Role of the enzyme

By protodetritiation of the thiazolium salt (152) and of 2 tritiothiamine (153) Kemp and O Brien (432) measured a kinetic isotope effect, of 2.7 for (152). They evaluated the rate of protonation of the corresponding yiides and found that the enzyme-mediated reaction of thiamine with pyruvate is at least 10 times faster than the maximum rate possible with 152. The scale of this rate ratio establishes the presence within the enzyme of a higher concentration of thiamine ylide than can be realized in water. Thus a major role of the enzyme might be to change the relative thermodynamic stabilities of thiamine and its ylide (432). 

Aldoximes are prepared from aldehydes and hydroxylamine by condensation reaction, and the dehydration reaction of aldoxime is one of the most important methods of nitrile synthesis in organic chemistry." We speculated that it would become one of the most important examples in Green Chemistry if the dehydration reaction could be realized by an enzymatic method, and started studies on a new enzyme, aldoxime dehydratase, and its use in enzymatic nitrile synthesis. Furthermore, we clarified the relationship between aldoxime dehydratase and nitrile-degrading enzymes in the genome of the microorganisms and the physiological role of the enzyme. 

The role of the enzyme s protein scaffold is to hold the zinc ion, coenzyme, and substrate in the three-dimensional array required to lower the energy of the transition state. 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

First, the true physiological substrates of most esterases are unknown. It is, therefore, hardly practicable to systematically name esterases according to the recommendations of the Enzyme Nomenclature Committee [1], i.e., based on the definite (physiological) role of the enzyme. The difficulty is that the use of nonphysiological substrates during purification and in characterization assays does not contribute to discovering the physiological role of an enzyme. 

N2-fixing organisms, the enzyme is found in both the vegetative cells and the heterocysts. In A. variabilis and A. nidulans the enzyme is thought to be membrane bound. Examination of the interrelationship between photosynthesis and respiration has led to speculation that the role of the enzyme may be to control electron flow in respiration and photosynthesis (Appel and Schulz 1998, 2000) (Fig. 2.2D). 

These principles are similar to those that govern the relationship between an enzyme and its catalytic activity. For the hormone, R is equivalent to the enzyme, H to the substrate, and hormone-receptor complex to the enzyme-substrate complex. The activity of the substrate effector system is similar to the transition state. The cellnlar response to the hormone is similar to the catalytic role of the enzyme in the cell (Chapter 3),... 

It should be noted that the genetic information for the P450 enzymes is present throughout in all tissues, but knowledge of the role of the enzymes in tissues other than the liver and gastrointestinal tract is unclear. For example, cytochrome P450 2D6 is found in the brain where it is linked to the dopamine transporter. Whether a deficit in the activity of this enzyme is responsible for predisposing some individuals to Parkinson s disease is a matter of conjecture. 

Table 3.1). Thus, for a redox reaction to be possible, the difference between the redox potential of the enzyme-cofactor system and that of the substrate must be above zero [3]. The catalytic role of the enzyme protein structure in a redox reaction is often to alter the electronic environment of the cofactor, thereby changing its redox potential and hence making the reaction more thermodynamically feasible. (For further in-depth discussion the reader is referred to the excellent text of Bugg [3].)...

It may be concluded that neither pH nor isozyme-specificity is likely to direct the exclusive formation of isodityrosine in the plant cell wall in vivo. Indeed, it might be argued that isozyme-specificity is intrinsically unlikely to direct the orientation of coupling (dityrosine vs. isodityrosine) since the role of the enzyme is thought to be merely the production of tyrosine free radicals (44) which then non-enzymically pair off. 

Carbonic anhydrase plays an important role in the secretion of aqueous humor [1,2]. This enzyme was first demonstrated to be present in the ciliary processes of the rabbit, and its presence was later confirmed in human ciliary processes [3,4]. Carbonic anhydrase is responsible for the generation of bicarbonate anions which are secreted from the ciliary process into the posterior chamber, with sodium being the counter ion. Inhibition of carbonic anhydrase in the ciliaiy processes of the eye decreases aqueous humor secretion, presumably by slowing the formation of bicarbonate ions with subsequent reduction in sodium and fluid transport. The role of the enzyme in aqueous humor secretion has been reviewed in detail by Maren [1]. 

