Mechanism of reactions catalyzed

Vederas JC, E Schleicher, M-D Tsai, HG Floss (1978) Stereochemistry and mechanism of reactions catalyzed by tryptophanase from Escherichia coli. J Biol Chem 253 5350-5354. 

It is very instructive to compare the kinetics and plausible mechanisms of reactions catalyzed by the same or related catalyst(s) in aqueous and non-aqueous systems. A catalyst which is sufficiently soluble both in aqueous and in organic solvents (a rather rare situation) can be used in both environments without chemical modifications which could alter its catalytic properties. Even then there may be important differences in the rate and selectivity of a catalytic reaction on going from an organic to an aqueous phase. TTie most important characteristics of water in this context are the following polarity, capability of hydrogen bonding, and self-ionization (amphoteric acid-base nature). 

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

Available evidence (14,15) favors the pathway for pyruvate kinase by way of phosphorylation of pyruvate enol. Furthermore, J. Knowles and his coworkers (16,17), using chiral thiophosphates and chiral (160,170,180) phosphate have shown that pyruvate kinase transfers phosphate from phosphoenolpyruvate to ADP with stereochemical inversion at phosphorus. Since monomeric metaphosphate is presumably planar, a chemical reaction by way of that ion should proceed with racemization. In the active site of an enzyme, however, all components might be held so rigidly that racemization need not occur. Furthermore, no information is yet available on the detailed mechanism of reactions catalyzed by cytidine synthetase our own experiments, designed to distinguish among the mechanisms here discussed, are as yet incomplete. 

Scheme XIX. Stereochemical mechanism of reactions catalyzed by cystathioninc-y-synthasc.

Haem peroxidases are globular proteins with an iron-porphyrin complex as a prosthetic group. These enzymes are widespread among prokaryotes and eukaryotes. They catalyze the oxidation of substrates by organic peroxides or hydrogen peroxide. During the past decades, considerable scientific effort has been put into elucidation ofthe mechanisms of reactions catalyzed by these enzymes. Pulse radiolysis technique has made an important contribution by providing information on the redox states of the enzymes and their interconversion, as well as on the properties ofthe free radical intermediates involved [23]. 

There is still another wide space for research activities related to titanium containing zeolites the synthesis of large pores Tl-zeolites, the activation of TS-1 with oxidants different from HjOj, the comprehension of the mechanism of reactions catalyzed by TS-... 

Fig. 1. Proposed mechanisms of reactions catalyzed by rice a-glucosidase with substrates of different anomcric configuration. Reproduced from II. Matsui et al., Carbohydr. Res., 250 (1989) 45-56, with permission from Elsevier Science. Ltd.

The mechanism of reactions catalyzed by enzymes has a complex character and has been conclusively established by no means in every case. However, in spite of their complexity, enzymatic reactions proceed in conformity with the general laws of conventional chemical transformations.Qualitative factors responsible for the catalytic effect of an enzyme are summarized in Reference (6) ... 

In a recent study, when the mechanism of reactions catalyzed by B INAP-ligated palladium complexes was revisited [39], it was shown that the bromoarene undergoes oxidative addition prior to a reaction of the palladium complex with the... 

Mechanism of Reactions Catalyzed by Complexes Other Than Palladium... 

Figure 1 Bodenstein s view about the principal mechanism of reactions catalyzed by solid surfaces.

Kinetic models to describe lipase-catalyzed reaction mechanisms have been proposed, and most have been extensions of the model developed by Michaelis and Menten (1913). However, normal Michaelis-Menten kinetics do not apply to lipase-induced changes, because the substrates (lipids) are not water-soluble and the enzyme operates at an interface (Brockman, 1984). However, rate expressions for the hydrolysis of emulsified lipids catalyzed by immobilized lipases resemble the rate expressions modeled with Michaelis-Menten mechanisms (Benzonana and Desnuelle, 1965). The kinetics and mechanisms of reactions catalyzed by immobilized lipases have been reviewed by Malcata et al. (1990 1992). 

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