine aminopeptidase from a hyperthermophile P. furiosus (PfMAP) was extremely high: the denaturation temperature was 106.2 degreesC at pH 10.2. To explore the contribution of electrostatic interaction to the superior thermostability of PfMAP, the thermostability of PfMAP was examined by differential scanning calorimetry (DSC) in various salt concentrations in the acidic region far from the isoelectric point of PfMAP. (1) In 20 mM glycine buffer, the DSC curve of PfMAP exhibited a single peak. Transition temperatures (Tm) were lowered with decreasing pH from 4 to 3. The heat denaturation of PfMAP was not reversible. (2) Denaturation enthalpy (DeltaH) measured at different pHs linearly correlated with Tm up to 102 degreesC, suggesting that the denaturation heat capacity (DeltaCp) for PfMAP is constant up to 100 degreesC. DeltaCp was estimated to be 0.82 J K-1 g-1. (3) In the presence of 10-100 mM KCl at pH 3.2, two peaks appeared on the DSC curves. The first peak shifted to lower temperatures with increasing concentration of KCl and, oppositely, the second one to higher temperatures. It was found that the first and second peaks originated from the heat denaturation of the native form of PfMAP and the melting of the non-native associated form having molten globule-like structure, respectively, judged from the CD spectra and ultracentrifugation analyses. This indicates the following: first, the attractive electrostatic interaction is an important factor in stabilizing the native form of PfMAP; second, the presence of KCl stimulates the formation of the molten globule-like state of PfMAP and stabilizes it. (4) In a comparison of the sequence and crystal structure of PfMAP, which has been recently determined (1xgs.pdb), with those of MAP from Escherichia coli (EcMAP), it was predicted that the extra four short-range ion pairs less than 3 A involved in PfMAP are crucial candidates as determinants for the superior thermostability of PfMAP.

High-resolution crystals of methionine aminopeptidase from Pyrococcus furiosus obtained by water-mediated transformation.

The monoclinic crystal form of methionine amino-peptidase from Pyrococcus furiosus (MAP-Pfu) has been crystallized from four different conditions. Native crystals belong to space group P2(1) with typical unit-cell dimensions a = 53.4, b = 85.1, c = 72.7 A, beta = 107.7 degrees and diffract to 2.9-4.5 A resolution. However, there is a problem of nonisomorphism among the crystals. Water-mediated transformation to low-humidity form occurs by reduction of the relative humidity of crystal environment to 79%. The unit-cell dimensions of transformed crystals are a = 51.9, b = 83.3, c = 70.3 A, beta = 105.9 degrees, and the calculated solvent content is 3.9% less than in original crystals. Transformation to low-humidity form is accompanied by 1.7 times reduction of overall temperature factors, extension of diffraction resolution up to 1.75 A, without change or reduction of crystal mosaicity, and improvement in stability to X-ray radiation. The water-mediated transformation also appears to relieve the problem of nonisomorphism among the original MAP-Pfu crystals.

Two hevein homologs isolated from the seed of Pharbitis nil L. exhibit potent antifungal activity.

Two antifungal peptides (Pn-AMP1 and Pn-AMP2) have been purified to homogeneity from seeds of Pharbitis nil. The amino acid sequences of Pn-AMP1 (41 amino acid0 residues) and Pn-AMP2 (40 amino acid residues) were identical except that Pn-AMP1 has an additional serine residue at the carboxyl-terminus. The molecular masses of Pn-AMP1 and Pn-AMP2 were confirmed as 4299.7 and 4213.2 Da, respectively. Both the Pn-AMPs were highly basic (pI 12.02) and had characteristics of cysteine/glycine rich chitin-binding domain. Pn-AMPs exhibited potent antifungal activity against both chitin-containing and non-chitin-containing fungi in the cell wall. Concentrations required for 50% inhibition of fungal growth were ranged from 3 to 26 micrograms/ml for Pn-AMP1 and from 0.6 to 75 micrograms/ml for Pn-AMP2. The Pn-AMPs penetrated very rapidly into fungal hyphae and localized at septum and hyphal tips of fungi, which caused burst of hyphal tips. Burst of hyphae resulted in disruption of the fungal membrane and leakage of the cytoplasmic materials. To our knowledge, Pn-AMPs are the first hevein-like proteins that show similar fungicidal effects as thionins do.

