Now showing 1 - 4 of 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new inhibitors have synthesized where the terminal α-carboxyl groups of Z-Ala-Ala-Phe-COOH and Z-Ala-Pro-Phe-COOH have been replaced by a proton to give Z-Ala-Ala-Phe-H and Z-Ala-Pro-Phe-H respectively.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.
      514Scopus© Citations 15
  • 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.
      518Scopus© Citations 2
  • 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.
      373Scopus© Citations 7
  • 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.
      521Scopus© Citations 2