Hindawi Publishing Corporation Enzyme Research Volume 2014, Article ID 317940, 9 pages http://dx.doi.org/10.1155/2014/317940
Research Article Sequential Statistical Optimization of Media Components for the Production of Glucoamylase by Thermophilic Fungus Humicola grisea MTCC 352 Vinayagam Ramesh and Vytla Ramachandra Murty Department of Biotechnology, Manipal Institute of Technology, Manipal University, Manipal, Karnataka 576104, India Correspondence should be addressed to Vinayagam Ramesh;
[email protected] Received 7 May 2014; Revised 25 June 2014; Accepted 28 June 2014; Published 9 July 2014 Academic Editor: Sunney I. Chan Copyright © 2014 V. Ramesh and V. Ramachandra Murty. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Glucoamylase is an industrially important enzyme which converts soluble starch into glucose. The media components for the production of glucoamylase from thermophilic fungus Humicola grisea MTCC 352 have been optimized. Eight media components, namely, soluble starch, yeast extract, KH2 PO4 , K2 HPO4 , NaCl, CaCl2 , MgSO4 ⋅7H2 O, and Vogel’s trace elements solution, were first screened for their effect on the production of glucoamylase and only four components (soluble starch, yeast extract, K2 HPO4 , and MgSO4 ⋅7H2 O) were identified as statistically significant using Plackett-Burman design. It was fitted into a first-order model (𝑅2 = 0.9859). Steepest ascent method was performed to identify the location of optimum. Central composite design was employed to determine the optimum values (soluble starch: 28.41 g/L, yeast extract: 9.61 g/L, K2 HPO4 : 2.42 g/L, and MgSO4 ⋅7H2 O: 1.91 g/L). The experimental activity of 12.27 U/mL obtained was close to the predicted activity of 12.15. High 𝑅2 value (0.9397), low PRESS value (9.47), and AARD values (2.07%) indicate the accuracy of the proposed model. The glucoamylase production was found to increase from 4.57 U/mL to 12.27 U/mL, a 2.68-fold enhancement, as compared to the unoptimized medium.
1. Introduction Glucoamylase or 1,4-𝛼-D-glucan glucohydrolase (EC 3.2.1.3) is an industrial enzyme which can degrade amylose and amylopectin by hydrolysis of both 𝛼-1,4 and 𝛼-1,6 glucosidic links, present in starch, resulting in production of 𝛽D glucose [1]. There are two stages in the production of industrial starch syrup: liquefaction and saccharification. In the first step, thermostable 𝛼-amylases are used to liquefy starch. Following this, saccharification is carried out at 55– 60∘ C with fungal glucoamylases. The glucoamylase from mesophilic fungi (e.g., Aspergillus niger) is unstable due to its exposure to higher temperatures for a prolonged duration [2]. This disadvantage necessitates the use of thermostable glucoamylases derived from thermophilic fungal sources [3] for industrial usage. There are a number of thermophilic fungi such as Thermomyces lanuginosus, Talaromyces duponti, Thermomucor indicae-seudaticae, and Humicola grisea which are capable
of producing glucoamylase [4–6]. Literature reveals that Humicola grisea is an attractive source for extracellular thermostable glucoamylase production [4, 7]. Humicola grisea possesses efficient hydrolytic system which is responsible for the production of many polysaccharide degrading enzymes such as cellulases, amylases, trehalase, beta-glucosidase, and xylanase [8]. Glucoamylase production depends on many media components such as carbon source, nitrogen source, mineral salts, and micronutrients. Therefore it is necessary to optimize the medium components for the enhanced production of glucoamylase [5, 9–11]. The classical one-variable-at-a-time (OVAT) approach involves altering the concentration of one of the components and maintaining the others, at a specified level. This is usually problematic since it is laborious and the interaction effects between the various media components are not taken into consideration. The shortcomings of this approach are overcome by the use of statistical techniques like
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Enzyme Research Table 1: Different levels of experimental variables used for the production of glucoamylase using Plackett-Burman design.
