BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.3. Cofactors and Coenzymes

7.3.2. Coenzymes

As noted previously, the catalytic activity of most Enzymes requires the presence of a coenzyme. An exception is represented by hydrolytic enzymes (proteases, lipases, Ribonuclease), which perform their function in the absence of a coenzyme.

Located within the catalytic region of the Active Site, the coenzyme takes a direct part in the chemical reaction, acting as an acceptor and donor of chemical groups, atoms, or electrons. A coenzyme can be bound to the protein moiety of the molecule via covalent or non-covalent bonds. In the former case, it is termed a prosthetic group (FAD, FMN, biotin, Lipoic Acid, etc.). However, instances are known where a coenzyme binds to the enzyme via non-covalent bonds so tightly that it does not dissociate from the protein molecule, such as Thiamine diphosphate. In the latter case, the coenzyme interacts with the enzyme only during the chemical reaction and can be regarded as a second substrate. Examples include NAD⁺ and NADP⁺.

The apoenzyme provides reaction Specificity and is responsible for selecting the type of chemical transformation of the substrate. The same coenzyme, when interacting with different apoenzymes, can participate in various Chemical transformations of the substrate. For instance, depending on the apoenzyme it interacts with, Pyridoxal phosphate participates in AMINO ACID Transamination or decarboxylation reactions.

The Chemical Nature of Coenzymes and Their Functions in enzymatic reactions are extraordinarily diverse. Traditionally, vitamin derivatives are classified as coenzymes, although aside from them, a significant Class of non-protein compounds participates in the expression of the catalytic function of enzymes. Coenzymes include the following compounds:

✵ vitamin derivatives;

✵ Hemes, which are components of Cytochromes, catalase, peroxidase, guanylyl cyclase, and NO synthase, serving as the prosthetic group of these enzymes;

✵ NUCLEOTIDES, which act as Donors and acceptors of phosphoric acid residues;

✵ ubiquinone, or coenzyme Q, involved in The transport of electrons and protons in the Respiratory Chain;

✵ phosphoadenosyl phosphosulfate, which participates in Sulfate transfer;

✵ S-adenosylmethionine (SAM), a methyl group donor;

✵ Glutathione, which participates in oxidation-reduction reactions.

The Structure AND FUNCTIONS of these coenzymes are examined in detail in the relevant chapters of the textbook.