Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Kinetics of Enzyme-Catalyzed Reactions
Regulation of Enzyme Activity

Not only substrates but also other substances interacting with Enzymes can alter, or modulate, their catalytic activity; these are called modulators or effectors. Effectors may be normal cellular components, or they may enter The Cell from the environment or act on isolated enzymes. This section will focus primarily on inhibitors—substances that decrease enzymatic activity. It should be noted, however, that cases of enzyme activation by substrates are also known.

The interaction of an enzyme with an effector is a chemical reaction and can therefore be fully reversible, partially reversible, or practically irreversible. Well-known irreversible inhibitors include poisons, such as the cyanide ion, which inactivates xanthine oxidase, and a group of compounds known as nerve agents; the latter irreversibly inactivate cholinesterases (enzymes involved in Nerve Impulse transmission systems and thus in motor activity). If the inhibition process is irreversible, The kinetics of inhibitor-dependent enzymatic reactions do not obey the Michaelis–Menten Equation, which is based on the existence of an equilibrium between the free and bound forms of the enzyme. Often, as more and more enzyme molecules are completely inactivated, The process of irreversible inhibition progresses over time. In other cases, which are more difficult to detect, the enzyme is only partially inactivated and retains catalytic activity, albeit to a lesser extent compared to the pure enzyme.

In Section 2.6, we already discussed the remarkable ability of most microorganisms to produce A wide variety of complex compounds from a relatively small number of simple precursors. To achieve this, a system is required for the highly efficient distribution of incoming precursors among many biosynthetic pathways leading to various End products of METABOLISM. In most cases, THE PRINCIPLE OF optimal utilization of available starting material requires that any end product be synthesized only in the strictly necessary amount. If, for example, a cell has a sufficient amount of a given monomer, its further accumulation must be halted so that cellular resources can be directed toward the Synthesis of Other compounds whose amounts are relatively low at that moment.

Class="center">

FIG. 3.19. Biosynthetic pathway of L-isoleucine, regulated by feedback inhibition; here, The activity of enzyme E₁ (L-Threonine deaminase) is reduced in the presence of the end product of the pathway, L-isoleucine. (P₁ = α-ketobutyrate; P₂ = α-acetohydroxybutyrate; P₃ = α,β-dihydroxy-β-methylvalerate; P₄ = α-keto-β-methylvalerate.)

One of The regulatory mechanisms used by the cell to achieve the most efficient utilization of nutrients is the reversible REGULATION OF ENZYMATIC Activity. (Another important control mechanism will be discussed in Chapter 6, Section 1.) The most interesting Examples of ENZYME ACTIVITY REGULATION involve a complex network of reactions with multiple regulatory loops; we will consider these examples later, after studying the BASICS OF CELL chemistry (Chapter 5). For now, an example of The regulation of a simple reaction pathway will suffice. Fig. 3.19 shows the five-step biosynthetic pathway of

The amino acid L-isoleucine. This reaction pathway is regulated via feedback inhibition, where the end product, L-isoleucine, inhibits the activity of The enzyme catalyzing the first step of the pathway. Thus, The Biosynthesis of the pathway's end product will be suppressed as it accumulates.

Table 3.7. Some types of reversible inhibition^a

^a From: Dixon M., Webb E., Enzymes. — Moscow: Mir, 1966.

Since the Reactions Catalyzed by enzymes E₂—E₅ are in equilibrium, and the first reaction in the cellular environment is "irreversible," the response rate of such a regulatory system is very high. In fact, most regulatory enzymes catalyze precisely these "irreversible" reactions. From the standpoint of cell biology, it is obvious that such "natural" Introduction/15.html">Regulation of enzyme Activity must be reversible. If, for example, all L-isoleucine has been consumed during Protein Synthesis, the inhibition of enzyme E₁ must cease, and the system must once again synthesize the required amount of this amino acid.

In the case of reversible inhibition, the approaches to kinetic analysis outlined in Section 3.2 have proven highly fruitful. Since many isolated enzyme-substrate systems obey Michaelis–Menten kinetics, it is customary to classify inhibitors according to their effect on the parameters of the Michaelis–Menten equation, v_max and K_m.

Reversible inhibitors are called competitive if they increase the K_m value while v_max remains unchanged. The effect produced by this type of inhibitor can be partially or completely overcome by increasing the Substrate Concentration. Non-Competitive Inhibitors, on the other hand, inactivate the enzyme or the enzyme-substrate complex by decreasing the v_max of the enzyme without affecting the K_m value. In this case, increasing the substrate concentration to an arbitrarily high level will no longer increase the reaction rate to the value characteristic of the uninhibited enzyme. Typical examples of non-competitive inhibitors are heavy Metal Ions, which react reversibly with the sulfhydryl (—SH) groups of Cysteine residues.

Several combinations and variations of these two MAIN TYPES OF reversible inhibitors are known; some of them are listed in Table 3.7. A bit later, we will look at experimental Methods for determining the type of inhibition, but first, we will briefly discuss some theories explaining the Mechanisms of action of enzyme activity regulators, along with the corresponding experimental data.