Symbol 𝐴 𝐵 𝐶 𝐷 𝐸 𝐹 𝐺 𝐻
Variable
Units
Soluble starch Yeast extract KH2 PO4 K2 HPO4 NaCl CaCl2 MgSO4 ⋅7H2 O Vogel’s solution
(g/L) (g/L) (g/L) (g/L) (g/L) (g/L) (g/L) (mL)
Plackett-Burman design (PBD), steepest ascent, and response surface methodology (RSM) [12]. Beginning from a large collection of factors, PBD helps to identify the main factors that would be taken up for further optimization processes, through lesser number of trials. The significant factors chosen from PBD are sequentially moved along the path of steepest ascent to target the maximum production of glucoamylase. The levels of the components obtained along the region of maximum response are used in central composite design (CCD), a response surface methodology technique. These form the right set of techniques leading to the optimal concentration of the various significant media components. This approach of arriving at the optimal media composition has been practiced by various researchers in many fermentation processes [13–15]. To the best of our knowledge, there are no available reports on the optimization of media components for glucoamylase production using Humicola grisea. Therefore, in the present report, the media components (soluble starch, yeast extract, KH2 PO4 , K2 HPO4 , NaCl, CaCl2 , MgSO4 ⋅7H2 O, and Vogel’s trace elements solution) for the production of glucoamylase by Humicola grisea MTCC 352 were optimized using response surface methodology that included a Plackett-Burman design, path of steepest ascent, and central composite design.
2. Materials and Methods 2.1. Microorganism, Inoculum Preparation, and Fermentation Conditions. The microorganism, used in the study, Humicola grisea MTCC 352, was obtained from Microbial Type Culture Collection, Chandigarh, India. The strain was maintained on potato dextrose agar (PDA) slant, grown at 45∘ C for 10 days before being stored at 4∘ C. The strain was subcultured, once every 2 months. The fermentation was started with 2 mL of conidial inoculum prepared using 0.15% Triton X-100, that was added to 250 mL Erlenmeyer flasks containing 100 mL of medium (glucose 1 g, yeast extract 0.3 g, KH2 PO4 0.1 g, K2 HPO4 0.1 g, NaCl 0.1 g, CaCl2 0.1 g, MgSO4 ⋅7H2 O 0.1 g, and 0.5 mL of Vogel’s trace element solution), adjusted to pH 6. The inoculum culture was incubated at 45∘ C for 4 days at 150 rpm. Vogel’s trace elements solution was constituted
−1 5 1 0.5 0.5 0.5 0.5 0.5 0.1
Coded level 0 10 3 1 1 1 1 1 0.5
1 15 5 1.5 1.5 1.5 1.5 1.5 1
by the following, as per literature [16]: citric acid monohydrate 5 g, ZnSO4 ⋅7H2 O 5 g, Fe(NH4 )2 (SO4 )2 ⋅6H2 O 1 g, CuSO4 ⋅5H2 O 0.25 g, MnSO4 ⋅H2 O 0.05 g, H3 BO3 0.05 g, and Na2 MoO4 ⋅2H2 O 0.05 g, dissolved in 95 mL distilled water. Based on preliminary experiments (data not shown), soluble starch and yeast extract showed better yields for enzyme production. Therefore, for the production medium, soluble starch was used as the carbon source in place of glucose. All the other media constituents and the culture conditions remained unaltered. Both the inoculum culture and production media were autoclaved for 15 minute at 121∘ C and 15 psi. The spore suspension of the fungal strain (5 mL) was inoculated in 100 mL of the production medium, taken in a 250 mL flask, for a period of 4 days. 2.2. Extraction of Extracellular Glucoamylase. After fermentation, the broth was subjected to filtration through Whatman No. 1 filter paper. The filtrate was centrifuged at 13000 rpm for 20 minutes to remove the fungal mycelia. The cell-free supernatant was assayed for glucoamylase activity. All the experiments were carried out in triplicate and the average values were reported. 2.3. Glucoamylase Activity Assay. 0.05 mL of cell-free supernatant was incubated with 0.7 mL of 50 mM citrate buffer (pH 5.5) and 0.25 mL starch solution (1%, w/v) at 60∘ C for 10 minutes. The reaction was stopped by placing the tubes in a boiling water bath for 10 minutes. After bringing back to room temperature, the concentration of glucose formed was determined by glucose oxidase/peroxidase (GOD/POD) method [17]. One unit of glucoamylase activity was defined as the amount of enzyme that releases 1 𝜇mol glucose from soluble starch per minute under assay conditions. 2.4. Plackett-Burman Design. Plackett-Burman design was used to screen the media components and identify the significant components that influence the higher production of glucoamylase. Eight independent variables were chosen with three different levels, namely, low, mid, and high factor settings, coded as −1, 0, and +1, respectively, with their actual values (Table 1). These variables were screened in 13 experimental runs that included a center point, according to
