Processive enzymes definition

The processes of prepropolypeptide synthesis, translocation, proteolytic processing and non-proteolytic modification can be enzymatically defined. These definitions are continuing to be developed and clarified. There are limited reports on insect neuropeptide processing (101.102. but these investigations should increase rapidly with the identification of precursor sequences via molecular genetics. The identification of processing enzymes, both proteolytic and non-proteolytic, will further open whole new areas for exploration. 

Thus, lAP is, like diphtheria and cholera toxins, a protein toxin with an A-B structure. The holotoxin is bound to particular sites on the cell surface via its B-oligomer moiety as the first step of its interaction with mammalian cells. The A-protomer (or holotoxin itself) is then inserted to the plasma membrane traversing the lipid bilayer gradually. This slow process of the entrance of the toxin molecule is reflected in a definite lag time invariably preceding the onset of the action of lAP on intact cells as analyzed with rat pancreatic islets by kinetic and immunological approaches [25]. The A-protomer is finally activated by certain processing enzyme(s) inside the membrane to catalyze ADP-ribosylation of the target membrane protein with intracellular NAD as substrate. 

Instead, the orthogonal multienzymatic reactions are always cascade processes by definition. However, this type of multienzymatic processes has been largely investigated in the past especially for the development of enzymecofactor regeneration systems. These studies not only allowed the wide exploitation of cofactor-dependent enzymes, such as NAD(P)H-dependent dehydrogenases, by making their reactions economically feasible but were also useful in identifying relevant process options for the development of effective multienzymatic reaction systems (3). 

Chemical Pathology. Also referred to as clinical chemistry, this monitoring procedure involves the measurement of the concentration of certain materials in the blood, or of certain enzyme activities in semm or plasma. A variety of methods exist that allow (to variable degrees of specificity) the definition of a particular organ or tissue injury, the nature of the injurious process, and the severity of the effect (76). 

Historically, dietary fiber referred to iasoluble plant cell wall material, primarily polysaccharides, not digested by the endogenous enzymes of the human digestive tract. This definition has been extended to iaclude other nondigestible polysaccharides, from plants and other sources, that are iacorporated iato processed foods. Cellulose [9004-34-6] (qv) is fibrous however, lignin [9005-53-2] (qv) and many other polysaccharides ia food do not have fiberlike stmctures (see also Carbohydrates). 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

Sidney Altman discovered this property of RNA in the course of studies on precursor transfer RNA. It was realized that the catalytic properties of RNA are not exactly the same as those of protein enzymes, since the ribozyme is itself active and thus undergoes change during the catalytic reaction. This does not correspond to the generally accepted definition of an enzyme. Later studies, however, showed that some ribozymes are capable of acting catalytically at other RNA molecules. The ribozymes remain completely unchanged in this process, and thus fulfil the definition of a real enzyme. 

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

The catalase-peroxidases present other challenges. More than 20 sequences are available, and interest in the enzyme arising from its involvement in the process of antihiotic sensitivity in tuherculosis-causing bacteria has resulted in a considerable body of kinetic and physiological information. Unfortunately, the determination of crystallization conditions and crystals remain an elusive goal, precluding the determination of a crystal structure. Furthermore, the presence of two possible reaction pathways, peroxidatic and catalatic, has complicated a definition of the reaction mechanisms and the identity of catalytic intermediates. There is work here to occupy biochemists for many more years. 

The use of enzymes to catalyze reversible reactions has proven to be an effective strategy for DCC. Enzymes work under physiological conditions (by definition), are reversible, and can also be applied to a variety of C-C and C-X bond-forming reactions. Venton and coworkers reported the first example of an enzyme-catalyzed process being used in a DCC context [32]. As their work preceded the codification of DCC in the literature, it contains little of the vocabulary that has come to define the field. It does, however, correspond perfectly with the conceptual framework of DCC, and has been widely cited as an influential early example of the DCC idea. 

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

Strictly speaking a catalytic cascade process is one in which all of the catalysts (enzymes or chemocatalysts) are present in the reaction mixture from the outset. A one-pot process, on the other hand, is one in which several reactions are conducted sequentially in the same reaction vessel, without the isolation of intermediates. However, not all of the reactants or catalysts are necessarily present from the outset. Hence, a cascade process is by definition a one-pot process, but the converse is not necessarily true. Clearly a cascade process is a more elegant solution, but a one-pot process that is not, according to the strict definihon, a cascade reaction may have equal practical uhlity. In this chapter we shall be primarily concerned with enzymatic cascade processes, but the occasional chemocatalytic step may be included where relevant and sometimes a sequential one-pot procedure may slip through the net. 

To develop a unifying view of iron center catalysis, properties of the iron center in individual enzymes must be determined. Obviously, the definitive solution for the structure is atomic resolution of the active enzyme and postulated intermediates determined by diffraction or nuclear magnetic resonance (NMR) spectroscopy. Just as obviously, these methods are limited by enormous time, effort, and instrumentation requirements as well as by practical and theoretical considerations. This point is emphasized by the paucity of available protein structures. In addition to the strictly structural details of the iron center, chemical and physical properties are required and, in some cases, these results augment diffraction or NMR structural studies. Discussed below are a few of the more common processes by which this information is obtained. 

R)- -Decalactone contributes much of the characteristic taste and aroma of peach and many other flavours. Chemically synthesised T -decalactone has been cheaply available for a long time, but the consumer demand for naturally flavoured food and beverages that arose in the mid 1980s created a strong demand for the (RJ-lf -decalactone isomer as a natural food flavour molecule. This definition of natural grade required its production by entirely enzyme-based steps. In turn this led to the development of a number of biotransformation processes to make natural f -decalactone. 

When H2O deacetylates the acyl-enzyme, phenylacetic acid is formed. When nucleophiles other than H2O deacylate the acyl-enzyme, a new condensation product, in this case phenylacetyl-O-R or phenylacetyl-NH-R is formed. By definition the hydrolysis of these condensation products can be catalyzed by the same enzyme that catalyzes their formation in equation 10.1. Thus, when the acyl-enzyme is formed from phenylacetyl-glycine or phenylacetyl-O-Me, this gives rise to an alternative process to produce Penicillin G, in addition to the thermodynamically controlled (= equilibrium controlled) condensation of phenylacetic acid and 6-aminopenicillanic acid (6-APA). This reaction that involves an activated side chain is a kinetically controlled (= rate controlled) process where the hydrolase acts as a transferase (Kasche, 1986 1989). 

Unlike proteins, polysaccharides generally do not have definite molecular weights. This difference is a consequence of the mechanisms of assembly of the two types of polymers. As we shall see in Chapter 27, proteins are synthesized on a template (messenger RNA) of defined sequence and length, by enzymes that follow the template exactly. For polysaccharide synthesis there is no template rather, the program for polysaccharide synthesis is intrinsic to the enzymes that catalyze the polymerization of the monomeric units, and there is no specific stopping point in the synthetic process. 

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