Sulbactam forms only minimal amounts of irreversible acrylate-enzyme with SHV-1 beta-lactamase

Abstract

Sulbactam is a mechanism-based inhibitor of beta-lactamase enzymes used in clinical practice. It undergoes a complex series of chemical reactions in the active site that have been studied extensively in the past three decades. However, the actual species that gives rise to inhibition in a clinical setting has not been established. Recent studies by our group, using Raman microscopy and X-ray crystallography, have found that large quantities of enamine-based acyl-enzyme species are present within minutes in single crystals of SHV-1 beta-lactamases which can lead to significant inhibition. The enamines are formed by breakdown of the cyclic beta-lactam structures with further transformations leading to imine formation and subsequent isomerization to cis and/or trans enamines. Another favored form of inhibition arises from attack on the imine by a second nucleophilic amino acid side chain, e.g., from serine 130, to form a cross-linked species in the active site that can degrade to an acrylate-like species irreversibly bound to the enzyme. Thus, the imine is at a branch point on the reaction pathway. Using sulbactam and 6,6-dideuterated sulbactam we follow these alternate paths in WT and E166A SHV-1 beta-lactamase by means of Raman microscopic studies on single enzyme crystals. For the unlabeled sulbactam, the Raman data show the presence of an acrylate-like species, probably 3-serine acrylate, several hours after the reaction is started in the crystal. However, for the 6,6-dideutero analogue the acrylate signature appears on the time scale of minutes. The Raman signatures, principally an intense feature near 1530 cm-1, are assigned based on quantum mechanical calculations on model compounds that mimic acrylate species in the active site. The different time scales observed for acrylate-like product formation are ascribed to different rates of reaction involving the imine intermediate. It is proposed that for the unsubstituted sulbactam the conversion from imine to enamine, which involves breaking a C-H bond, is aided by quantum mechanical tunneling. For the 6,6-dideutero-sulbactam the same step involves breaking a C-D bond, which has little or no assistance from tunneling. Consequently the conversion to enamines is slower, and a higher population of imine results, presenting the opportunity for the competing reaction with the second nucleophile, serine 130 being the prime candidate. The hydrolysis of the resulting cross-linked intermediate leads to the observed rapid buildup of the acrylate product in the Raman spectra from the dideutero analogue. The protocol used here, essentially running the reactions with the two forms of sulbactam in parallel, provides an element of control and enables us to conclude that, for the unsubstituted sulbactam, the formation of the cross-linked intermediate and the final irreversible acrylate product is not a significant route to inhibition of SHV-1.

Figure 1

Class A β-lactamase inhibitors

Figure 2

Raman difference spectra of E166A SHV-1 β-lactamase reacting with sulbactam 5mM at different time points. The peak near 1045 cm⁻¹ is due to HEPES buffer.

Figure 3

Time dependence of the 1530 cm⁻¹ feature height given by sulbactam reacting inside an E166A SHV-1 β-lactamase crystal. The estimated standard deviation in the intensity values is ± 12%.

Figure 4

Calculated Raman spectrum of 3-methoxyacrylate, obtained by using the Gaussian 03 software, where the main features are; 1629 cm⁻¹ C=O and C=C stretch, 1536 cm⁻¹ C=C stretch and delocalized motions over the entire chain, 1330 cm⁻¹ C=C stretch and C-H bending, 1240 cm⁻¹ C=C and C=O stretch, C-H bending

Figure 5

Raman difference spectra of E166A SHV-1 β-lactamase reacting with 6,6 dideuterated sulbactam 10 mM at different time points

Figure 6

Time dependence of the 1530 cm⁻¹ feature height given by 6, 6 dideuterated sulbactam reacting inside an E166A SHV-1 β-lactamase crystal. The estimated standard deviation in the intensity values is ± 12%.

Figure 7

Raman difference spectra of wild type SHV-1 β-lactamase reacting with sulbactam (red) at 75 minutes or 6,6 dideuterated sulbactam (green) at 80 minutes

Figure 8

Time dependence of the 1502 cm⁻¹ feature height given by 6, 6 dideuterated sulbactam reacting inside a WT SHV-1 β-lactamase crystal. The estimated standard deviation in the intensity values is ± 12%.

Scheme 1

Simplified reaction mechanism for Class A β-lactamase with sulbactam