Oxytetracycline belongs to a structurally diverse class of aromatic polyketide antibiotics produced by Streptomyces via type II polyketide synthases (PKSs) which are also known as bacterial aromatic polyketides.
Other compounds produced via type II PKSs are important bioactive compounds which span from anticancer agents doxorubicin to antibiotics such as tetracycline.
The biosynthesis of oxytetracycline can be broken down into three general portions; first is the formation of an amidated polyketide backbone with minimal PKS's, second is the cyclization of the polyketide backbone and finally, the formation of anhydrotetracycline - a shared intermediate with tetracycline - to produce oxytetracycline.


The biosynthesis of oxytetracycline begins with the utilization of PKS enzymes ketosynthase (KS), the chain length factor (CLF), the acyl carrier protein (ACP), and an acyltransferase (encoded as OxyA, OxyB, OxyC and OxyP in the oxytetracycline gene cluster) to catalyze the extension of the malonamyl-CoA starting unit with 8 malonyl-CoA extender units.
The process of elongating the polypeptide skeleton occurs through a series of Claisen-like decarboxylation reactions until the linear tetracyclic skeleton is formed.
Thus, minimal PKS's form a completed amidated polyketide backbone without any additional post-synthase tailoring enzymes (Figure 1).

Following the formation of the linear tetracyclic skeleton, four successive cyclization reactions must occur in a regioselective manner to produce the aromatic natural product known as pretetramid - a common precursor to both oxytetracycline and other tetracycline antibiotics.

In the oxytetracycline gene cluster, these enzymes are encoded as OxyK (aromatase), OxyN (cyclase), and OxyI (cyclase).
Formation of pretetramid allows for one of the most important intermediates en route to the biosynthesis of oxytetracycline; this is the generation of anhydrotetracycline.
Anhydrotetracycline contains the first functionalized A ring in this biosynthetic pathway.

After the formation of anhydrotetracycline, ATC monooxygenase (OxyS) oxidizes the C-6 position in an enantioselective manner in the presence of the cofactor NADPH and atmospheric oxygen to produce 5a,11a-dehydrotetracycline.
Next, a hydroxylation occurs at the C-5 position of 5a,11a-dehydrotetracycline via the oxygenase encoded as OxyE in the oxytetracycline gene cluster.
This produces the intermediate 5a,11a-dehydro-oxytetracycline.
However, the exact mechanism of this step remains to be unclear.
The final step of this biosynthesis occurs through the reduction of a double bond in the α, β - unsaturated ketone of 5a,11a-dehydro-oxytetracycline.
In this final step, the cofactor NADPH is employed by TchA (reductase) as the reducing agent.
Upon reduction, the enol form is favored due to conjugation, thus producing the aromatic polyketide oxytetracycline.
Figure 2 shows the biosynthesis as described above, as well as an arrow-pushing mechanism of NADPH being used as the final cofactor in the biosynthesis of oxytetracycline.
Oxytetracycline belongs to a structurally diverse class of aromatic polyketide antibiotics produced by Streptomyces via type II polyketide synthases (PKSs) which are also known as bacterial aromatic polyketides. Other compounds produced via type II PKSs are important bioactive compounds which span from anticancer agents doxorubicin to antibiotics such as tetracycline. The biosynthesis of oxytetracycline can be broken down into three general portions; first is the formation of an amidated polyketide backbone with minimal PKS's, second is the cyclization of the polyketide backbone and finally, the formation of anhydrotetracycline - a shared intermediate with tetracycline - to produce oxytetracycline. 

The biosynthesis of oxytetracycline begins with the utilization of PKS enzymes ketosynthase (KS), the chain length factor (CLF), the acyl carrier protein (ACP), and an acyltransferase (encoded as OxyA, OxyB, OxyC and OxyP in the oxytetracycline gene cluster) to catalyze the extension of the malonamyl-CoA starting unit with 8 malonyl-CoA extender units. The process of elongating the polypeptide skeleton occurs through a series of Claisen-like decarboxylation reactions until the linear tetracyclic skeleton is formed. Thus, minimal PKS's form a completed amidated polyketide backbone without any additional post-synthase tailoring enzymes (Figure 1). 
Following the formation of the linear tetracyclic skeleton, four successive cyclization reactions must occur in a regioselective manner to produce the aromatic natural product known as pretetramid - a common precursor to both oxytetracycline and other tetracycline antibiotics.
In the oxytetracycline gene cluster, these enzymes are encoded as OxyK (aromatase), OxyN (cyclase), and OxyI (cyclase). Formation of pretetramid allows for one of the most important intermediates en route to the biosynthesis of oxytetracycline; this is the generation of anhydrotetracycline. Anhydrotetracycline contains the first functionalized A ring in this biosynthetic pathway. 
After the formation of anhydrotetracycline, ATC monooxygenase (OxyS) oxidizes the C-6 position in an enantioselective manner in the presence of the cofactor NADPH and atmospheric oxygen to produce 5a,11a-dehydrotetracycline. Next, a hydroxylation occurs at the C-5 position of 5a,11a-dehydrotetracycline via the oxygenase encoded as OxyE in the oxytetracycline gene cluster. This produces the intermediate 5a,11a-dehydro-oxytetracycline. However, the exact mechanism of this step remains to be unclear. The final step of this biosynthesis occurs through the reduction of a double bond in the α, β - unsaturated ketone of 5a,11a-dehydro-oxytetracycline. In this final step, the cofactor NADPH is employed by TchA (reductase) as the reducing agent. Upon reduction, the enol form is favored due to conjugation, thus producing the aromatic polyketide oxytetracycline. Figure 2 shows the biosynthesis as described above, as well as an arrow-pushing mechanism of NADPH being used as the final cofactor in the biosynthesis of oxytetracycline.
Oxytetracycline