Kinetic analysis for suicide-substrate inactivation of microperoxidase-11: A modified model for bisubstrate enzymes in the presence of reversible inhibitors

An outlook on suicide enzyme inhibition and drug design

There have been recent renewed interests in the importance of suicide enzyme inhibition. The principal objective of this review is to investigate all types of suicide inhibitions for natural enzymes, artificial biocatalysts as well as therapeutic potential of enzyme suicide inhibition. It is discussed the suicide inhibition beneficial in drug design and treatments and non-beneficial achievements for some industrial enzymes such as HRP peroxidase enzyme. The design of biomimetic artificial enzymes explained to prevent inhibition by protecting the active site via environmental conditions. Suicide enzyme inhibition development can be the key mechanism against sever diseases such as SARS. In this report, suicide enzyme inactivation classes are classified based on target enzyme groups via their substrates.

Experimental and Computational Insight into the Mechanism of NO Binding to Ferric Microperoxidase. The Likely Role of Tautomerization to Account for the pH Dependence

According to the current paradigm, the metal-hydroxo bond in a six-coordinate porphyrin complex is believed to be significantly less reactive in ligand substitution than the analogous metal-aqua bond, due to a much higher strength of the former bond. Here, we report kinetic studies for nitric oxide (NO) binding to a heme-protein model, acetylated microperoxidase-11 (AcMP-11), that challenge this paradigm. In the studied pH range 7.4-12.6, ferric AcMP-11 exists in three acid-base forms, assigned in the literature as [(AcMP-11)FeIII(H2O)(HisH)] (1), [(AcMP-11)FeIII(OH)(HisH)] (2), and [(AcMP-11)FeIII(OH)(His-)] (3). From the pH dependence of the second-order rate constant for NO binding (kon), we determined individual rate constants characterizing forms 1-3, revealing only a ca. 10-fold decrease in the NO binding rate on going from 1 (kon(1) = 3.8 × 106 M-1 s-1) to 2 (kon(2) = 4.0 × 105 M-1 s-1) and the inertness of 3. These findings lead to the abandonment of the dissociatively activated mechanism, in which the reaction rate can be directly correlated with the Fe-OH bond energy, as the mechanistic explanation for the process with regard to 2. The reactivity of 2 is accounted for through the existence of a tautomeric equilibrium between the major [(AcMP-11)FeIII(OH)(HisH)] (2a) and minor [(AcMP-11)FeIII(H2O)(His-)] (2b) species, of which the second one is assigned as the NO binding target due to its labile Fe-OH2 bond. The proposed mechanism is further substantiated by quantum-chemical calculations, which confirmed both the significant labilization of the Fe-OH2 bond in the [(AcMP-11)FeIII(H2O)(His-)] tautomer and the feasibility of the tautomer formation, especially after introducing empirical corrections to the computed relative acidities of the H2O and HisH ligands based on the experimental pKa values. It is shown that the "effective lability" of the axial ligand (OH-/H2O) in 2 may be comparable to the lability of the H2O ligand in 1.

4-Aminobenzoic acid hydrazide inhibition of microperoxidase-11: catalytic inhibition by reactive metabolites

Inhibition of human peroxidase enzymes such as myeloperoxidase or eosinophil peroxidase represents a novel therapeutic area, for which there are no current clinical therapeutics. We utilized 4-aminobenzoic acid hydrazide which was reported to be a potent irreversible inhibitor of myeloperoxidase to gain insight into the role of reactive metabolites in catalytic inhibition. In order to carry out detailed studies, we used a model peroxidase, microperoxidase-11 (MP-11). We investigated the heme spectrum of MP-11 in the presence of 4-ABAH and found that heme bleaching occurred that was irreversible. This coincided with an absence of catalytic activity. The spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was able to significantly prevent inactivation of peroxidase activity, therefore, we performed ESR spin trapping studies and detected a carbonyl carbon-centered radical of 4-ABAH. In order to determine if the free radical metabolites became bound to MP-11, we performed high-resolution MALDI with elemental analysis to determine the change in elemental composition that occurred in these reactions. These masses were assigned to free radical metabolites of 4-ABAH and were not observed in reactions containing DMPO. We conclude that the 4-ABAH free radical metabolites which were bound to MP-11 were involved in the catalytic inhibition and were scavenged by DMPO.