Biotechnology - Yu.O. Sazykin 2006

Applied Biotechnology
Challenges in the discovery, development, and application of antibiotics in medical practice
Mechanisms of antibiotic biosynthesis

Three key factors determine the features of Biosynthesis common to all Antibiotics:

✵ antibiotics are not direct products of Translation or template synthesis in general;

✵ antibiotics, as secondary metabolites, are formed from primary metabolites;

✵ The biosynthesis of any antibiotic molecule involves a series of Enzymes.

The coordination of enzyme activity—meaning the Regulation of the proper sequence of enzymatic reactions—is achieved in various ways. One mechanism, demonstrated in the biosynthesis of cyclopeptide and certain Other Antibiotics, involves the synthesis or assembly of the antibiotic molecule within multienzyme complexes featuring ordered spatial arrangements of enzymes. Primary metabolites "enter" the multienzyme complex, where they undergo a series of transformations. Either a completed antibiotic molecule or a large fragment thereof (such as a specific aglycone of a given antibiotic) "exits" the complex. During the "assembly" of the antibiotic carbon Skeleton, various reactions may take place, including methylation or demethylation, carboxylation or decarboxylation, and amination or deamination.

Beta-lactam antibiotics are derived from Amino Acids. The formation of the beta-lactam molecule begins with the Synthesis of the so-called LLD-tripeptide from three L-amino acids—primary metabolites—namely L-alpha-aminoadipic acid, L-Cysteine, and L-valine:

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The Formation of the latter involves an enzyme that closes the peptide bonds, as well as an enzyme that converts L-valine into its optical antipode, D-valine. These are specialized enzymes of antibiotic biosynthesis, distinct from those that catalyze The conversion of primary metabolites in standard metabolic cycles.

Next, the LLD-tripeptide is converted into a monocyclic beta-lactam, meaning the beta-lactam ring is closed. The subsequent stage involves the formation of a five-membered sulfur-containing ring condensed with the beta-lactam ring. All of this indicates the participation of novel enzymes in the biosynthesis of the antibiotic. In the case of benzylpenicillin production, phenylacetic acid (in an activated form) must be present, which results in the release of aminoadipic acid and coenzyme A.

The second pathway involves "expansion"—the enlargement of the five-membered ring into a six-membered one—catalyzed by a specific enzyme known as "expandase." Subsequently, a series of further reactions leads to the formation of the cephalosporin C molecule.

The biosynthetic reactions outlined above serve as an example to re-emphasize a fundamental principle: the molecule of any antibiotic is synthesized with the mandatory participation of a series of enzymes (ranging from 5–10 to many more).

Analysis of the structural formulas of Aminoglycosides—such as streptomycin, gentamicins, and others—suggests that Glucose serves as their precursor. Indeed, numerous studies have shown that glucose gives rise not only to the sugar residues in aminoglycoside molecules, but also to their aminocyclitol moiety. Two other fragments of the streptomycin molecule, a pentose (streptose) and L-glucosamine, are likewise synthesized from glucose via a series of enzymatic reactions. Finally, the assembly of the streptomycin molecule from its three components—streptidine, streptose, and L-glucosamine (via the formation of glycosidic bonds between them)—requires specific enzymes. No fewer than 20 enzymes are involved in the biosynthesis of streptomycin and most other aminoglycoside antibiotics.

When describing the enzymatic reactions involved in the biosynthesis of the tetracycline Structure, researchers typically draw an analogy with the biosynthesis of primary metabolites, such as Fatty acids. These are built from acetate or propionate units following a "HEAD-to-tail" principle, where a bond forms between the carbon of the carboxyl group and the carbon of the methyl group (or methylene group) of the adjacent unit. Coenzyme A participates in these enzymatic reactions.

At the same time, there are fundamental differences between the synthesis of Fatty Acids and that of antibiotics (secondary metabolites). Unlike FATTY ACID BIOSYNTHESIS, antibiotic biosynthesis does not involve the reduction of carbonyl groups following Condensation reactions, or such reduction occurs prior to the formation of hydroxyl groups or double bonds. Full reduction yields aromatic structures, whereas partial reduction yields macrocyclic lactones. Thus, a distinct relationship exists between the "biogenesis" of compounds such as Tetracyclines and macrolide antibiotics. The carbon skeleton of a tetracycline molecule is constructed from one malonamide unit and eight malonate units.

The structural foundation of the erythromycin macrocyclic lactone ring is formed through the enzymatic polymerization of one propionate unit and six methylmalonate units. The sugars of erythromycin originate from glucose via a series of enzymatic transformations. The biosynthesis involves enzymes responsible for assembling the molecule from the macrocyclic lactone and its constituent sugars.

Introduction/32.html">Genetic Engineering Methods are successfully employed to produce recombinant Proteins, given that proteins are the direct products of translation. Overall, the foundational rule remains valid: one Gene, one protein—meaning that a single "structural" gene determines the structure (Amino Acid Sequence) of a single protein.

Unlike the enzymes involved in antibiotic biosynthesis, antibiotics themselves are not direct products of translation. The number of such enzymes reaches several dozen. Consequently, dozens of "structural" genes take part in the biosynthesis of a single antibiotic molecule.