Latest revision as of 09:18, 23 September 2023

T7-RBS-SpyTag-PCLase2-SpyCatcher-Tr

Polycaprolactone (PCL) is a biodegradable plastic material that has received FDA approval. It's extensively used in various applications such as tissue engineering, drug delivery, food packaging, and agricultural coverings¹. Typically, products made from PCL take 2-3 years to decompose. As a result, there's a growing interest in identifying novel enzymes that can degrade PCL or in optimizing those that are currently available commercially². However, most of them can’t tolerate at extremely high temperatures, including the boiling temperatures encountered during the thermoforming process of shaping PCL plastic.

In Prof. Fan Li's laboratory, novel PCL-degrading enzymes, PCLase I and PCLase II, were identified and purified from Pseudomonas hydrolytica³. This discovery was made possible by cultivating the bacteria in a PCL-emulsified medium. The team conducted an in-depth study of the PCLase enzymes, examining aspects such as enzyme activity, the influence of pH and temperature, substrate specificity, degradation products, as well as the associated gene sequences and protein structures. Learned with this comprehensive data and the enzymes' impressive PCL-degrading efficiency, our aim is to enhance their thermostability.

SpyRing cyclization technique to enhance enzyme thermal resilience was clarified by Dr. Mark Howarth’s team⁴. SpyRing harbors genetically modified SpyTag (13 amino acids) on the N-terminus and SpyCatcher (12kDa) on the C-terminus on the protein of interest. This context spontaneously reacts together through an irreversible isopeptide bond. SpyRing cyclization was demonstrated successfully to increase stress resilience of β-lactamase and some industrially important enzymes. With this synthetic biology tool, we plan to employ SpyRing cyclization techniques and expect to make PCLase thermal resistent, as demonstrated in our work with the SpyTag-GFP-SpyCatcher construct (Part:BBa_K4652002).

PLASMID CONSTRUCTION

PCLase I and PCLase II gene sequences with N-terminal SpyTag and C-terminal SpyCatcher were synthesized by Integrated DNA Technologies, Inc. (IDT) and then cloned into pSB1C3, respectively (SpyTag-PCLase1-SpyCatcher, Part:BBa_K4652008; SpyTag-PCLase2-SpyCatcher, Part:BBa_K4652012). Then, the parts were connected with a T7 promoter (Part:BBa_K1833999), a strong RBS (Part:BBa_B0030), and a double terminator (Part:BBa_B0015). The final construct was verified using colony PCR (Figure 1) and further validated through DNA sequencing. These resultant constructs were designated as T7-SpyTag-PCLase1-SpyCatcher (Part:BBa_K4652010) and T7-SpyTag-PCLase2-SpyCatcher (Part:BBa_K4652013), respectively.





Figure 1. Verification of T7-SpyTag-PCLase1-SpyCatcher (Part:BBa_K4652010) and T7-SpyTag-PCLase2-SpyCatcher (Part:BBa_K4652013) using colony PCR. PCR was performed using a CmR-specific forward primer from the vector and a PCLase-specific reverse primer from the gene. The expected size of the amplified DNA fragments is 2395 bp for T7-SpyTag-PCLase1-SpyCatcher and 2561 bp for T7-SpyTag-PCLase2-SpyCatcher, respectively. The rightmost lane displays a 1 kb DNA ladder. The numbers correspond to selected colonies, with one control (lane 7) derived from a mock pick from a clear zone on the plate.

PCL-DEGRADING LIPASE COMPARISON

To compare the lipase activities of PCLase I, PCLase II, and other commercially available PCR-degrading enzymes such as BCLA⁵ and CALB⁶, we conducted a pNPB assay. In this assay, a potential lipase breaks down the ester bond of p-nitrophenylbutyrate (pNPB), producing p-Nitrophenol. The concentration of p-Nitrophenol can be measured at 405nm, and these measurements are corresponding to the lipase activity.

Lysates from E. coli BL21, which carried the T7 promoter-driven expression plasmid with the indicated genes in the same context, were collected after being induced with 0.3 mM IPTG at 25°C for 20 hours. The lipase activities within these lysates were assessed using the pNPB assay⁵. As shown in Figure 2, PCLase I exhibited the significantly highest readings at 405 nm. This suggests that under our experimental conditions, PCLase I is the most effective lipase, demonstrating potential activity in decomposing PCL through the hydrolysis of the ester bonds between polymers. Consequently, we chose to investigate the characteristics of PCLase I (hereafter referred to as PCLase for short) in terms of its thermostability, protein structure, PCL degradation capability, and its potential use in real-world products.

Figure 2. Comparison between lipase activities of BCLA, CALB, PCLase I, and PCLase II using pNPB assay. E. coli BL21 was transformed using the indicated T7 promoter-driven gene expression plasmid. The bacteria were induced by 0.3 mM of IPTG at 25°C for 20 hours. Subsequently, the lysates were harvested using 0.1 mm Disruptor Beads (Scientific Industries, Inc). A 20 µL aliquot of these lysates was combined with 175 µL of Tris-HCl buffer (20mM, pH=8) and 5 µL of pNPB (40 mM dissolved in 2-methyl-2-butanol). Lipase activity was read at 405 nm based on p-Nitrophenol production. The obtained readings were normalized with the OD600 values at the time of bacterial lysate collection.

Sequence and Features