Enzyme reversible action

As already oudined, inhibition is essentially complete when caused by reagents that react with sulfhydryl groups (for example, p-chloro-mercuribenzoate, p-mercuribenzoate, o-iodosobenzoate, L-ascorbic acid silver, cupric, and mercuric ions iodine, and ferricyanide) this inactivation can be reversed to some extent by hydrogen sulfide and by cysteine. Lineweaver—Burk graphs have shown that the action of L-ascorbic acid is noncompetitive, and L-ascorbic acid acting in the presence of cupric ions probably causes formation of an inactive cuprous-enzyme. The action of p-chloromercuribenzoate on barley beta-amylase has been shown to be a competitive inhibition. In contrast, the soya-bean p-chloromercuribenzoate inhibition is noncompetitive, and the extent of inhibition is inversely related to the concentration of acetate ion. The latter exhibits a protective effect, and there... 

All replication-competent retroviruses possess a characteristic enzyme, reverse transcriptase (RT), which is present at 20-70 mol/virus particle (1-3). The enzyme is cleaved, and thereby activated, from an inactive precursor by the action of another retroviral enzyme, the viral protease. All RTs possess three distinct enzymatic activities (1) an RNA-dependent DNA polymerase, which is the RT in the strict sense of the word, (2) an RNase H, and (3) a DNA-dependent DNA polymerase. After infection of a new host cell, these different activities serve in turn to synthesize a cDNA of the viral RNA, to degrade RNA from the cDNA-RNA heteroduplex, and to duplicate the cDNA strand (reviewed in ref. 4). 

Thromboxane produced by platelet COX-1 in concert with a downstream enzyme is prothrombotic, so aspirin and other NSAIDs cause platelet dysfunction and increase bleeding time. Aspirin is unusual in that it causes covalent, irreversible inhibition of the COX protein, whereas other NSAIDs have noncovalent, reversible actions. Thus, platelets, because they cannot synthesize... 

The acyl enzyme is an inert form of the enzyme for which—by an intramolecular mechanism—the active enzyme is generated. Therefore, the reversibly acylated enzyme could be used as a delivery system for the active enzyme whose action is both delayed and prolonged in plasma (depot effect). 

Some of quercetin s potentially useful vs. potentially harmful properties have been reviewed by Alan Gaby, MD., in an editorial in the May 1998 issue of the Townsend Letter for Doctors Patients. It acts as an inhibitor for the enzyme phosphodiesterase, and hence may possibly act against asthma. Also, it may act against allergies by inhibiting the release of histamine. Quercetin is also an inhibitor for the enzyme aldose reductase, which is involved in diabetic complications. Perhaps most interestingly, it inhibits the enzyme reverse transcriptase (or RNA-directed DNA polymerase), involved in the action of retroviruses such as the AIDS virus or HIV (and retroviruses may also be involved in cancer formation). 

HIV is a single-stranded RNA virus that contains a virally encoded DNA polymerase enzyme, reverse transcriptase (RT). The HIV RT is the enzyme responsible for transcribing viral RNA into DNA, a cytoplasmic event. Following reverse transcription, the HIV DNA enters the cell nucleus and, through the action of another viral enzyme (integrase), is incorporated into the host cell genome. It is this event, the incorporation into host DNA that makes HIV infection a chronic, lifelong disease, because each time the lymphocyte is activated and the DNA transcribed and translated, new HIV virions are created and released. 

After having proved that it was, indeed, the same enzyme which effected either hydrolysis or synthesis, Bourquelot demonstrated the reversible action of other glycosidases a-glucosidase and a-galactosidase from bottom yeast, /3-galactosidase from almonds, and a-mannosidase from germinated alfalfa seeds. 

Melittin, a toxin in bee venom, is a pentacationic peptide composed of 26 amino acids. Four of the basic residues occur in a cluster near the C-terminus. The retromer made of the enantiomeric amino acids in a reversed sequence (Scheme 9.6.1) forms helices in water containing 20% hexafluoro-2-propanol as a cosolvent. The CD spectra of L-melittin and retroenantio melittin (retro d-melittin) are approximately mirror images, but both are equally effective in their antibacterial activity. The toxic action of melittin can therefore not function via a (protein) receptor or enzyme-type action. The only other possibility is formation of ion-conducting pores by self-aggregation (Juvvadi, 1997). 

