The Regulation of Enzyme Activity

Enzymes function as biocatalysts and, as such, are involved in all metabolic reactions. Characteristic for enzymes is their high efficiency, high specificity and extreme stereoselectivity, as well as their ability to be regulated. Analogous to chemical catalysts, enzymes do not alter the equilibrium of a reaction, but only accelerate the establishment of the equihbrium of the reaction. 

The mechanism of action of enzymes can best be described with the aid of transition state theory. On the pathway from substrate A to product B in a reaction catalyzed by a chemical catalyst or an enzyme A passes through a transition state A which is found at the highest point of the energy diagram (fig. 2.1). The energy difference between the ground state of A and the transition state A represents the activation energy. The transition state as such can not be isolated. It is the state of A in which the bonds 

For the tight binding of the transition state the binding surface of the enzyme must be complementary to the structure of the transition state, so that optimal interactions between the enzyme and the transition state are possible. This demand imphes that enzymes display a high affinity to molecules which are chemically similar to the transition state of the reaction. Complexes of such transition state analogues with enzymes are well suited for X-ray structure analysis to elucidate the structural principles of the active site and the catalytic mechanism. 

The pathway from enzyme-bound substrate to the transition state involves changes in the electronic configuration and geometry of the substrate. The enzyme itself is also not static. The ability to tightly bind the transition state requires flexibUity in the active site. Such flexibility has been experimentally demonstrated in many cases. A corollary to this is that the effectivity of enzyme catalysis can easily be influenced and regulated by conformational changes in the enzyme. An extensive consideration of the mechanisms of enzymes can be found in the works by J. Kraut (1988) and A. Fersht (1998). 

The ability of proteins to exist in different conformations is termed allostery (see 2.2). Allosteric enzymes can assume various conformations which differ in catalytic activity and/or substrate binding capacity. 

In a broad sense, enzyme regulation operates at two main levels, either by changing the concentration of an enzyme or by altering the activity of preexisting enzymes. The concentration of enzymes can be regulated, e.g., via 

In contrast, regulation via modulation of the activity of preexisting enzymes is often used for a quick response to internal or external stimuli, allowing short-term changes in the rates of biochemical reactions. 

Biochemistry of Signal Transduction and Regulation. 3rd Edition. Gerhard Krauss Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30591-2 

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Box 3.7 Discovery of reversible phosphorylation in the regulation of enzyme activity... 

Hilhorst, R., Spruijt, R., Laane, C., and Veeger, C. (1984). Rules for the regulation of enzyme-activity in reversed micelles as illustrated by the conversion of apolar steroids by 20-beta-hydroxysteroid dehydrogenase. Eur. J. Biochem., 144, 459-66. 

The phosphorylation of enzymes by specific protein kinases is a widespread mechanism for the regulation of enzyme activity. It represents a flexible and reversible means of regulation and plays a central role in signal transduction chains in eucaryotes. 

Proteins are phosphorylated mainly on Ser/Thr residues and on Tyr residues. Occasionally Asp or His residues are phosphorylated, the latter especially in procaryotic signal transduction pathways (see chapter 7, chapter 12). For the regulation of enzyme activity the phosphorylation of Ser and Thr residues is most significant. Apart from regulation of Tyr kinases, Tyr phosphorylation serves the function of creating specific attachment sites for proteins. Both of these functions will be discussed in more detail in chapter 8. 

We will consider in this chapter the general processes by which enzymes achieve enhancement of reaction rates, basic chemical and enzymatic kinetics and inhibition, the roles of cofactors and coenzymes, the effects of environmental factors, the regulation of enzyme activity, and some clinical applications of enzymology. 

In addition, PolyPs are most likely involved in the regulation of enzyme activities by participation in their phosphorylation. A protein phosphorylation process, using not ATP but high-polymer PolyPs, was revealed in the archae Sulfolobus acidocaldarius (Skorko, 1989). Tripolyphosphate was observed to be a phosphodonor of selective protein phosphorylation of rat liver microsomal membrane (Tsutsui, 1986). 

Table 15-1 Established Forms of Covalent Modification in the Regulation of Enzyme Activity...

Sucrose is the sole source for synthesis of D-fructan in artichoke tubers.206 Thus, the level of this disaccharide can play an important role in the regulation of enzyme activities.202,203,207 Sucrose strongly inhibits the enzymic hydrolysis of inulin by D-fructan hydrolases, as... 

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