Fundamentals of Biochemistry - A. A. Anisimov 1986

Enzymes
Types of Enzymatic Reactions

Depending on the number of participants, enzymatic reactions are divided into single-substrate and two-substrate reactions. Reactions involving a larger number of substrates are quite rare. The most common type of enzymatic reaction is a two-substrate reaction yielding two products: A + В ⇄ C + Z). This type accounts for approximately 60% of all known reactions, primarily group transfer reactions.

Single-substrate, single-product reactions are relatively rare; Examples include isomerization reactions, notably glucose-1-phosphate⇄glucose-6-phosphate. However, many reactions are close to single-substrate reactions in their properties. This occurs when the concentration of only one of the two substrates varies. For instance, hydrolytic reactions can be treated as single-substrate because the concentration of the second substrate, Water, can be considered constant, as it decreases insignificantly during the reaction.

Reactions with a single substrate are essentially monomolecular Chemical Reactions (A→P). Most of them belong to 1st-order reactions, since the reaction rate is proportional to the concentration of only one reacting substance (V=k₁[S]). The reaction order is determined by The Nature of its dependence on Substrate Concentration. The rate of a monomolecular reaction may be independent of the substrate concentration (when the substrate is in excess), in which case the reaction order is zero (V = k₀, where k₀ is the zero-order reaction rate constant).

First-Order Reactions also include bimolecular (two-substrate) reactions. This happens when the concentration of one of the substrates is very high compared to that of the other, as is the case in Hydrolysis reactions. Most bimolecular reactions are 2nd-order reactions, because their rate is proportional to the concentrations of two reacting substances or, less frequently, to the square of the substrate concentration: (V = k₂[S₁][S₂] or V = k₂[S]², where k₁ and k₂ are the rate constants of the corresponding reaction order).

It should be noted that actual reaction rates do not necessarily correspond strictly to a specific order, and mixed-type reactions are frequently observed. Furthermore, the reaction order may change over time and, as will be shown below, depend on the substrate concentration. Therefore, the reaction order is determined with respect to the "current" substrate concentration. In addition, enzymatic reactions proceed in several consecutive mono- and bimolecular steps, and thus the molecularity of the overall reaction does not necessarily coincide with its order.

Depending on the order of substrate binding in two-substrate reactions (A + B ⇄ C+D), Two main mechanisms of these reactions are distinguished.

1. The double-displacement mechanism, which is well illustrated by the following equation:

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First, one substrate (A) binds to the enzyme to form the EA complex; As a result of the first reaction step, it is converted into product C and intermediate F (a slightly modified form of E). This intermediate then binds the second substrate (B) to form the FB complex, which dissociates into the free enzyme (E) and the second product (D). The double-displacement mechanism is also known as the "ping-pong" mechanism. Transamination reactions, for example, proceed via this pathway.

Thus, in the transamination reaction (glutamate + oxaloacetate → a-ketoglutarate + aspartate) catalyzed by aspartate aminotransferase (B), glutamate serves as the first substrate (A) to bind to the enzyme containing Pyridoxal phosphate as its non-protein moiety. This forms the EA complex containing an aldimine, which is subsequently converted into the FC complex, where the non-protein moiety of the enzyme and the bound substrate form a ketimine. Hydrolysis of the ketimine releases the first reaction product (C)—a-ketoglutarate (2-oxogluturate)—and intermediate F, which represents a modified form of enzyme E in which pyridoxamine phosphate replaces pyridoxal phosphate as the non-protein moiety. Intermediate F then binds the second substrate (B)—oxaloacetate—yielding the FB complex containing a ketimine. The ketimine is then converted into an aldimine, after which the second product (D), aspartate, is formed and the free enzyme (E) is released.

2. The sequential mechanism, in which both substrates must bind to the enzyme to form a ternary complex before any product is released. The sequential mechanism is divided into two types: Ordered and Random. The ordered mechanism has the following equation:

In the case of a random sequential mechanism, the enzyme features two independent substrate-binding sites in its active center, and their binding to the enzyme can occur in any sequence. Similarly, there is no fixed order for the release of reaction products from the active center. Such reactions are represented by the following scheme:

NAD(P)⁺-dependent dehydrogenases operate via an ordered mechanism, for instance. Thus, The sequence of reactions occurring with the participation of LDH is written as follows:

Certain Glycosyltransferases and creatine kinase also operate via an ordered mechanism.

Reactions involving three and four substrates proceed via one of the two aforementioned mechanisms or through a mixed mechanism.