Enzyme action, reversal

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 ... 

But the moment one becomes interested in applying his knowledge of metabolism, enzyme action, or endocrine influences to modifying (ameliorating or the reverse) the life processes of any organism (human or subhuman), biochemical individuality enters as a potent factor which must be reckoned with. 

Enzyme inhibitors inhibit the action of enzymes either reversibly or irreversibly. Since enzymes are such pervasive, powerful biological catalysts, inhibitors can act as potent drugs. Broadly categorized, enzyme inhibitors may be either irreversible or reversible. 

The ES complex is the key to understanding this kinetic behavior, just as it was a starting point for our discussion of catalysis. The kinetic pattern in Figure 6-11 led Victor Henri, following the lead of Wurtz, to propose in 1903 that the combination of an enzyme with its substrate molecule to form an ES complex is a necessary step in enzymatic catalysis. This idea was expanded into a general theory of enzyme action, particularly by Leonor Michaelis and Maud Menten in 1913. They postulated that the enzyme first combines reversibly with... 

The process of reversible inhibition is described by an equilibrium interaction between enzyme and inhibitor. Most inhibition processes can be classified as competitive or noncompetitive, depending on how the inhibitor impairs enzyme action. A competitive inhibitor is usually similar in structure to the substrate and is capable of reversible binding to the enzyme active site. In contrast to the substrate molecule, the inhibitor molecule cannot undergo chemical transformation to a product however, it does interfere with substrate binding. A noncompetitive inhibitor does not bind in the active site of an enzyme but binds at some other region of the enzyme molecule. Upon binding of the noncompetitive inhibitor, the enzyme is reversibly converted to a nonfunctional conformational state, and the substrate, which is fully capable of binding to the active site, is not converted to product. 

Fructose-1,6-bisphosphate is cleaved by aldolase into two molecules of triose phosphate. This reaction represents the reversal of an aldol condensation (fig. 12.20). Most aldolases are highly specific for the upper end of the substrate molecule, requiring a phosphate group at C-l, a carbonyl at C-2, and specific steric configurations at C-3 and C-4. The nature of the remainder of the molecule is unimportant as far as the enzyme action is concerned. 

TIs also inhibit the reverse transcriptase enzyme s ability to perform one of the initial steps in HIV replication. The NNRTIs, however, directly inhibit the active (catalytic) site on this enzyme, whereas zidovudine and other NRTIs serve as false substrates that take the place of the substance (thymidine) normally acted on by this enzyme (see Reverse Transcriptase Inhibitors Mechanism of Action ). Hence, NNRTIs provide another way to impair one of the key steps in HIV replication, and these drugs can be used along with other agents (NRTIs, protease inhibitors) to provide optimal benefits in preventing HIV replication and proliferation (see the next section). 

Enzyme inhibitors (I) may have either a reversible or irreversible action. Reversible inhibitors tend to bind to an enzyme (E) by electrostatic bonds, hydrogen bonds and van der Waals forces, and so tend to form an equilibrium system with the enzyme. A few reversible inhibitors bind by weak covalent bonds, but this is the exception rather than the rule. Irreversible inhibitors... 

These are inhibitors that form a dynamic equilibrium system with the enzyme. The inhibitory effects of reversible inhibitors are normally time dependent because the removal of unbound inhibitor from the vicinity of its site of action by natural processes will disturb this equilibrium to the left. As a result, more enzyme becomes available, which causes a decrease in the inhibition of the process catalysed by the enzyme. Consequently, reversible enzyme inhibitors will only be effective for a specific period of time. 

Non-competitive inhibitors bind reversibly to an allosteric site (see Appendix 7) on the enzyme. In pure non-competitive inhibition, the binding of the inhibitor to the enzyme does not influence the binding of the substrate to the enzyme. However, this situation is uncommon, and the binding of the inhibitor usually causes conformational changes in the structure of the enzyme, which in turn affects the binding of the substrate to the enzyme. This is known as mixed noncompetitive inhibition. The fact that the inhibitor does not bind to the active site of the enzyme means that the structure of the substrate cannot be used as the basis of designing new drugs that act in this manner to inhibit enzyme action. 

In the cases where S is not in excess, the reverse direction of enzyme action (i.e., P - S) will decrease appreciably the net rate of P formation. It follows from eq. 3 that ... 

P-Enzyme is widely distributed in the plant kingdom, and relatively large amounts are present in many starch-synthesizing cells, so that the in vivo importance of this enzyme has become generally accepted. Nevertheless, this view is not supported by certain experimental observations. For example, P-enzyme action is freely reversible, and amylose synthesis will only occur when the ratio (inorganic phosphate) (a-D-glucosyl phosphate) is less than the equilibrium value [10.8 at pH 5, 6.7 at pH 6, and... 

Since the coupling reaction is the reverse reaction of cyclization, the enzyme is able to degrade CDs in the presence of suitable acceptor molecules. This activity can have a negative effect on conversion of starch into CDs. Reaction conditions are essential for directing the enzyme action to CD production [39]. 

Another important class of RNA virus comprises the retroviruses, so called because the genetic information flows from RNA to DNA rather than from DNA to RNA. This class includes human immunodeficiency virus 1 (HIV-1), the cause of AIDS, as well as a number of RNA viruses that produce tumors in susceptible animals. Retrovirus particles contain two copies of a single-stranded RNA molecule. On entering the cell, the RNA is copied into DNA through the action of a viral enzyme called reverse transcriptase (Figure 5.23). The resulting double-helical DNA version of the viral... 

Another important pathway for reversing the message is through protein phosphatases, enzymes that reverse the action of kinases by removing phosphate groups... 

I.V. Berezin, Dejstvie fermentov v obraschennykh micellakh (Action of Enzymes in Reverse Micelles), Nauka, Moscow, 1985. 

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