Now showing 1 - 8 of 8
  • Publication
    Quantifying tetrahedral adduct formation and stabilization in the cysteine and the serine proteases
    Two new papain inhibitors have been synthesized where the terminal α-carboxyl groups of Z-Phe-Ala-COOH and Ac-Phe-Gly-COOH have been replaced by a proton to give Z-Phe-Ala-H and Ac-Phe-Gly-H. We show that for papain, replacing the terminal carboxylate group of a peptide inhibitor with a hydrogen atom decreases binding 3–4 fold while replacing an aldehyde or glyoxal group with a hydrogen atom decreases binding by 300,000–1,000,000 fold. Thiohemiacetal formation by papain with aldehyde or glyoxal inhibitors is shown to be ~ 10,000 times more effective than hemiacetal or hemiketal formation with chymotrypsin. It is shown using effective molarities, that for papain, thiohemiacetal stabilization is more effective with aldehyde inhibitors than with glyoxal inhibitors. The effective molarity obtained when papain is inhibited by an aldehyde inhibitor is similar to the effective molarity obtained when chymotrypsin is inhibited by glyoxal inhibitors showing that both enzymes can stabilize tetrahedral adducts by similar amounts. Therefore the greater potency of aldehyde and glyoxal inhibitors with papain is not due to greater thiohemiacetal stabilization by papain compared to the hemiketal and hemiacetal stabilization by chymotrypsin, instead it reflects the greater intrinsic reactivity of the catalytic thiol group of papain compared to the catalytic hydroxyl group of chymotrypsin. It is argued that while the hemiacetals and thiohemiacetals formed with the serine and cysteine proteases respectively can mimic the catalytic tetrahedral intermediate they are also analogues of the productive and non-productive acyl intermediates that can be formed with the cysteine and serine proteases.
      322Scopus© Citations 7
  • Publication
    A new Lysine derived glyoxal inhibitor of trypsin, its properties and utilization for studying the Stabilization of Tetrahedral adducts by Trypsin
    New trypsin inhibitors Z-Lys-COCHO and Z-Lys-H have been synthesised. Ki values for Z-Lys-COCHO, Z-Lys-COOH, Z-Lys-H and Z-Arg-COOH have been determined. The glyoxal group (–COCHO) of Z-Lys-COCHO increases binding ~300 fold compared to Z-Lys-H. The α-carboxylate of Z-Lys-COOH has no significant effect on inhibitor binding. Z-Arg-COOH is shown to bind ~2 times more tightly than Z-Lys-COOH. Both Z-Lys-13COCHO and Z-Lys-CO13CHO have been synthesized. Using Z-Lys-13COCHO we have observed a signal at 107.4 ppm by 13C NMR which is assigned to a terahedral adduct formed between the hydroxyl group of the catalytic serine residue and the 13C-enriched keto-carbon of the inhibitor glyoxal group. Z-Lys-CO13CHO has been used to show that in this tetrahedral adduct the glyoxal aldehyde carbon is not hydrated and has a chemical shift of 205.3 ppm. Hemiketal stabilization is similar for trypsin, chymotrypsin and subtilisin Carlsberg. For trypsin hemiketal formation is optimal at pH 7.2 but decreases at pHs 5.0 and 10.3. The effective molarity of the active site serine hydroxyl group of trypsin is shown to be 25300 M. At pH 10.3 the free glyoxal inhibitor rapidly (t1/2=0.15 h) forms a Schiff base while at pH 7 Schiff base formation is much slower (t1/2=23 h). Subsequently a free enol species is formed which breaks down to form an alcohol product. These reactions are prevented in the presence of trypsin and when the inhibitor is bound to trypsin it undergoes an internal Cannizzaro reaction via a C2 to C1 alkyl shift producing an α-hydroxycarboxylic acid.