The role of the enzymes is three-fold. Firstly there is the use of very thermostable df-amylases to pre-thin the gelatinised starch, reducing its viscosity so that it can be easily handled and further processed. This process is conceptually very similar to many other commercial uses of hydrolases, especially proteases and glycosidases. Pre-thinning takes place at 105°C and the thermostable df-amylase from B. licheniformis actually has a temperature optimum of almost 100°C. 

The sequential and cooperative roles of the enzymes are shown in Figure 1. Endoglucanases, often called cellulases or carboxymethylcellu-lases, provide nonreducing chain ends from which the exoCBH s cleave cellobiosyl residues. The cellobiose, which is inhibitory to the depoly-merizing enzymes, is cleaved to glucose by the / -glucosidase. Since the product of one enzyme may become the substrate for another, enzyme purification is an essential prerequisite to any clear description of the system. 

The catalytic role of the enzyme is influenced by the metal ion in the active site of the proteins and the binding mode of the ligands to this metal ion, and the study of this influence is the main theme of the present monograph. The catalytic... 

Mechanism (1), first suggested almost three decades ago [150], continues to be the most invoked explanation of the role of the enzyme in promoting homolysis. Cleavage of the C—Co bond of sterically hindered alkylcobalamins (e.g., neopentylcobalamin) was markedly increased by diol dehydrase [72], Such cobal-amins do not function as coenzymes but convert to enzyme-bound hydroxocobal-amin in stoichiometric first-order reactions. The strong competitive inhibition by AdoB 12 indicates that labilization occurs at the active site of the enzyme and is suggested to be caused by a steric distortion of the corrin ring. 

The absence in halobacteria of the oxoacid dehydrogenase complexes creates another puzzle. In most known systems, the role of the enzyme lipoamide dehydrogenase is to reoxidize the lipoic acid that is involved in the oxidation of the oxoacids in the oxoacid dehydrogenase complexes. This enzyme was nonetheless found in H. halobium and purified to homogeneity by Danson et al. (1986). What, then, is its function It is likely that lipoamide dehydrogenase assumes a different role in halobacteria. Another reducing system unique to... 

A diverse cross section of enzymes has been studied in Jerusalem artichoke (Table 10.8), some simply because the tuber provides a convenient source that can be easily stored. In other instances, especially where there are relatively unique properties involved, the focus has been on the role of the enzyme system in the species. The following describes several such enzymes. 

The reverse aldol reaction is catalyzed by an enzyme called aldolase. One of the roles of the enzyme is to stabilize the enolate anion intermediate because such ions are too basic to be produced under physiological conditions. In animals, aldolase accomplishes this task by forming an inline bond between the carbonyl group of fructose-1,6-bisphosphate and the amino group of a lysine amino acid of the enzyme. As a result, the product of the reverse aldol step is an enamine derived from DHAP rather than its enolate anion. (Section 20.8 shows that enamines are the synthetic equivalents of enolate anions.) The formation of the strongly basic enolate anion is avoided. This process is outlined here ... 

The role of the enzyme in these types of reactions is to accelerate the rate of product formation. However, the substrate possesses a binding site on the enzyme, and so in order... 

Generally speaking, the role of the enzyme consists of the selective and specific attraction of substrate and the highly efficient catalysis. Every enzyme has its own characteristic feature for example, the specificity in the binding and a charge-relay action in the catalysis in a-chymotrypsin, the contribution of the imidazole moiety as an electron donor to the electrophilicity of zinc ion in carboxypeptidase, the change in the spin state and the reactivity of the transition metal ion by the coordination of the imidazole in the hemochrome. These typical characteristic features are the result of the cooperative actions of the constituents. 

While the relatively low cost of many amino acids does not seem to justify the preparation of supported catalysts derived from them, other reasons may drive the immobilization of chiral catalysts, such as those mentioned above and the possibility of experimenting with different solubility properties, easy separation of the products from the catalysts and the catalyst s recyclability. The immobilization of these compounds on a support can also be seen as an attempt to develop a minimalistic version of an enzyme, with the amino acid playing the role of the enzyme s active site and the polymer that of an oversimplified peptide backbone not directly involved in catalytic activity. 

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