Study of Class I and Class III Polyhydroxyalkanoate (PHA) Synthases with Substrates Containing a Modified Side Chain.

Polyhydroxyalkanoates (PHAs) are carbon and energy storage polymers produced by a variety of microbial organisms under nutrient-limited conditions. They have been considered as an environmentally friendly alternative to oil-based plastics due to their renewability, versatility, and biodegradability. PHA synthase (PhaC) plays a central role in PHA biosynthesis, in which its activity and substrate specificity are major factors in determining the productivity and properties of the produced polymers. However, the effects of modifying the substrate side chain are not well understood because of the difficulty to accessing the desired analogues. In this report, a series of 3-(R)-hydroxyacyl coenzyme A (HACoA) analogues were synthesized and tested with class I synthases from Chromobacterium sp. USM2 (PhaCCs and A479S-PhaCCs) and Caulobacter crescentus (PhaCCc) as well as class III synthase from Allochromatium vinosum (PhaECAv). It was found that, while different PHA synthases displayed distinct preference with regard to the length of the alkyl side chains, they could withstand moderate side chain modifications such as terminal unsaturated bonds and the azide group. Specifically, the specific activity of PhaCCs toward propynyl analogue (HHxyCoA) was only 5-fold less than that toward the classical substrate HBCoA. The catalytic efficiency (kcat/Km) of PhaECAv toward azide analogue (HABCoA) was determined to be 2.86 × 10(5) M(-1) s(-1), which was 6.2% of the value of HBCoA (4.62 × 10(6) M(-1) s(-1)) measured in the presence of bovine serum albumin (BSA). These side chain modifications may be employed to introduce new material functions to PHAs as well as to study PHA biogenesis via click-chemistry, in which the latter remains unknown and is important for metabolic engineering to produce PHAs economically.