Enzyme Research
3 Table 2: Plackett-Burman experimental design matrix for glucoamylase production.
Run 1 2 3 4 5 6 7 8 9 10 11 12 13
𝐴 1 −1 −1 −1 −1 1 −1 0 1 −1 1 1 1
𝐵 1 1 1 −1 −1 −1 1 0 1 −1 −1 1 −1
𝐶 −1 1 1 −1 −1 1 −1 0 1 1 1 −1 −1
𝐷 1 1 −1 −1 1 1 −1 0 −1 1 −1 1 −1
𝐸 −1 −1 1 −1 1 −1 −1 0 1 1 −1 1 1
𝐹 −1 1 −1 −1 1 1 1 0 1 −1 −1 −1 1
𝐺 −1 1 −1 −1 −1 −1 1 0 −1 1 1 1 1
𝐻 1 −1 −1 −1 1 −1 1 0 1 1 1 −1 −1
Glucoamylase activity (U/mL) 6.31 4.87 3.78 3.02 3.97 4.94 4.83 4.57 5.43 4.74 4.79 6.56 5.01
Table 3: Experimental results of the path of steepest ascent. Variable Base point (zero level in the PBD) Origin step unit (concentration range of unity level) Slope (estimated coefficient ratio from equation) New step unit (slope × origin step unit) Run 1 Run 2 Run 3 Run 4 Run 5 Run 6 Run 7 Run 8 Run 9 Run 10
Soluble starch (g/L)
Yeast extract (g/L)
K2 HPO4 (g/L)
MgSO4 ⋅7H2 O (g/L)
10 5 0.6525 3.26 13.26 16.52 19.78 23.04 26.3 29.56 32.82 36.08 39.34 42.6
3 2 0.4425 0.89 3.89 4.78 5.67 6.56 7.45 8.34 9.23 10.12 11.01 11.9
1 0.5 0.3775 0.19 1.19 1.38 1.57 1.76 1.95 2.14 2.33 2.52 2.71 2.9
1 0.5 0.2792 0.14 1.14 1.28 1.42 1.56 1.7 1.84 1.98 2.12 2.26 2.4
PBD (Table 2) along with the response (glucoamylase activity).The center point experiment was performed to obtain the standard error of the coefficients. The Plackett-Burman design was based on the first-order model, shown in 𝐺 = 𝑔0 + ∑ 𝑔𝑖 𝑍𝑖 ,
(1)
where 𝐺 is the glucoamylase activity (U/mL), 𝑔0 is the model intercept, 𝑔𝑖 is the linear coefficient, and 𝑍𝑖 is the level of the independent variable [12]. 2.5. Path of Steepest Ascent. Following the first-order model based on PBD, new sets of experiments were performed in the direction of maximum response as described by steepest ascent method [12]. In this approach, the experiments were started at the midlevel of the statistically significant factors that were picked from PBD. The levels of the each factor
Glucoamylase activity (U/mL)
5.16 5.91 6.88 8.13 9.42 10.48 11.64 10.28 9.67 9.21
were increased depending on their magnitude of the main effect. Experiments were continued until no further increase in response was observed (Table 3). 2.6. Central Composite Design and Response Surface Methodology. In order to obtain the optimum values of each factor, a CCD was performed. The CCD was used to obtain a quadratic model consisting of factorial points (−1, +1), star points (−2, +2), and central point (0) to estimate the variability of the process with glucoamylase yield as the response (Table 4). Response surface methodology was employed to optimize the four selected significant factors, namely, soluble starch (𝑍1 ), yeast extract (𝑍2 ), K2 HPO4 (𝑍3 ), and MgSO4 ⋅7H2 O (𝑍4 ), which increase the glucoamylase production. In this methodology, a 4-factor, 5-level CCD with 31 runs was employed (Table 5).
