Enhanced enzyme activity and stability through immobilization of recombinant chitinase on sodium alginate-modified rice husk beads for efficient decolorization of synthetic dyes

Abstract

Background: The energy efficiency and environmental friendliness of recombinant chitinase A make it a promising candidate for industrial applications as a sustainable catalyst. For the first time, a very stable and an efficient biocatalyst was developed to decolorize synthetic dyes by immobilizing Serratia marcescens chitinase A (SmChiA) onto beads comprised of sodium alginate (SA) and modified rice husk powder (mRHP). The mRHP was produced by treating rice husk powder with citric acid, which was then combined with SA at three different concentrations (25, 50 and 100% of SA weight) and cross-linked with calcium chloride to form the beads. 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide facilitates the formation of amide bonds that covalently bind SmChiA to the beads. The effectiveness of the synthesis and immobilization processes was confirmed using characterization methods (scanning electron microscopy, SEM and Fourier transform infrared spectroscopy, FTIR).

Results: Beads with 50% mRHP and 1.75 UmL^{- 1} of enzyme solution achieved the highest immobilization after 5 h of activation. The immobilized SmChiA demonstrated superior pH, temperature, and storage stability in respect to its free relative. The K_m value was 3.33 mg/mL, while the V_max was 4.32 U/mg protein/min. Activation energy (Ea), denaturation (E_d), half-lives (T_1/2), and decimal reduction time (D-values) were evaluated for immobilized and free SmChiA. The immobilization of SmChiA increased its affinity for the substrates by around 2.12 to 2.18 times. Compared to free chitinase, immobilized chitinase demonstrated greater durability after 22 reuses, maintaining its full activity. This proved the suitability of SA-mRHP beads as a cross-linker for chitinase immobilization. Crystal violet, malachite green, safranin, and methylene blue were more effectively decolorized from aqueous solutions by the immobilized SmChiA at a contact period of 84-h, dosage of 2.625 U/1.5 g, and temperature of 30 ^◦C. Using an immobilized biocatalyst, the biodegradation was also examined using UV, FTIR, and SEM-EDX. The results confirmed the dye degradation.

Conclusion: A variety of dyes could be safely removed from the environment using our bioremediation procedures. To the best of our knowledge, no studies had been conducted on the application of immobilized chitinase for dye removal.

Keywords: Serratia marcescens; Dye removal; Immobilization; Recombinant chitinase A; Rice husk.

Fig. 1

Schematic representation of the synthesis of SA-mRHP beads (A) and the immobilization of recombinant enzyme onto SA-mRHP beads (B)

Fig. 2

(A) FTIR of RHP and mRHP. (B) FTIR of SA beads, SA-mRHP beads before and after enzyme immobilization

Fig. 3

SEM images of SA beads: (A) before enzyme immobilization, and (B, C) after enzyme immobilization at low and high magnification, respectively

Fig. 4

Response surface 3D and contour plots possessing the interactive effect between different variables on immobilized recombinant SmChiA; (A) Polymer concentration & Enzyme concentration (B) Enzyme concentration & Time

Fig. 5

Effect of the reaction pH (control is pH 4.5) (A), preincubation at different pH values for differenttime intervals (the activity of the immobilized recombinant enzyme without preincubation was considered 100% activity), on the activity of the immobilized enzyme (B), the reaction temperature (50 °C is the control) (C), the activation energy (Ea) (D), and preincubation at different temperatures for different time intervals (the activity of the enzyme without preincubation was considered 100% activity) on the activity of the immobilized enzyme (E) and free enzyme (F). First order of thermal deactivation of immobilized (G) and free (H). Arrhenius plot to calculate activation energy for denaturation (Ed) immobilized (I) and free enzyme (J). The activity of the immobilized enzyme using different substrate concentrations immobilized enzyme (K) and free enzyme (L). Lineweaver–Burk plots were used to determine the values of the Michaelis–Menten constant (k_m) and maximum reaction rate (V_max) for both immobilized (M) and free SmChiA enzymes (N)

Fig. 5

Effect of the reaction pH (control is pH 4.5) (A), preincubation at different pH values for differenttime intervals (the activity of the immobilized recombinant enzyme without preincubation was considered 100% activity), on the activity of the immobilized enzyme (B), the reaction temperature (50 °C is the control) (C), the activation energy (Ea) (D), and preincubation at different temperatures for different time intervals (the activity of the enzyme without preincubation was considered 100% activity) on the activity of the immobilized enzyme (E) and free enzyme (F). First order of thermal deactivation of immobilized (G) and free (H). Arrhenius plot to calculate activation energy for denaturation (Ed) immobilized (I) and free enzyme (J). The activity of the immobilized enzyme using different substrate concentrations immobilized enzyme (K) and free enzyme (L). Lineweaver–Burk plots were used to determine the values of the Michaelis–Menten constant (k_m) and maximum reaction rate (V_max) for both immobilized (M) and free SmChiA enzymes (N)

Fig. 6

UV/vis spectra of malachite green (MG), crystal violet (CV), methylene blue (MB), and safranin (S) dyes before and after treatment with immobilized recombinant chitinase enzyme

Fig. 7

FTIR of Crystal Violet (CV), Methylene Blue (MB), Malachite Green (MG), and Safranin (S) before and after recombinant enzyme addition (CV2, MB2, MG2, and S2)

Fig. 8

SEM-EDX spectra of the malachite green, crystal violet, methylene blue, and safranin loaded immobilized SmChiA