  • Publication
    pH stability of the stromelysin-1 catalytic domain and its mechanism of interaction with a glyoxal inhibitor
    The Stromelysin-1 catalytic domain83-247 (SCD) is stable for at least 16 hours at pHs 6.0-8.4. At pHs 5.0 and 9.0 there is exponential irreversible denaturation with half lives of 38 and 68 min respectively. At pHs 4.5 and 10.0 irreversible denaturation is biphasic. At 25°C, C-terminal truncation of stromelysin-1 decreases the stability of the stromelysin-1 catalytic domain at pH values > 8.4 and < 6.0. We describe the conversion of the carboxylate group of (βR)-β-[[[(1S)-1-[[[(1S)-2-Methoxy-1-phenylethyl]amino]carbonyl]-2,2-dimethylpropyl]amino]carbonyl]-2-methyl-[1,1'-biphenyl]-4-hexanoic acid (UK-370106-COOH) a potent inhibitor of the metalloprotease stromelysin-1 to a glyoxal group (UK-370106-CO13CHO). At pH 5.5 - 6.5 the glyoxal inhibitor is a potent inhibitor of stromelysin-1 (Ki = ~1 μM). The aldehyde carbon of the glyoxal inhibitor was enriched with carbon-13 and using Carbon-13 NMR we show that the glyoxal aldehyde carbon is fully hydrated when it is in aqueous solutions (90.4 ppm) and also when it is bound to SCD (~92.0 ppm). We conclude that the hemiacetal hydroxyl groups of the glyoxal inhibitor are not ionised when the glyoxal inhibitor is bound to SCD. The free enzyme pKa values associated with inhibitor binding were 5.9 and 6.2. The formation and breakdown of the signal at ~92 ppm due to the bound UK-370106-CO13CHO inhibitor depends on pKa values of 5.8 and 7.8 respectively. No strong hydrogen bonds are present in free SCD or in SCD-inhibitor complexes. We conclude that the inhibitor glyoxal group is not directly coordinated to the catalytic zinc atom of SCD.
      429Scopus© Citations 2
  • Publication
    Hemiacetal stabilization in a chymotrypsin inhibitor complex and the reactivity of the hydroxyl group of the catalytic serine residue of chymotrypsin
    The aldehyde inhibitor Z-Ala-Ala-Phe-CHO has been synthesized and shown by 13C-NMR to react with the active site serine hydroxyl group of alpha-chymotrypsin to form two diastereomeric hemiacetals. For both hemiacetals oxyanion formation occurs with a pKa value of ~ 7 showing that chymotrypsin reduces the oxyanion pKa values by ~ 5.6 pKa units and stabilizes the oxyanions of both diastereoisomers by ~ 32 kJ mol− 1. As pH has only a small effect on binding we conclude that oxyanion formation does not have a significant effect on binding the aldehyde inhibitor. By comparing the binding of Z-Ala-Ala-Phe-CHO with that of Z-Ala-Ala-Phe-H we estimate that the aldehyde group increases binding ~ 100 fold. At pH 7.2 the effective molarity of the active site serine hydroxy group is ~ 6000 which is ~ 7 × less effective than with the corresponding glyoxal inhibitor. Using 1H-NMR we have shown that at both 4 and 25 °C the histidine pKa is ~ 7.3 in free chymotrypsin and it is raised to ~ 8 when Z-Ala-Ala-Phe-CHO is bound. We conclude that oxyanion formation only has a minor role in raising the histidine pKa and that the aldehyde hydrogen must be replaced by a larger group to raise the histidine pKa > 10 and give stereospecific formation of tetrahedral intermediates. The results show that a large increase in the pKa of the active site histidine is not needed for the active site serine hydroxyl group to have an effective molarity of 6000.
      424Scopus© Citations 9
  • Publication
    Mechanism of the binding of Z-L-tryptophan and Z-L-phenylalanine to thermolysin and stromelysin-1 in aqueous solutions
    The chemical shift of the carboxylate carbon of Z-tryptophan is increased from 179.85 to 182.82 ppm and 182.87 on binding to thermolysin and stromelysin-1 respectively. The chemical shift of Z-phenylalanine is also increased from 179.5 ppm to 182.9 ppm on binding to thermolysin. From pH studies we conclude that the pKa of the inhibitor carboxylate group is lowered by at least 1.5 pKa units when it binds to either enzyme. The signal at ~183 ppm is no longer observed when the active site zinc atom of thermolysin or stromelysin-1 is replaced by cobalt. We estimate that the distance of carboxylate carbon of Z-[1-13C]-L-tryptophan is ≤ 3.71 Å from the active site cobalt atom of thermolysin. We conclude that the side chain of Z-[1-13C]-L-tryptophan is not bound in the S2' subsite of thermolysin. As the chemical shifts of the carboxylate carbons of the bound inhibitors are all ~183 ppm we conclude that they are all bound in a similar way most probably with the inhibitor carboxylate group directly coordinated to the active site zinc atom. Our spectrophotometric results confirm that the active site zinc atom is tetrahedrally coordinated when the inhibitors Z-tryptophan or Z-phenylalanine are bound to thermolysin.
