Enzyme inhibition/inhibitors transition-state analog

Figure 8.24. Inhibition by Transition State Analogs. (A) The isomerization of 1-proline to d-proline by proline racemase, a bacterial enzyme, proceeds through a planar transition state in which the a carbon is trigonal rather than tetrahedral. (B) Pyrrole 2-carboxylate, a transition state analog because of its trigonal geometry, is a potent inhibitor of proline racemase.

Interestingly, although many transition state analogs bind noncovalently to the target enzyme s active site via a one-step kinetic mechanism (Scheme la) and would therefore be expected to exhibit no time-dependent properties of inhibition, inhibitors with Kj values of < 10 10 M (like coformy-cin) usually have a slow onset of inhibition kobserved < 10 2 s 1 (i.e., an approach to equilibrium inhibition of > 1 min).161 This is merely an assay artifact due to... 

Some boronic acid-based enzyme inhibitors undergo strong yet reversible covalent attachment to a nucleophile at the enzyme s active site, while others simply act as competitive inhibitors in their borate conjugate base form. Boronic acid-based inhibition of thrombin has been achieved <93MI109>, and that of P-lactamases has been particularly effective <95TL8399, 96M1688>. When compared to other covalent transition-state analog inhibitors of P-lactamases like phos-... 

Extrapolating from well-characterized enzymatic inhibition in test tubes, numerous mechanistic ideas concerning the in vivo effects of vanadium compounds have been advanced. The effects of vanadium compounds as transition-state analogs of certain enzymes with a phosphoprotein intermediate in their reaction scheme is proposed to account for the action of vanadium [11] in many biological systems. Unfortunately, it is often difficult to determine if the inhibition observed in the test tube occurs in vivo. For example, although vanadate is a potent inhibitor of plasma membrane ion pumps (such as the sodium potassium ATPase) in the test tube, it is difficult to determine if these pumps are actually inhibited in animals exposed to vanadium compounds. Currently, the role of vanadium compounds as protein phosphatase (PTP) inhibitors is believed to be related to the metabolic effects of this... 

Several 0-GlcNAcase inhibitors have been described, the most popular are streptozotocin (STZ) and PUGNAc. Both of these compounds are substrate mimetics that inhibit 0-GlcNAcase activity by resembling the natural substrate s ox-azoline transition state after its entrance into the active site (50, 51). Unfortunately, the above inhibitors are nonselective and can inhibit other glycosyl hydrolases therefore, they are a detriment to a multitude of cell pathways. Another, more potent transition state analog, A-acetylglucosamine-thiazoline (NAG-thiazoline), has been described recently (5). The increased potency likely is because the compound already resembles the oxazoline intermediate before it is exposed to the enzyme. Macauley et al. 

Multiple efforts have been made to replace phosphorous-containing linkages with sulfur-containing isosteres in the context of enzyme inhibition. In a search for nonionic transition state analog inhibitors of restriction enzymes, Blattler et al. (34) found that nucleic acid duplexes that incorporate a dimethyl sulfone in place of a phosphodiester have distorted backbones similar to those in restriction enzyme bound DNA. Chimeric DNAs that incorporate sulfone hnkages were synthesized, and depending on the location of the dimethylene sulfone hnker, either between the first AT unit or the second AT unit in the EcoRV recognition site, values were 20 nM and 120... 

We turn now to compounds that provide the most intimate views of the catalytic process itself Linus Pauling proposed in 1948 that compounds resembling the transition state of a catalyzed reaction should be very effective inhibitors of enzymes. These mimics are called transition-state analogs. The inhibition of proline racemase is an instructive example. The racemization of proline proceeds through a transition state in which the tetrahedral a- carbon atom has become trigonal by loss of a proton (Figure 8.24). In the trigonal form, all three bonds are in the same plane C also carries a... 

Since disturbed acid phosphatase activity has been associated with pathological conditions, the research has focused on the development of diagnostic methods for detection of activity as a marker for the onset of the disease, and in some extent to the development of inhibitors rather than activators to treat those conditions in which the increase in enzyme activity has a direct effect on the evolution of the disease. In particular, only the development of bisphospho-nate derivatives as inhibitors for tartrate-resistant acid phosphatase found their way to the market for treatment of osteoporosis [41], Typical inhibition of phosphatase activity includes anionic species such as L-(+)-tartrate, phosphate, vanadate, molybdate, and fluoride and neutral molecules such as formaldehyde. Vanadate ion,, is a competitive unspecific inhibitor for acid phosphatases by forming transition state analogs. Other oxoanions such as molybdate and tungstate also show inhibitory effects on... 

The kinetic parameters of arylsulfohydrolase A with p-nitrocatechol sulfate are shown in Table 2. Optimal activity is at pH 5.0 with a value of 0.4 mM. This enzyme is strongly inhibited by sulfate, sulfite, and phosphate ions with Kj vines of 0.052, 0.026, and 0.034 mM, respectively (Table 3). Thus sulfite is the most potent inhibitor, followed by phosphate and sulfate. Such a powerful inhibition by sulfite suggests that this ion may be a transition-state analog intermediate of the arylsulfohydrolase A catalyzed reaction. This enzyme is also strongly inhibited by ascorbic acid 2-phosphate (Carlson et ah, 1976). The inhibition by ascorbic acid 2-phosphate is competitive with a Kj value of 0.3 xM. This metabolite binds very tightly to the enzyme molecule and may serve as controlling metabolite for arylsulfohydrolases. 

A report describes a member of a new class of very effective non-[3-lactam (3-lactamase inhibitors. This inhibitor is a monoester of m-hydroxybenzoic acid and phenylac-etamidomethylphosphonic acid. It appears to be specific against Class C enzymes (e.g., from E. colaceae). The compound showed little inhibition of [3-lactamases of Class A and B categories. This phosphonate ester appears to act as a transition-state analog inhibitor (see PALA, Chapter 2). Incubation with penicillin G in the presence of this potential drug reduced enzyme activity to zero within 10 minutes. 

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