Types of Inhibitor

Enzyme inhibitors can be classified into four categories  

Integrated Michaelis-Menten Equation for Substrate and Product Inhibition 

In this section, we will discover that inhibition, which tends to be a nuisance at low conversion in initial-rate kinetics studies, can become very detrimental if high degrees of conversions are sought, such as in any situation in synthesis or large-scale processing. 

To investigate the impact of inhibition on reactor design, the procedure again is as follows (i) we have to develop the pertinent kinetic rate equation, (ii) insert this rate law into the equation for the reactor we intend to operate, and (iii) integrate over all applicable degrees of conversion. As the two most frequent cases of inhibition are the occurrence of substrate and (even more often) product inhibition, we will treat those two cases in the following section. We will just mention the equation for substrate inhibition, but we will develop step-by-step the equation for product inhibition. 

Suicide or irreversible inhibitors of GST Pl-1 include agents that bind covalently to glutathione, thereby forming thioether adducts that are stabilized at the active site of the enzyme. These agents include activated aromatic systems (2, 3), epoxides (4, 5), esters (6), and Michael acceptors such as ethacrynic acid (7), cycloalkenones (8, 9), and haloenol lactones (10-13), among others [3,48,54-57], 

Several Michael acceptors have been studied as irreversible GST Pl-1 inhibitors. A recent SAR study of analogs of 7 in HL-60 cells revealed that substitution at the Rj position is essential for GST Pl-1 inhibition (7,19, 20 89-94% inhibition at 40pmol/L), while substitution at the R2 position did not contribute significantly towards activity (21-23 19% inhibition at 40pmol/L) [59]. 

The DNA minor groove binder brostallicin (PNU-166196, 24), currently in Phase 2 clinical trials for the treatment of soft-tissue sarcoma, exhibited potent 

Competitive inhibitors of GST Pl-1 fall under two categories non-glutathione-and glutathione-based compounds. The former group covers a broad range of chemical structures such as tricyclic-based dibenzazepines, polyphenolic natural products, alkaloids, pyrimethamine, and dyes. The latter group, as its name indicates, covers compounds whose main structure or backbone is that of GSH. 

Tricyclic antidepressants such as imipramine (25), clomipramine (26), amitriptyline (27), and doxepine (28) were found to be weak inhibitors of GST Pl-1 in vitro. Inhibition of GST Pl-1 was enhanced with the introduction of a chloro group on the dibenzazepine ring (25 40% inhibition at 15 mM 26 70% inhibition at 10 mM). The same result was observed with the substitution of an oxygen for a carbon in the heptadiene ring (27 18% inhibition at 10 mM 28 48% inhibition at 15 mM) [35], 

---

Inhibitors act and are classified in a variety of ways (1,3,37,38). The classifications used herein closely foUow the discussion in Reference 37. Types of inhibitors include (/) anodic, (2) cathodic, (3) organic, (4) precipitation, and (5) vapor-phase inhibitors. 

The three most common types of inhibitors in enzymatic reactions are competitive, non-competitive, and uncompetitive. Competitive inliibition occurs when tlie substrate and inhibitor have similar molecules that compete for the identical site on the enzyme. Non-competitive inhibition results in enzymes containing at least two different types of sites. The inhibitor attaches to only one type of site and the substrate only to the other. Uncompetitive inhibition occurs when the inhibitor deactivates the enzyme substrate complex. The effect of an inhibitor is determined by measuring the enzyme velocity at various... 

The Effect of Various Types of Inhibitors on the Michaelis-Menten Rate Equation and on Apparent K, and Apparent F ,ax ... 

The extent of inhibition afforded to metals other than mild steel depends on the metal and the inhibitor see The Nature of the Metal, and Dissimilar Metals in Contact). The cathodic type of inhibitor is perhaps less susceptible than the anodic type to the nature of the metal. However, cathodic inhibitors are usually less efficient (although performing quite satisfactorily in many systems) in terms of reduction in corrosion rate, than are anodic inhibitors. The latter, when used in adequate concentrations, can often achieve 100% protection. 

Temperature of the system When inhibitors are used in the 0-100°C range it is usually found that higher concentrations become necessary at the higher temperatures Other inhibitors can lose their effectiveness altogether as the temperature is raised. A prime example of this is the polyphosphate type of inhibitor. This is effective in circulating systems at temperatures below about 40°C, but at higher temperatures reversion to orthophosphate can occur and this species is ineffective at the concentrations at which it will then be present. If calcium ions are present, additional loss of inhibitor will occur due to calcium phosphate precipitation. 

The principles and practice of corrosion inhibition have been described in terms of the factors affecting inhibitor performance and selection (principles) and the more important practical situations in which inhibitors are used (practice). For the latter a brief account is given of the nature of the system, the reasons for inhibitor application and the types of inhibitor in use. 

It is important to recognize that different types of inhibitors often function by different mechanisms, and that a given antioxidant may react in more than one way. Thus, a material that acts as an antioxidant under one set of conditions may become a pro-oxidant in another simation. The search for possible synergistic combinations of antioxidants can be conducted more logically and efficiently if we seek to combine the effects of different modes of action. Five general modes of oxidation inhibition are commonly recognized ... 

The importance of a basic nitrogen atom for strong inhibition by the indolizine type of inhibitor is demonstrated by the finding that I for inhibition by castanospermine A -oxide of the ) -D-glucosidase from almonds is 500-fold larger than of castanospermine itself (760 fiM V5. 1.5 /lA/at pH... 

