Cofactors and coenzymes

Structure of conjugated enzymes and classification

Globally, all cofactors are divided into two large groups: inorganic (metal ions) and organic (coenzymes).

A conjugated enzyme operates according to a strictly defined scheme. Its structure includes several obligatory components:

• Apoenzyme — the protein moiety exclusively. By itself, it lacks catalytic activity, meaning that in the absence of the coenzyme, the enzyme does not function.
• Coenzyme (or another cofactor) — the non-protein moiety located in the active site.
• Holoenzyme — the assembled, complete, catalytically active complex.

The activation process can be described by an equilibrium reaction: the coenzyme binds to the inactive apoenzyme, resulting in the formation of the working holoenzyme.

The bond between the protein and non-protein parts is of two types:

1. Temporary — the molecule binds to the apoenzyme exclusively at the moment of the chemical reaction.
2. Strong (covalent) — the non-protein part is permanently and very reliably bound to the protein. In this case, it is given a special name — prosthetic group.

Functions of metal ions

Metal ions act as cofactors and participate in enzyme function in four main ways:

1. Change in substrate conformation. The metal interacts with the substrate, ensuring its ideal (complementary) interaction with the active site of the protein. A classic example is when the substrate is not a pure molecule, but the Mg²⁺—ATP complex.
2. Provision of the native conformation of the active site. Metal ions (such as Mg²⁺, Mn²⁺, Zn²⁺, Co²⁺, Mo²⁺) stabilize the active site itself. This facilitates the attachment of the organic coenzyme.
3. Stabilization of the quaternary structure of the protein molecule. Metals help maintain the complex conformation of the entire protein. For example, zinc ions are critically required for the stabilization of alcohol dehydrogenase, the enzyme that catalyzes the oxidation of ethanol.
4. Direct involvement in enzymatic catalysis. Here, metals can function in two ways. First, they mediate electrophilic catalysis (ions such as Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺ are known for this). Second, variable-valence metals participate in oxidation-reduction (redox) reactions by mediating electron transfer. A prime example is cytochromes (heme-containing proteins), where the iron ion continuously accepts and donates an electron according to the scheme: $Fe^{2+} \rightleftarrows Fe^{3+} + e^-$.

Organic coenzymes and reaction types

Coenzymes are complex organic substances, most commonly vitamin derivatives. They are located directly within the active site of the enzyme and take direct part in the catalytic act.

Various coenzymes specialize in strictly defined types of chemical transformations.

Examples of matches between coenzymes and chemical reactions:

• Oxidation-reduction (redox): coenzymes such as NAD⁺, FAD, and NADP⁺ traditionally participate in these processes.
• Carboxylation: reactions involving the addition of a carboxyl group typically proceed with the mandatory participation of biotin.
• Transamination, acylation, and acetylation: these types of reactions require specific chemical group carriers, among which pyridoxal phosphate, biotin, and NAD⁺ appear (depending on the specific pathway).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of electrophilic catalysis involving zinc and copper ions
• Structure and functions of heme-containing proteins (cytochromes)
• Vitamins as the main precursors of organic coenzymes
• The role of the Mg²⁺—ATP complex in biochemical processes
• Detailed mechanism of pyridoxal phosphate action in transamination

Other topics in the "Enzymology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.