      403Scopus© Citations 1
  • Publication
    A 13C-NMR study of azacryptand complexes
    An azacryptand has been solubilised in aqueous media containing 50% (v/v) dimethyl sulphoxide. 13C-NMR has been used to determine how the azacryptand is affected by zinc binding at pH 10. Using 13C-NMR and 13C-enriched bicarbonate we have been able to observe the formation of 4 different carbamate derivatives of the azacryptand at pH 10. The azacryptand was shown to solubilise zinc or cadmium at alkaline pHs. Two moles of zinc are bound per mole of azacryptand and this complex binds 1 mole of carbonate. By replacing the zinc with cadmium-113 we have shown that the 13C-NMR signal of the 13C-enriched carbon of the bound carbonate is split into two triplets at 2.2 °C. This shows that two cadmium complexes are formed and in each of these complexes the carbonate group is bound by two magnetically equivalent metal ions. It also demonstrates that these cadmium complexes are not in fast exchange. From temperature studies we show that in the zinc complexes both complexes are in fast exchange with each other but are in slow exchange with free bicarbonate. HOESY is used to determine the position of the carbonate carbon in the complex. The solution and crystal structures of the zinc–carbonate–azacryptand complexes are compared.
      414Scopus© Citations 4
  • Publication
    The importance of tetrahedral intermediate formation in the catalytic mechanism of the serine proteases chymotrypsin and subtilisin
    Two new inhibitors have synthesized where the terminal α-carboxyl groups of Z-Ala-Ala-Phe-COOH and Z-Ala-Pro-Phe-COOH have been replaced by a proton to give Z-Ala-Ala-Phe-H and Z-Ala-Pro-Phe-H respectively. Using these inhibitors we estimate that for α-chymotrypsin and subtilisin Carlsberg the terminal carboxylate group decreases inhibitor binding 3-4 fold while a glyoxal group increases binding by 500-2000 fold. We show that at pH 7.2 the effective molarity of the catalytic hydroxyl group of the active site serine is 41,000-229,000 and 101,000 to159,000 for α-chymotrypsin and subtilisin Carlsberg respectively. It is estimated that oxyanion stabilisation and the increased effective molarity of the catalytic serine hydroxyl group can account for the catalytic efficiency of the reaction. We argue that substrate binding induces the formation of a strong hydrogen bond or low barrier hydrogen bond between histidine-57 and aspartate-102 that increases the pKa of the active site histidine enabling it to be an effective general base catalyst for the formation of the tetrahedral intermediate and increasing the effective molarity of the catalytic hydroxyl group of serine-195. A catalytic mechanism for acyl intermediate formation in the serine proteases is proposed.
      406Scopus© Citations 15
  • Publication
    Oxyanion and tetrahedral intermediate stabilization by subtilisin : detection of a new tetrahedral adduct
    The peptide-derived glyoxal inhibitor Z-Ala-Ala-Phe-glyoxal has been shown to be ~10 fold more effective as an inhibitor of subtilisin than Z-Ala-Pro-Phe-glyoxal. Signals at 107.2 p.p.m. and 200.5 p.p.m. are observed for the glyoxal keto and aldehyde carbons of the inhibitor bound to subtilisin, showing that the glyoxal keto and aldehyde carbons are sp3 and sp2 hybridized respectively. The signal at 107.2 p.p.m. from the carbon atom attached to the hemiketal oxyanion is formed in a slow exchange process that involves the dehydration of the glyoxal aldehyde carbon. Two additional signals are observed one at 108.2 p.p.m. and the other at 90.9 p.p.m. for the glyoxal keto and aldehyde carbons respectively at pHs 6-8 demonstrating that subtilisin forms an additional tetrahedral adduct with Z-Ala-Ala-Phe-glyoxal in which both the glyoxal keto and aldehyde carbons are sp3 hybridised. For the first time we can quantify oxyanion stabilisation in subtilisin. We conclude that oxyanion stabilisation is more effective in subtilisin than in chymotrypsin. Using 1H-NMR we show that the binding of Z-Ala-Ala-Phe-glyoxal to subtilisin raises the pKa of the imidazolium ion of the active site histidine residue promoting oxyanion stabilisation. The mechanistic significance of these results are discussed.
      270Scopus© Citations 7