Coated Bacterial Enzymes: A one-step approach for enzymatic purification and immobilization

Technical Summary: Coated Bacterial Enzymes (CBEs) for One-Step Purification and Immobilization

Problem Statement
The industrial production of lactose-reduced milk (LRM) requires high-purity $\beta$-galactosidase (lactase) free from contaminating proteases, which can cause off-flavors and undesirable Maillard reaction products. Conventional enzyme purification relies on costly, multi-step chromatographic protocols (e.g., gel filtration, ion exchange) that are time-consuming and yield low recovery rates. While one-step affinity purification methods exist (e.g., polyhistidine tags), they often require expensive affinity matrices. Furthermore, industrial applications demand enzymes with high thermal stability and tolerance to cations naturally present in milk (e.g., Ca$^{2+}$, Fe$^{2+}$), properties often lacking in free recombinant enzymes.

Methodology
The authors developed a novel platform termed Coated Bacterial Enzymes (CBEs) to achieve simultaneous purification and immobilization. The methodology involved:

1. Enzyme Design: A chimeric enzyme, BbgII-SlpA, was constructed by fusing the $\beta$-galactosidase from Bifidobacterium bifidum (BbgII) with the cell wall-binding domain (SlpA) derived from the S-layer protein of Lactobacillus helveticus. The gene was codon-optimized and expressed in Escherichia coli BL21.
2. Expression Optimization: Expression conditions were optimized to maximize soluble protein yield. The optimal conditions were determined to be induction at 22°C with 0.1 mM IPTG for 16 hours, resulting in approximately 70% of the protein being in the soluble fraction.
3. Carrier Preparation: Bacillus subtilis var. natto cells were harvested, washed, and chemically inactivated using 2.5% glutaraldehyde to serve as a low-cost biological carrier.
4. CBE Production: The soluble BbgII-SlpA fraction from the E. coli lysate was incubated with the inactivated B. subtilis cells. The SlpA domain facilitated specific binding to the bacterial cell wall. Unbound proteins were removed via washing, and the bound enzyme was either used directly as a biocatalyst or eluted for purity analysis.
5. Characterization: The resulting CBEs were characterized for binding kinetics ($K_d$, $B_{max}$), catalytic parameters ($K_m$, $V_{max}$, $k_{cat}$), optimal pH and temperature, thermal stability, and tolerance to various metal cations.

Key Results

• Purification and Binding: The CBE platform successfully captured BbgII-SlpA directly from crude lysate. SDS-PAGE analysis confirmed the removal of contaminants, yielding a highly purified enzyme. The binding interaction was characterized by a dissociation constant ($K_d$) of 16.2 $\mu$M and a maximum binding capacity ($B_{max}$) of 144 $\mu$mol/g of dry cell weight. Immunofluorescence microscopy confirmed a uniform distribution of the enzyme on the bacterial surface.
• Kinetic Parameters: The immobilized enzyme retained catalytic activity. For the synthetic substrate ONPG, the CBE showed a $k_{cat}$ of 15,451 s$^{-1}$ and a catalytic efficiency ($k_{cat}/K_m$) of $2.62 \times 10^4$ M$^{-1}$s$^{-1}$. For the natural substrate lactose, the $k_{cat}$ was 47.05 s$^{-1}$ with an efficiency of $4.40 \times 10^2$ M$^{-1}$s$^{-1}$. The authors note that while the apparent $K_m$ values increased upon immobilization (likely due to mass transfer limitations), the enzyme maintained significant activity on lactose.
• Thermal and pH Stability: Immobilization significantly enhanced stability.
  • pH: The CBE exhibited a broader pH activity profile (retaining >70% activity between pH 5.5–7.5) compared to the free enzyme.
  • Temperature: While optimal temperatures were similar (60°C for ONPG, 50°C for lactose), the CBE retained >90% activity at 50°C with lactose, whereas the free enzyme retained <10%.
  • Thermostability: After pre-incubation at 45°C for 390 minutes, the CBE retained ~80% of its initial activity, compared to only ~20% for the free enzyme.
• Cation Tolerance: The CBE demonstrated improved tolerance to inhibitory cations found in milk. At 100 mM, Ca$^{2+}$ completely inhibited the free enzyme but the CBE retained 36% activity. Similarly, at 100 mM Fe$^{2+}$, the free enzyme retained only 5% activity, while the CBE retained 73%.

Significance and Claims
The paper claims that the CBE platform offers a cost-effective, one-step alternative for producing high-purity, immobilized enzymes, addressing the economic and yield limitations of traditional purification methods. By utilizing chemically inactivated B. subtilis as a carrier, the system decouples enzyme production from the final biocatalyst, avoiding the regulatory and metabolic burdens associated with live genetically modified organisms (GMOs) in the final product.

The authors assert that the CBE system successfully expands the scope of SlpA-based tagging technology from protein purification to functional enzyme immobilization. The enhanced thermal stability and cation tolerance suggest the CBEs are robust candidates for industrial bioprocesses, specifically for the hydrolysis of lactose in dairy applications. However, the authors modestly note that while the platform shows promise, further investigation is required regarding scalability, enzyme leakage, long-term storage stability, and performance in complex matrices like real milk or whey. Additionally, they acknowledge that the use of glutaraldehyde for inactivation limits immediate food-grade application, suggesting future work should explore food-compatible cross-linkers.