A tridimensional representation of enzyme inhibition useful for diagnostic purposes

Partial Reversible Inhibition of Enzymes and Its Metabolic and Pharmaco-Toxicological Implications

Partial reversible inhibition of enzymes, also called hyperbolic inhibition, is an uncommon mechanism of reversible inhibition, resulting from a productive enzyme-inhibitor complex. This type of inhibition can involve competitive, mixed, non-competitive and uncompetitive inhibitors. While full reversible inhibitors show linear plots for reciprocal enzyme initial velocity versus inhibitor concentration, partial inhibitors produce hyperbolic plots. Similarly, dose-response curves show residual fractional activity of enzymes at high doses. This article reviews the theory and methods of analysis and discusses the significance of this type of reversible enzyme inhibition in metabolic processes, and its implications in pharmacology and toxicology.

Models of enzyme inhibition and apparent dissociation constants from kinetic analysis to study the differential inhibition of aldose reductase

In order to explain the negative slope of KappM/kappcat versus inhibitor concentration observed in the study of epigallocatechin gallate acting as an inhibitor of aldose reductase, a kinetic analysis was performed to rationalise the phenomenon. Classical and non-classical models of complete and incomplete enzyme inhibition were devised and analysed to obtain rate equations suitable for the interpretation of experimental data. The results obtained from the different approaches were discussed in terms of the meaning of the emerging kinetic constants. A decrease of KappM/kappcat versus the inhibitor concentration was revealed to be a valuable indication of the occurrence of an incomplete inhibition. This indication, which is univocal in the case of an uncompetitive inhibition, may be especially useful when the residual activity resulting from inhibition is rather low.

Alleviated Inhibition of Single Enzyme in Confined and Crowded Environment

Most proteins perform functions in intracellular milieu. The crowding, compartmentalized cytosol environment affects the protein structure, folding, conformational stability, substrate diffusion, and substrate-enzyme binding. Moreover, enzymes are available at single or very low copy numbers in a cell, and thus the conformation fluctuations of a single enzyme in a crowding environment could also greatly influence its kinetics. However, the crowding effect is poorly understood in the kinetical aspect of enzymatic reactions. In the present study, individual horseradish peroxidase (HRP) is encapsulated in a liposome containing crowding reagents as mimics of viscous cytosol. The confined crowding environment possesses a profound influence on both the catalytic activity and the product inhibition of enzymes. By analyzing the correlation between product generation and product inhibition, we find that the allosteric noncompetitive inhibition of HRP is alleviated in the crowded and confined milieu. Small-angle X-ray scattering experiments provide straightforward proofs of structural changes of enzymes in crowding environments, which are responsible for the reduced enzyme activity and increased enzyme-substrate affinity. We expect that this work may deepen the understanding of correlations between enzymatic conformations and activity performance in real cellular environments.

The many faces of partial inhibition: Revealing imposters with graphical analysis

Partial inhibition occurs when an enzyme-inhibitor complex is capable of producing a product, as opposed to complete inhibition where the enzyme-inhibitor complex is completely inactive and cannot produce a product. While not frequently encountered, partial inhibition is also not a rare phenomenon and can potentially have significant repercussions later on in a research project. It is therefore important to recognize partial inhibition when it occurs. However, it cannot be recognized from the standard velocity versus substrate concentration plots or even from other primary plots such as Lineweaver-Burk plots. Fortunately, partial inhibition is easily identified by using replots which can be generated from the data already gathered to produce the primary plots. Partial inhibition phenomena are also not usually addressed in introductory textbooks and are often given only brief mention in some articles or books dealing with enzyme kinetics. The same types of approaches can be used to study enzyme activation phenomena, which can be considered the flip-side of inhibition. This review presents a graphical approach to identifying partial inhibition in an attempt to provide a comprehensive treatment for those wishing for or needing additional information in carrying out steady-state kinetic inhibition analyses.

Simplified velocity equations for characterizing the partial inhibition or nonessential activation of bireactant enzymes

The steady state velocity equation for a bireactant enzyme in the presence of a partial inhibitor or nonessential activator, M, contains squared substrate concentration and higher-ordered M concentration terms. The equation is too complex to be useful in kinetic analyses. Simplification by the method of Cha (J. Biol. Chem. 243, 820 825 (1968)) eliminates squared substrate concentration terms, but retains higher-ordered terms in [M]. It is shown that if strict equilibrium is assumed between free E, M, and EM and for all but one other M-binding reaction, a velocity equation is obtained for an ordered bireactant enzyme that is first degree in all ligands in the absence of products. The equation is an approximation (because it was derived assuming only one M-binding reaction in the steady state), but it contains five inhibition (or activation) constants associated with M, all of which can be obtained by diagnostic replots and/or curve-fitting procedures. The equation also provides a framework for obtaining limiting constants (V'max, K'ia, K'mA, K'mB) that characterize the enzyme at saturating M. The same approach is applicable to an enzyme that catalyzes a steady state ping pong reaction.