Enzymatic catalysis three-enzyme process

As plant tissues senesce and die, three processes may ensue almost simultaneously. First, enzymes within the dead but sterile and physically intact cells cause proteolysis and other autolytic degradations. The released amino acids, sugars, tannins, phenols, and quinones may be oxidized by chemical or enzymatic catalysis to produce humus-like pigments, or proto-humus as discussed by Stevenson in Chapter 2. This was well illustrated by Cohen (Given and Dickinson, 1975) who observed cellular material of partially polymerized eaco-anthocyanins in residues of Rhizophora mangle deposited in a mangrove swamp in Florida. The autolytic reactions may be prominent in situations where microbial decomposition is slow due to acidity, anaerobiosis, or lack of basic nutrients. 

The variety of enzymatic reactions is so large that a comprehensive discussion of enzymatic mechanisms is not appropriate here, but it is worthwhile considering the general enzymatic process in terms of some of the elementary steps commonly involved in enzyme catalysis. Three types of reactions are now considered in detail ... 

The latter method, called the PI-FEP/UM approach, allows accurate primary and secondary kinetic isotope effects to be computed for enzymatic reactions. These methods are illustrated by applications to three enzyme systems, namely, the proton abstraction and reprotonation process catalyzed by alanine race-mase, the enhanced nuclear quantum effects in nitroalkane oxidase catalysis, and the temperature (in)dependence of the wild-type and the M42W/G121V double mutant of dihydrofolate dehydrogenase. These examples show that incorporation of quantum mechanical effects is essential for enzyme kinetics simulations and that the methods discussed in this chapter offer a great opportunity to more accurately model the mechanism and free energies of enzymatic reactions. 

This special volume on the enzyme-catalyzed synthesis of polymers focuses on various methods of polymer synthesis using enzymes as catalysts. There are three cases for such synthetic processes (1) In hving cells (in vivo) enzymes catalyze the synthesis of all biopolymers besides other biological substances via biosynthetic (metabolic) pathways, hi test tubes (in vitro) enzymatic catalysis is achieved for the synthesis of polymers via (2) biosynthetic pathways or (3) non-biosynthetic pathways. The present volume is concerned with case (3). Therefore, studies such as the synthesis of polyesters via fermentation using micro-organisms and synthesis of proteins using E. coli are not included. 

In order to accelerate the rate of a chemical reaction the enzyme must find a way to interact with the substrate(s) and to change the potential surface so that the resulting activation barrier is lower than that of the original process in water. There is a continuous debate on the importance of various factors in enzymatic rate acceleration however, most authors attribute enzyme catalysis to transition-state stabilization. There are three main factors that are likely to contribute to enzyme catalysis entropy, general acid-base catalysis, and electrostatics. ... 

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