In theory, enzyme reactions may tolerate reversibility, the activation/ inhibition by substrates/products, and even thermo-inactivation of enzyme. From a mathematic view, it is still feasible to estimate parameters of an enzyme reaction system by kinetic analysis of reaction curve if the roles of all those factors mentioned above are included in a kinetic model (Baywenton, 1986 Duggleby, 1983, 1994 Moruno-Davila, et al., 2001 Varon, et al, 1998). However, enzyme kinetics is usually so complex due to the effects of those mentioned factors that there are always some technical challenges for kinetic analysis of reaction curve. Hence, most methods for kinetic analysis of reaction curve are reported for enzymes whose actions suffer alterations by those mentioned factors as few as possible. 

In (i) and (2) the extracts have the same coagulating powers however, it is only in (i) that plastein appears. In (3) and (4) the conditions are reversed. The two liquids have equal digestive powers, but they differ enormously as to their coagulating powers. This time it is still in (3) that coagulation takes place, the quantity of pepsin alone in (4) being too small to act, as previously in (2) the quantity of rennet alone was insufficient. The Danilewsky reaction is therefore determined, either by pepsin alone, or by rennet alone. In the presence of the two enzymes, the actions are superimposed. When one enzyme is removed from the mixture it is necessary, in order to obtain a result, to replace the quantity eliminated by an equivalent quantity of the other. 

Industrial enzymes are usually hydrolases that catalyse hydrolysis or synthesis reactions in aqueous environments. For thermodynamically controlled hydrolysis reactions, the equilibria can - in principle - be shifted completely to the product-side by dilution (increasing entropy of product formation). Thermodynamically controlled synthesis reactions using the reverse action of hydrolases can be enhanced by using excess of the cheaper reactants. This does not lead to compact processes, and affects their economic feasibiUty in a negative maimer. Therefore, possibilities to selectively remove reaction products from each other or fi om the reactors during the reaction are very attractive. 

A pathway for the metabolism of aspartic acid which appears to be virtually limited to microorganisms is through the action of the enzyme aspartase. This enzyme reversibly catalyzes the reaction... 

AZT can only stop or retard the proliferation of HIV, but cannot cure AIDS. You can guess this from what we talked about it above. Besides, AZT has a number of side effects the truth of matter is that no drug is without side effects. AZT is to block the action of an enzyme, reverse transcriptase. This enzyme is to polymerize... 

All reactions mediating fixation of CO by reductive carboxylation are readily reversible. Their equilibrium constant is sUghtly in favor of carboxylation, but at the low tensions of CO prevailing in cells and biological fluids the reverse action, i.e., oxidative decarboxylation, is favored unless appropriate mechanisms are broi t into play to displace the equilibrium position in favor of carboxylation. The two primary processes in each of these reactions, oxidation and decarboxylation or carboxylation and reduction, are intiinately interconnected and appear to be catalyzed by the same enzyme protein. The reactions to be considered in this section are the reductive car-boxylations of pyruvate, a-ketoglutarate, and ribulose 5-phosphate to L-malate, d-isocitrate, and 6-phosphogluconate, respectively. 

Since an enzyme is a biological catalyst and therefore merely accelerates a reaction, it cannot alter the position of equilibrium in a reversible reaction. The hydrolysis of p-methylglucoside is reversible and emulsin should therefore be capable also of synthesising this compound frc n glucose and methanol. This synthesis can actually be carried out by the action of the enzyme on glucose dissolved in an excess of methanol, the excess of the alcohol throwing the equilibrium over to the left. Owing to experimental difficulties, this reaction is not here described. 

Lenore Michaelis and Maud L. Menten proposed a general theory of enzyme action in 1913 consistent with observed enzyme kinetics. Their theory was based on the assumption that the enzyme, E, and its substrate, S, associate reversibly to form an enzyme-substrate complex, ES ... 

AMP reverses the inhibition due to ATP, and AMP levels in cells can rise dramatically when ATP levels decrease, due to the action of the enzyme adenylate kinase, which catalyzes the reaction... 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

---