Catalyzed Enzymatic Reactions

Table 1 Coenzyme B12 catalyzed enzymatic reactions involving namral substrates...

The homolytic cleavage of the Co - C bond of the protein-boimd organo-metallic cofactor AdoCbl (2) is the initial step of the coenzyme Bi2-catalyzed enzymatic reactions. Halpern quoted that adenosyl cobamides can be considered as reversibly functioning sources for organic radicals [119]. A neutral aqueous solution of 2 is remarkably stable with a half-Ufe of 10 s (in the dark at room temperature), but decomposes, mainly with the homolysis of the Co-C bond, at higher temperatures [119,123]. The coenzyme B12-catalyzed enzyme reactions occur with maximal rates of approximately 100 s [173,239]. Rapid formation of Co(ll)corrins occurs only with addition of substrate to a solution of holoenzyme (or of apoenzymes and 2), as demonstrated in most of the known coenzyme Bi2-dependent enzymes, e.g., in methyl-malonyl-CoA mutase [121], glutamate mutase [202] and ribonucleotide reductase [239]. 

With the exception of the cobamide-dependent ribonucleotide reductase, the 5 -deoxyadenosyl radical is indicated to be the direct activator of the substrate. In ribonucleotide reductases, a protein-derived thiyl radical takes this role (see the following text). Accordingly, two vmusual ftinctions would be given to the apoen-zyme in the course of the coenzyme Bi2-catalyzed enzymatic reactions first, it would have to contribute to the activation of the bound coenzyme toward the (reversible) formation of radicals, then it would have to help harness the reactivity of the radical intermediates generated (64). 

The parameters n, are the specific reaction rates for the uptake of sugar i. These reactions are first-order in the biomass concentration. The specific rates must refiect the catalyzed enzymatic reactions that occur within yeast cells that convert sugar into alcohol. These rates are given by the Monod expression for glucose,... 

Sakai S, Yamada Y, Zenke T, Kawakami K (2009) Novel chitosan derivative soluble at neutral pH and in situ gellable via peroxidase-catalyzed enzymatic reaction. J Mater Chem 19 230-235... 

Chorismate Mutase catalyzed Claisen Rearrangement- 10 rate enhancement over non-enzymatic reaction... 

One approach called enzymatic resolution, involves treating a racemic mixture with an enzyme that catalyzes the reaction of only one of the enantiomers Some of the most commonly used ones are lipases and esterases enzymes that catalyze the hydrol ysis of esters In a typical procedure one enantiomer of the acetate ester of a racemic alcohol undergoes hydrolysis and the other is left unchanged when hydrolyzed m the presence of an esterase from hog liver... 

Aldol Additions. These reactions catalyzed by lyases are perhaps the most synthetically useful enzymatic reactions for carbon—carbon bond formation. Because of the broad synthetic utiUty of this method, the enzymatic aldol reactions have received considerable attention in recent years and have been extensively covered in a number of books and reviews (10,138—140). 

This chapter presents the implementaiton and applicable of a QM-MM method for studying enzyme-catalyzed reactions. The application of QM-MM methods to study solution-phase reactions has been reviewed elsewhere [44]. Similiarly, empirical valence bond methods, which have been successfully applied to studying enzymatic reactions by Warshel and coworkers [19,45], are not covered in this chapter. 

Citrate synthase catalyzes the metabolically important formation of citrate from ace-tyl-CoA and oxaloacetate [68]. Asp-375 (numbering for pig CS) has been shown to be the base for the rate-limiting deprotonation of acetyl-CoA (Fig. 5) [69]. An intennediate (which subsequently attacks the second substrate, oxaloacetate) is believed to be formed in this step the intermediate is thought to be stabilized by a hydrogen bond with His-274. It is uncertain from the experimental data whether this intermediate is the enolate or enol of acetyl-CoA related questions arise in several similar enzymatic reactions such as that catalyzed by triosephosphate isomerase. From the relative pK values of Asp-375... 

Modern variants are the enzyme-catalyzed and the transition-metal-catalyzed Baeyer-Villiger reaction, allowing for an oxidation under mild conditions in good yields, with one stereoisomer being formed predominantly in the enzymatic reaction ... 

Although the fundamental chemomechanical transduction processes seem to be the same in all types of vertebrate muscle, contraction in smooth muscle is characterized by much greater involvement of enzymatically catalyzed control reactions. In smooth muscle the control reactions themselves involve the use of phosphorylation-dephosphorylation cycles. Moreover, they are futile in the sense they cause the expenditure of bond energy without a tangible work resultant, i.e., compounds synthesized or external work done. 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Chemoselectivity of enzymatic reaction is the enzyme-catalyzed transformation of one type of functional group in the presence of other sensitive groups present in the substrate molecule. As a result, reactions catalyzed by enzymes generally tend to be cleaner and purification of product(s) from impurities can largely be omitted. Therefore, all enzymatic reactions generate less by-products and waste compared to chemical transformations. This simplifies all operations and reduces costs of transformation, which is of great importance for the industry. 

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