Enzyme catalysis mediators

The mechanism of enzyme catalysis mediated by MPO is similar to other heme-containing peroxidases MPO reacts with hydrogen peroxide to yield several spectroscopically distinct forms of the enzyme. MPO-compound I is a short-lived... 

As discussed above for ferrocene derivatives, small water-soluble ruthenium and osmium complexes are good candidates for redox enzyme catalysis mediation for their reversible (II/III) behavior and relative stabiKty in the two-oxidation state in water. The alteration of the aromatic rings is a means of tuning of the redox potential/structure characteristics of the complexes, which is important for efficient redox enzyme mediation [75, 76]. Table 1 gives the redox potentials in acetonitrile of a series of neutral osmium(II) dichloride complexes with different substituted ligands [77]. 

Common Mechanistic Properties in the Enzymatic Catalysis for Enzyme-NAD+ Mediated Hydrogen Transfer Reactions—A Model System for the Study of the Evolutionary Process of Enzyme Catalysis ... 

The mechanism and theory of bioelectrocatalysis is still under development. Electron transfer and variation of potential in the electrodeenzyme-electrolyte system has therefore to be investigated. Whether the enzyme is soluble and the electron transfer process occurs through a mediator, or whether there is direct enzyme immobilization on the electrode surface, the homogeneous process in the enzyme active centre has to be described by the laws of enzyme catalysis, and the heterogeneous processes on the electrode surface by the laws of electrochemical kinetics. Besides this there are other aspects outside electrochemistry or... 

In an investigation of the role of water in enzymic catalysis. Brooks and Karplus (1989) chose lysozyme for their study. Stochastic boundary molecular dynamics methodology was applied, with which it was possible to focus on a small part of the overall system (i.e., the active site, substrate, and surrounding solvent). It was shown that both structure and dynamics are affected by solvent. These effects are mediated through solvation of polar residues, as well as stabilization of like-charged ion pairs. Conversely, the effects of the protein on solvent dynamics and... 

So far, only very little attention has been focussed on the use of zeolites in biocatalysis, i.e., as supports for the immobilization of enzymes. Lie and Molin [116] studied the influence of hydrophobicity (dealuminated mordenite) and hydrophilicity (zeolite NaY) of the support on the adsorption of lipase from Candida cylindracea. The adsorption was achieved by precipitation of the enzyme with acetone. Hydrolysis of triacylglycerols and esterification of fatty acids with glycerol were the reactions studied. It was observed that the nature of the zeolite support has a significant influence on enzyme catalysis. Hydrolysis was blocked on the hydrophobic mordenite, but the esterification reaction was mediated. This reaction was, on the other hand, almost completely suppressed on the hydrophilic faujasite. The adsorption of enzymes on supports was also intensively examined with alkaline phosphatase on bentolite-L clay. The pH of the solution turned out to be very important both for the immobilization and for the activity of the enzyme [117]. Acid phosphatase from potato was immobilized onto zeolite NaX [118]. Also in this study, adsorption conditions were important in causing even multilayer formation of the enzyme on the zeolite. The influence of the cations in the zeolite support was scrutinized as well, and zeolite NaX turned out to be a better adsorbent than LiX orKX. 

The amino acid side chains in the active site of enzymes catalyze proton transfers and nucleophilic substitutions. Other reactions require a group of nonprotein cofactors, that is, metal cations and the coenzymes. Metal ions are effective electrophiles, and they help orient the substrate within the active site. In addition, certain metal cations mediate redox reactions. Coenzymes are organic molecules that have a variety of functions in enzyme catalysis. 

Receptor-mediated endocytosis 18.11 Membrane transport resembles enzyme catalysis because both processes exhibit a high degree of specificity. 

The main features of the reaction mechanism for YADH are in all probability essentially the same as in LADH. The structural similarities of the catalytic domain, including the catalytic zinc and its protein ligands, are strong indications that both enzymes perform electrophilic catalysis mediated by zinc. Involvement of zinc in the catalytic action of YADH was suggested almost 20 years ago (365,366,452), but evidence for a direct participation has been obtained only recently (329,449). 

The theoretical description of electrocatalysis that takes into account electron and ion transfer and the transport process, the permeations of the substrates, and their combined involvement in the control over the overall kinetics has been elaborated by Albeiy and Hillman [312,313,373] and by Andrieux and Saveant [315], and a good summary can be found in [314]. Practically all of the possible cases have been considered, including Michaelis-Menten kinetics for enzyme catalysis. Inhibition, saturation, complex mediation, etc., have also been treated. The different situations have also been represented in diagrams. Based on the theoretical models, the respective forms of the Koutecky-Levich eqrration have been obtained, which make analyzing the resirlts of voltarrrmetry on stationary artd rotating disc electrodes a straightforward task. 

L. (1993) Enzyme catalysis at hydrogel-modified electrodes with redox polymer mediator. /. Chem. Soc., Faraday Trans., 89, 377-384. 

Cofactor Substitution, Mediated Electron Transfer to Enzymes, Fig. 1 Examples of artificial cofactor regeneration and cofactor substitution used in in vitro enzyme catalysis. Cofaclor regeneration (middle) replaces the metabolic cofaclor regeneration found in vivo (top), while cofactor substitution (bottom) shortcuts the natural system and... 

The masters of catalysis are enzymes. Enzymes are biomolecules typically based on proteins and often associated with small organic molecules or metal ions known as cofactors. In recent years it has become clear that RNA molecules can also catalyze important reactions, and such catalytic RNA molecules are referred to as ribozymes. Our focus here, however, will be on the more well known, protein-based enzymes, which mediate the overwhelming majority of biochemical transformations. These are nature s catalysts, and they can be incredibly efficient. As just one example, the hydrolysis of a phosphoester such as that used to link nucleotides together in DNA is estimated to have a half-life of hundreds of millions of years in water at neutral pH. Yet, the enzyme staphylococcal nuclease can catalyze this hydrolysis reaction with a half-life of a few minutes. Since this is a physical organic textbook, not a biochemistry textbook, we do not look at the structures of enzymes and how they are formed. Instead, we simply focus upon the mechanisms and kinetics of enzymatic catalysis. 

The properties of the carriers involved in both active and passive transport suggest that they are proteins. Apart from the fact that no other type of molecule has the necessary ability for specific recognition of the substance to be carried, carrier-mediated transport mechanisms show features that are reminiscent of enzyme catalysis, i.e. they are pH dependent, can be competitively inhibited by structurally similar compounds and poisoned by other compounds. Moreover, they show a relationship to the concentration of transported material that is essentially similar to that of substrate concentration on enzyme activity. It appears therefore that passive carrier-mediated transport may be a catalysed permeability in which the equilibrium of the reaction is not affected although the rate of attainment of equilibrium is greatly enhanced while in the case of active transport, there is a modification of the diffusion equilibrium as a result of the coupling of the transport process to an energy-yielding reaction. 

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