4. Elementary Mechanisms of Enzyme Reactions (Part 1)

The outstanding catalytic, regulatory, and self-preserving properties of enzymes have been developed over the course of several million years of molecular evolution. Proximity and Oriental Effects are an important part of these mechanisms due to the pre-binding of the substrate by unique orientation relative to the catalytic groups. For example, cyclization of aliphatic molecule provides decrease of entropy of DS₀ = – 3.6 − 4.7 eu. Difference in rates is 3–4 orders of magnitude (intramolecular versus intermolecular). In a concerted reaction, when a substrate is simultaneously attracted by different active donor and acceptor reagents such as acid and basic groups, nucleophile and electrophile, or reducing and oxidizing agents a significant decrease in the activation energy is expected. General theoretical considerations in favor of synchronous or sequential mechanisms and specific cases are discussed in a separate section. In the case of effective concerted mechanism, the decrease in the synchronization probability with increasing number of atoms or groups, participating in an elementary stage, can be compensated for by an appreciable decrease in the activation energy due to the inclusion of nuclei of donor and acceptor groups in the process. This consideration has led us to the formulation of the principle of optimum motion (POM). In 1975, the author of this monograph has suggested that rapid electron transfer in photosynthetic reaction centers in the forward direction and significantly slower transfer in the reverse direction may account for the cascade structure of RC which provides tunneling (long-distance) mechanism of the photoseparated charges. The discovery and experimental confirmation of Long-Range Electron Transfer (LRET) between donor (D) and acceptor (A) centers in model and biological systems were described. Data on Photosystem I and photosystem II which are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyze the primary photosynthetic endergonic reactions producing high energy compounds were briefly reviewed. Magnetic isotope effects arise when a chemical reaction involves spin-selective processes proceeded via the radical pair mechanism was described.