Chemistry and Biology of Proteins - F. Haurowitz 1953

Proteins with Enzymatic Properties
Enzymatic catalysis of oxidation-reduction reactions

Unlike Hydrolysis, oxidation-reduction reactions invariably involve two substrates: one undergoes oxidation, while the other is reduced. Oxygen frequently acts as the latter substrate, being reduced to hydrogen peroxide and Water.

It was initially believed that the primary role in these processes belonged to the prosthetic group of the enzyme. It was assumed that this group binds simultaneously to both substrates, accepting electrons (or hydrogen atoms) from the oxidizable substrate and transferring them to the reducible substrate. This assumption, however, was not confirmed, as it turned out that the Specificity of dehydrogenases is often determined by their protein component, which indicates that at least one of the substrates binds to the apoenzyme [104].

Many redox reactions are irreversible. This applies particularly to reactions in which oxygen is reduced, since reversing such a reaction (i.e., forming oxygen from water) requires a substantial energy input. It is worth recalling that catalysts do not affect THE POSITION OF equilibrium of a catalyzed reaction, but merely facilitate and accelerate the attainment of equilibrium. Let us consider The formation of water from hydrogen and oxygen as an example:

Class="center">О₂ + 2Н₂ ⇄ 2Н₂О.

For this reaction, the equilibrium is heavily shifted toward the right side of the equation; consequently, the enzyme practically catalyzes only the forward reaction, rather than the reverse.

For many Enzymes catalyzing oxidation-reduction reactions, the Chemical Nature of their prosthetic groups has been elucidated. Some of these are known to contain a metal as their prosthetic group, which can exist in various valence states, while others contain organic molecules capable of transitioning between oxidized and reduced forms. Therefore, some authors suggest that the prosthetic group of oxidoreductases, by alternately undergoing Oxidation and reduction, thereby transfers electrons from one substrate to another. This can be schematically represented by the following reactions:

where DH₂ is the hydrogen and electron donor, A is the acceptor, and E is the enzyme. It is also possible that intermediate, readily dissociating enzyme-substrate complexes are formed in the process

(DH₂) Е → D (ЕН₂) → (EH₂) А → Е (АН₂).

According to the author [105, 106], in many cases there is no need to assume the obligatory oxidation or reduction of the enzyme itself; the enzyme likely catalyzes the redox process without undergoing true oxidation and reduction [107]. This can be visualized as E accepting hydrogen atoms without being reduced to EH₂, instead catalyzing the reaction DH₂ + А → D + АН₂, during which the Formation of the following intermediates is possible [32, 33, 107, 108]:

From this perspective, the enzyme binds both substrates (both the hydrogen donor and acceptor), thereby bringing these substances into the close proximity required for the reaction to proceed.

In cases where one of the substrates is an electron, the enzyme will indeed be reduced upon accepting it. If the reactive group of the enzyme capable of accepting an electron is an organic molecular residue, two electrons and two protons are typically added to this group, and the enzyme thus undergoes divalent reduction. This reaction is reversible; otherwise, the enzyme would be incapable of transferring electrons from the oxidizable substrate. The combination of the enzyme with an electron or hydrogen is usually referred to not as an enzyme-substrate complex, but as a reduced enzyme, or dihydroenzyme.

If the prosthetic group of the enzyme contains a metal atom capable of accepting only a single electron, this atom will transition between the oxidized and reduced (ferrous/cuprous) states. For example, ferric iron or cupric copper in such enzymes is converted into ferrous iron and cuprous copper, respectively. According to Michaelis [109], the reversible reduction of enzymes always proceeds as a univalent reaction, even when the enzyme accepts two or more electrons. In the latter case, electron addition occurs sequentially, with the intermediates of this stepwise reduction being free radicals, as evidenced by their intense coloration and paramagnetic susceptibility.

In many instances, however, it is difficult to determine whether the metal atom in metalloenzymes undergoes oxidation and reduction or merely binds to one of the substrates to form an enzyme-substrate complex. Admittedly, the distinction between these two reactions is rather subtle, as the following formulas show:

where is the reactive trivalent iron atom of the iron-containing enzyme, and (: D) is the electron donor.

In formula (I), the substrate is bound to the trivalent iron atom to form a complex, whereas in formula (II), the trivalent iron accepts a single electron, forming divalent iron and a radical (• D). It is easy to conceive of the existence of intermediates between (I) and (II), such as Fe (• D) or Fe⁺ • (• D). In these complexes, the iron atom is neither divalent nor trivalent, but possesses an intermediate valence [106]. METABOLISM/2.html">THE CONCEPT OF intermediate valence was proposed by Pauling [111] to explain certain properties of metals. By analogy, one might assume that a similar state of metal atoms occurs during the catalytic action of enzymes.

Subsequent sections will examine only those oxidation-reduction enzymes that have been obtained in a more or less purified state and whose prosthetic group nature has been established; the Classification of these enzymes is therefore based on the chemical nature of their prosthetic groups.