**1. Introduction**

Enzymes (E) is a group of biologically active polymers (mainly proteins) that catalyze almost all metabolic reactions in all living organisms. Enzymes are able to accelerate chemical reaction dividing it into separate steps. Because each step of enzymatic reaction has a value of activation energy significantly lower than the value of activation energy for the same chemical reaction, enzymes can increase a rate of reaction 10<sup>6</sup> –1018 folds. According to contemporary hypothesis, high conformational mobility of the enzymes allows them to adopt their active sites to substrate(s) and intermediates of the reaction in the best way [1, 2]. Multiple conformers of enzymes with close values of free energy preexist in the solution simultaneously. Along the reaction way, a conformer is picked out, the structure of which can stabilize definite intermediate that makes a reaction more thermodynamically profitable [3].

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© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This hypothesis is supported by unsuccessful attempts to create catalytically effective low molecular enzymes having needed active site (molecular mass should be higher than 10,000 Da) or enzyme with correct active site but with restricted conformational mobility (antibodies with needed active site, so-called *abzymes* [4]). In the context of described hypothesis, enzyme inhibitors being bound to the enzyme "freeze" it in definite conformation that makes impossible selection of conformers participating in numerous steps of enzymatic reaction. By this way, inhibitors stop enzymatic reaction.

On the other hand, the binding of enzyme activators may lead to the creation of more profitable conformers that can be more effective in carrying out definite steps of the reaction. Therefore, they will accelerate enzymatic reaction. Taking into account this information about enzymes in this chapter, we consider contemporary knowledge about enzyme inhibitors and activators.
