**2. Choice of the target enzyme**

The sequencing of the human genome has promised a revolution in medicine. The genome encodes 20,000- 25,000 human genes, and thousands more proteins as a result of alternative gene splicing. Many of these hold the keys to treating disease, especially numerous enzymes of undefined so far physiologic functions [Gonzaga-Jagureui et al., 2012]. Out of 1200 registered drugs over 300 act as enzyme inhibitors. Most of them are simple analogs of substrates of cer‐ tain enzymatic reaction. Analogy to transition state as a mean to obtain effective inhibitors emerged in 1970s [Lienhard, 1973]. Through the 1970s and 1980s, most of the known examples were natural products [Wolfenden, 1976]. The situation has changed in 1990s when synthetic inhibitors became the predominate examples of transition-state inhibitors. In 1995, there were transition-state analogues for at least 130 enzymes [Radzicka & Wolfenden, 1995].

**Figure 1.** Progress of the enzymatic reaction versus uncatalyzed one.

The design of transition-state inhibitors is likely to become more frequent in the future, alongside with the development of theory and technology for understanding enzyme transi‐ tion states. Today the sequence of information required to obtain transition state analog of enzymatic reaction considers: choice of the suitable enzyme (most likely suited to kinetic isotope effect measurement), selection of presumable mechanism(s) of catalyzed reaction, measurement of kinetic isotope effects (KIE), computer-aided calculations matching the in‐ trinsic KIEs, construction of steric and electronic map of transition state and synthesis of sta‐ ble compound(s) matching this map [Schramm, 2007]. This procedure has been developed gradually in parallel with the advances in KIE enzymology, computational chemistry, and synthetic organic chemistry.
