The activity of the dinuclear cobalt-beta-lactamase from Bacillus cereus in catalysing the hydrolysis of beta-lactams

Abstract

Metallo-beta-lactamases are native zinc enzymes that catalyse the hydrolysis of beta-lactam antibiotics, but are also able to function with cobalt(II) and require one or two metal-ions for catalytic activity. The hydrolysis of cefoxitin, cephaloridine and benzylpenicillin catalysed by CoBcII (cobalt-substituted beta-lactamase from Bacillus cereus) has been studied at different pHs and metal-ion concentrations. An enzyme group of pK(a) 6.52+/-0.1 is found to be required in its deprotonated form for metal-ion binding and catalysis. The species that results from the loss of one cobalt ion from the enzyme has no significant catalytic activity and is thought to be the mononuclear CoBcII. It appears that dinuclear CoBcII is the active form of the enzyme necessary for turnover, while the mononuclear CoBcII is only involved in substrate binding. The cobalt-substituted enzyme is a more efficient catalyst than the native enzyme for the hydrolysis of some beta-lactam antibiotics suggesting that the role of the metal-ion is predominantly to provide the nucleophilic hydroxide, rather than to act as a Lewis acid to polarize the carbonyl group and stabilize the oxyanion tetrahedral intermediate.

Scheme 1

Figure 1. Schematic representation of the two zinc-binding sites in the MBL from B. cereus, BcII

Scheme 2

Figure 2. pH-rate profile for CoBcII-catalysed hydrolysis of cefoxitin

Plot of log k_cat against pH for CoBcII-catalysed hydrolysis of cefoxitin in 0.025 M buffer, I=0.25 M, in the presence of 10⁻⁴ M cobalt ions (×) and 10⁻³ M cobalt ions (■); the solid lines are the calculated values using eqn (1) and the parameters in Table 1 (Scheme 4).

Figure 3. pH-K_m profile for CoBcII-catalysed hydrolysis of cefoxitin

Plot of pK_m against pH for CoBcII-catalysed hydrolysis of cefoxitin in 0.025 M buffer, I=0.25 M, in the presence of 10⁻⁴ M cobalt ions (×) and 10⁻³ M cobalt ions (■); the solid lines are the calculated values using eqn (2) and the parameters in Table 1 (Scheme 4).

Figure 4. pH-rate profile for CoBcII-catalysed hydrolysis of cephaloridine

Plot of log k_cat against pH for CoBcII-catalysed hydrolysis of cephaloridine in 0.025 M buffer, I=0.25 M, in presence of 10⁻⁴ M cobalt ions (×) and 10⁻³ M cobalt ions (■); the solid lines are the calculated values using eqn (1) and the parameters in Table 1 (Scheme 4).

Figure 5. pH-rate profile for CoBcII-catalysed hydrolysis of benzylpenicillin

Plot of log k_cat against pH for CoBcII-catalysed hydrolysis of benzylpenicillin in 0.025 M buffer, I=0.25 M, in the presence of 10⁻⁴ M cobalt ions (×) and 10⁻³ M cobalt ions (■); the solid lines are the calculated values using eqn (1) and the parameters in Table 1 (Scheme 4).

Scheme 3

Scheme 4