## 1 Introduction
### PHA Synthesis  
Some options for production improvement and the synthesis of new PHAs are the control of synthesis through the manipulation of regulatory elements such as enzymes and proteins and the engineering of biosynthesis pathways [25]. Consequently, it is necessary to comprehend these mechanisms in order to visualize the process in its entirety, as well as to use this knowledge to increase the yield and purity of the compounds produced. Some of the central metabolic pathways, such as the Krebs cycle, glycolytic and pentose-phosphate pathways, and degradation and biosynthesis pathways of fatty acids and amino acids, are directly or indirectly linked to the biosynthetic pathways of PHAs. The 14 pathways of PHA synthesis described so far demonstrate the importance of acetyl-CoA as a precursor to the biosynthesis of several SCL- and MCL-PHAs, such as PHB synthesis (Figure 3) which consists of three reactions catalyzed by acetyl-CoA acetyltransferase (\(\beta\)-ketothiolase; PHAA) catalyzing the condensation of two acetyl-CoA molecules into acetacetyl-CoA, acetoacetyl-CoA reductase (PhaB) converting acetoacetyl-CoA into (\(R\))-3-hydroxybutyl-CoA[(\(R\))-3-HB-CoA], and PHA synthase (PhaC) that polymerizes (\(R\))-3-HB-CoA in a PHB chain [21]. Under normal growth conditions, \(\beta\)-ketothiolase would be inhibited by the free coenzyme-A from the Krebs cycle. However, during the scarcity of noncarbonic nutrients in the substrate, the entry of acetyl-CoA becomes restricted, and the surplus is directed to the synthesis of PHB [26]. Figure 2: Examples of most common SCL-PHAs. The synthesis of SCL-PHA occurs in Gram-positive, Gram-negative, and some haloarchae species, with the majority of SCL-PHA producers able to accumulate PHB, and some species capable of producing the aforementioned copolymers. As stated earlier, acetyl-CoA plays a crucial part in the synthesis of SCL-PHA due to its function as a precursor for the 3 HB monomer, which forms the PHB. Succinyl-CoA serves as the precursor for the synthesis of 4-hydroxybutyrate (4 HB), an essential monomer. Conversely, 3-hydroxyvalerate (3 HV) is derived from propionyl-CoA, acetyl-CoA, and, in certain instances, intermediates of \(\beta\)-oxidation of valerate or other fatty acids. The previously published mechanism for the production of 3 HB monomer provides a broad understanding of the biosynthesis of SCL-PHA. Nevertheless, it is important to note that certain modifications may arise, particularly in situations when, in the first stage, condensation takes place between an acetyl-CoA and a propionyl-CoA molecule. This process results in the formation of 3-ketovaleryl-CoA subsequent to the reduction phase facilitated by 3-ketoacyl-CoA reductase, leading to the production of a molecule known as (R)-3-HV-CoA. The conversion of succinyl-CoA to succinate semialdehyde is facilitated  
Figure 3: Synthesis of PHB with acetyl-CoA from the Krebs cycle as the precursor. by the activity of succinate semialdehyde dehydrogenase, which acts as a catalyst for this reaction. Subsequently, the molecule succinate semialdehyde undergoes oxidation through the action of 4-hydroxybutyrate dehydrogenase, leading to the formation of 4 HB. In both cases, the monomers will ultimately be polymerized by the PHA synthases into PHB or one of its copolymers, PHBV, poly(3-hydroxybutyrate-co-4-hydroxybutyrate), and poly(3-hydroxybutyrate-co-3-hydroxyalerate-co-4-hydroxybutyrate) (PHBV4 HB) [25]. MCL-PHA can be synthesized in two ways: \(\beta\)-oxidation and new synthesis of fatty acids. These two pathways work interactively, with \(\beta\)-oxidation being the main route for the synthesis of MCL-PHA from fatty acids through their degradation, while the fatty acid synthesis pathway functions as the main metabolic pathway to obtain MCL-PHA from carbohydrates.