Methionine aminopeptidase from the hyperthermophilic Archaeon Pyrococcus furiosus: molecular cloning and overexpression in Escherichia coli of the gene, and characteristics of the enzyme.

A gene for a methionine aminopeptidase (MAP; EC 3.4.11.18), which catalyzes the removal of amino-terminal methionine from the growing peptide chain on the ribosome, has been cloned from the hyperthermophilic Archaeon, Pyrococcus furiosus, by a novel method effectively using its cosmid protein library, sequenced and expressed in Escherichia coli. The DNA sequence encodes a protein containing 295 amino acid residues with methionine at the N-terminus. From protein analyses of the recombinant protein expressed in E. coli, by using both amino acid sequence analysis from the N-terminus by automated Edman degradation and analyses of molecular masses of the peptides generated by two enzymatic cleavages performed independently, digestions with lysylendopeptidase and Endoproteinase Asp-N, with ionspray mass spectrometry, the primary structure of the protein has been elucidated to be completely identical with that deduced from its DNA sequence. Comparison of the amino acid sequence of P. furiosus MAP (P.f. MAP) with those of other MAPs from Eukarya and Bacteria showed that the protein has a high degree of sequence homology in the stretches surrounding the five cobalt-binding residues fully preserved in all of MAPs determined so far, but P.f. MAP belongs to Type II because it has an extra long insertion of about 60 amino acid residues between the fourth and fifth cobalt-binding ligands, similar to MAPs from human and rat, and to Met-AP2 from Saccharomyces cerevisiae, in comparison to Type I MAPs from Bacteria. Therefore, P.f. MAP seems to be rather close to those from Eukarya, although it is distinct in lacking the N-terminal extension of about 90-150 residues universally found in MAPs from Eukarya. These findings suggest that P.f. MAP is evolutionally located at the Eukarya-Bacteria boundary. The enzyme expressed in E. coli exhibits a considerable thermostability, with a half-life of approximately 4.5 h at 90 degrees C and an optimum temperature of around 90 degrees C.

Paracatalytic inactivation of L-2-haloacid dehalogenase from Pseudomonas sp. YL by hydroxylamine. Evidence for the formation of an ester intermediate.

Asp10 of L-2-haloacid dehalogenase from Pseudomonas sp. YL was proposed to act as a nucleophile to attack the alpha-carbon of L-2-haloalkanoic acids to form an ester intermediate, which is hydrolyzed by nucleophilic attack of a water molecule on the carbonyl carbon (Liu, J.-Q, Kurihara, T., Miyagi, M., Esaki, N., and Soda, K. (1995) J. Biol. Chem. 270, 18309-18312). We have found that the enzyme is paracatalytically inactivated by hydroxylamine in the presence of the substrates monochloroacetate and L-2-chloropropionate. Ion spray mass spectrometry demonstrated that the molecular mass of the enzyme inactivated by hydroxylamine during the dechlorination of monochloroacetate is about 74 Da greater than that of the native enzyme. To determine the increase of the molecular mass more precisely, we digested the inactivated enzyme with lysyl endopeptidase and measured the molecular masses of the peptide fragments. The molecular mass of the hexapeptide Gly6-Lys11 was shown to increase by 73 Da. Tandem mass spectrometric analysis of this peptide revealed that the increase is due to a modification of Asp10. When the enzyme was paracatalytically inactivated by hydroxylamine during the dechlorination of L-2-chloropropionate, the molecular mass of the hexapeptide was 87 Da higher. Hydroxylamine is proposed to attack the carbonyl carbon of the ester intermediate and form a stable aspartate beta-hydroxamate carboxyalkyl ester residue in the inactivated enzyme.