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Enzyme Research Table 4: Ranges of the independent variables used in central composite design.
Symbol 𝑍1 𝑍2 𝑍3 𝑍4
Variable
Unit
Soluble starch Yeast extract K2 HPO4 MgSO4 ⋅7H2 O
(g/L) (g/L) (g/L) (g/L)
−2 26.3 7.45 1.95 1.7
−1 29.56 8.34 2.14 1.84
Coded level 0 32.82 9.23 2.33 1.98
1 36.08 10.12 2.52 2.12
2 39.34 11.01 2.71 2.26
Table 5: Central composite design matrix for the experimental design and predicted responses for glucoamylase activity. Trial 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
𝑍1 −1 0 1 0 0 0 1 −1 −1 −1 −1 1 −2 0 1 1 2 0 0 0 1 −1 0 0 0 −1 0 1 1 0 −1
𝑍2 −1 0 1 0 0 0 1 −1 −1 1 1 1 0 0 −1 −1 0 0 0 0 −1 1 0 −2 0 1 0 1 −1 2 −1
Coded variable level 𝑍3 1 0 1 0 2 0 −1 −1 1 1 −1 1 0 0 −1 −1 0 0 0 0 1 −1 −2 0 0 1 0 −1 1 0 −1
𝑍4 −1 0 1 0 0 2 −1 −1 1 −1 1 −1 0 0 1 −1 0 0 −2 0 −1 −1 0 0 0 1 0 1 1 0 1
A quadratic equation was used to fit the response to the independent variables as given in (2) 𝐺 = 𝑔0 + ∑ 𝑔𝑖 𝑍𝑖 + ∑ 𝑔𝑖𝑖 𝑍𝑖2 + ∑ 𝑔𝑍𝑖 𝑍𝑗 ,
(2)
where 𝐺 is the predicted response of the glucoamylase activity (U/mL), 𝑔0 is the offset term (constant), 𝑔𝑖 is the linear effect, 𝑔𝑖𝑗 is the quadratic effect when 𝑖 = 𝑗 and interaction effect when 𝑖 < 𝑗, 𝑔𝑖𝑖 is the squared term, and 𝑍𝑖 and 𝑍𝑗 are
Glucoamylase activity (U/mL) Observed Predicted 10.73 10.498 11.56 11.65 10.2 10.156 11.72 11.65 9.42 9.845 8.41 9.031 8.38 8.186 10.56 10.362 8.93 8.883 11.87 11.737 10.22 9.779 10.4 10.191 11.35 11.703 11.82 11.65 9.42 9.311 8.96 8.996 9.54 9.758 11.62 11.65 9.77 9.72 11.48 11.65 9.02 9.219 9.56 9.82 8.52 8.666 8.69 8.935 11.71 11.65 11.1 10.735 11.64 11.65 9.21 9.113 9.16 8.571 9.65 9.976 9.83 9.71
the coded independent variables for statistical calculations according to 𝑍=
𝑅 − 𝑅0 , Δ𝑅
(3)
where 𝑍 is the coded value of the independent variable, 𝑅 is the real value of the independent variable, 𝑅0 is the real value of the independent variable on the center point, and Δ𝑅 is the step change value [12].
Enzyme Research
5 Table 6: Statistical analysis of PBD on glucoamylase activity.
Variables Intercept 𝐴 𝐵 𝐶 𝐷 𝐸 𝐹 𝐺 𝐻
1.3050 0.8850 −0.1917 0.7550 0.1217 −0.0250 0.5583 0.3150
Coefficients 4.8542 0.6525 0.4425 −0.0958 0.3775 0.0608 −0.0125 0.2792 0.1575
𝑡 value 74.68 10.04 6.81 −1.47 5.81 0.94 −0.19 4.29 2.42
Standard error 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065
𝑃 value