The effect of various types of inhibitors with respect to structure and solubility on the formation of N-Nitrosodiethanolamine was studied in a prototype oil in water anionic emulsion, Nitrosation resulted from the action of nitrite on diethanolamine at pH 5.2-5.A, Among the water soluble inhibitors incorporated into the aqueous phase, sodium bisulfite and ascorbic acid were effective. Potassium sorbate was much less so. The oil soluble inhibitors were incorporated into the oil phase of the emulsion. 

Another interesting target for this type of inhibitors is the dipeptidyl peptidase IV (DPP IV). This exodipeptidase, which can cleave peptides behind a proline residue is important in type 2 diabetes as it truncates the glucagon-like peptide 1. Taking into account the P2-Pi( Pro)-P,1 cleavage and the requirement for a free terminal amine, the synthesis of a suicide inhibitor was planned. It looked as if the the e-amino group of a P2 lysine residue could be cyclized because of the relative little importance of the nature of the P2 residue on the rate of enzymatic hydrolysis of known synthetic substrates. Therefore, anew series of cyclopeptides 11 was synthesized (Fig. 11.8). 

The process of inhibition becomes somewhat more complex if no restriction is made with respect to the concentration of modulators. As seen in Fig. 20.13, compounds with a low affinity to the transporter (EUh < 2) are able to activate if applied at low concentrations. However, if these compounds are applied at high concentration, ATP hydrolysis slows down (low V2 values) and they act as inhibitors (cf. Fig. 20.9). Compounds with EUh < 2 seem not to be transported (see Fig. 20.11). This may lead to an obstruction of the transport route and thus to a slow down of the activation cycle. Examples of this type of inhibitor include pro-... 

In summary, results of in vivo experiments showed the effectiveness of carbosilane dendrimers having clustered Pk carbohydrate moieties, and the complete neutralization potency against STL-II was discovered when dumbbell-shaped dendrimers were identified as potent candidate inibitors. Although the precise mechanism of action remains to be elucidated, this type of inhibitor provided a new strategy for the detoxification of SLTs present in circulation. 

Plotting 1/V versus 1/[S], one obtains a straight line having a slope of Km/Vmax with a y-axis intercept of l/VmAX and an x-intercept of - 1/Km as shown in Figure 2.13. Lineweaver-Burk plots of enzyme activity in the presence of an inhibitor can distinguish the type of inhibitor. Competitive inhibitors have a molecular structure similar to that of the substrate and will alter Km but not VnrAX because they compete with the substrate for binding at the enzyme s active site but do not change the enzyme s affinity for substrate. Noncompetitive inhibitors bear no structural similarity to the substrate but bind the free enzyme or enzyme-substrate... 

Suicide substrates and quiescent affinity labels, unlike the other types of inhibitors discussed in this chapter, form covalent bonds with active site nucleophiles and thereby irreversibly inactivate their target enzymes. A suicide substrate,191 also described by Silverman in a comprehensive review1101 as a mechanism-based inactivator, is a molecule that resembles its target enzyme s true substrate but contains a latent (relatively unreactive) electrophile. When the target enzyme attempts to turn over the... 

Another type of inhibitor combines with the enzyme at a site which is often different from the substrate-binding site and as a result will inhibit the formation of the product by the breakdown of the normal enzyme-substrate complex. Such non-competitive inhibition is not reversed by the addition of excess substrate and generally the inhibitor shows no structural similarity to the substrate. Kinetic studies reveal a reduced value for the maximum activity of the enzyme but an unaltered value for the Michaelis constant (Figure 8.7). There are many examples of non-competitive inhibitors, many of which are regarded as poisons because of the crucial role of the inhibited enzyme. Cyanide ions, for instance, inhibit any enzyme in which either an iron or copper ion is part of the active site or prosthetic group, e.g. cytochrome c oxidase (EC 1.9.3.1). 

Use the data in the table above to plot Michaelis-Menten, Lineweaver-Burke and Eadie-Hofstee graphs to determine Km and Vm DC values. State the type of inhibitor which is present. Calculate the K based on Equations 2.10 (for competitive inhibition) or 2.11 (non-competitive inhibition) asappropriate assuming the [I] = 10mmol/l. 

There is ongoing research on another type of inhibitor, which targets the glycoprotein gpl20 on the HIV. 

There are several such toxic agents that cause considerable medical, public and political concern. Two examples are discussed here the heavy metal ions (e.g. lead, mercury, copper, cadmium) and the fluorophosphonates. Heavy metal ions readily form complexes with organic compounds which are lipid soluble so that they readily enter cells, where the ions bind to amino acid groups in the active site of enzymes. These two types of inhibitors are discussed in Boxes 3.5 and 3.6. There is also concern that some chemicals in the environment, (e.g. those found in industrial effluents, rubbish tips and agricultural sprays), although present at very low levels, can react with enhanced reactivity groups in enzymes. Consequently, only minute amounts concentrations are effective inhibitors and therefore can be toxic. It is suggested that they are responsible for some non-specific or even specific diseases (e.g. breast tumours). 

Irreversible inhibition in an organism usually results in a toxic effect. Examples of this type of inhibitor are the organophosphorus compounds that interfere with acetylcholinesterase (see Box 7.26). The organophosphorus derivative reacts with the enzyme in the normal way, but the phosphory-lated intermediate produced is resistant to normal hydrolysis and is not released from the enzyme. 

---