Identification of free Amino Acids in Brewer's Yeast after Heavy Metals Biosorption

Abstract

Yeasts of genera Saccharomyces are efficient biosorbents for heavy metal ions. The aim of this study was to identify if the free amino acids present in brewer yeast are involved in metal biosorption due to their capacity to coordinate metal ions. As biosorbent was used non-living brewer's yeast type Saccharomices cerevisae at 0.5% yeast dose. Copper, lead and zinc solution of 1mg/L concentrations were prepared using their salts. The experiments were conducted at pH=6, which is the most appropriate for amino acids extraction. The amino acids were identified by HPLC-DAD/-ESI-MS chromatography. The experiments were conducted by mixing metals solution with yeast and shaken at a constant speed of 120 rpm at 200C for 120 minute. The samples were centrifuged at 2500 rpm for 15 minute and the pellet were analysed for amino acids identification. The amino acids extraction from pellets was performed using HCl 0,05M/deionized water (v/v) 1/1. The HPLC analysis was performed on a Agilent 1200 system equipped with a binary pump delivery system LC-20 AT, a degasser DGU-20 A3, diode array SPD-M20 A, UV-VIS detector (DAD). Amino acids were identified using an EEZ:Faast Kit for free amino acids, The amino acids identified by HPLC method were glycine, glutamic acid, leucine, isoleucine, ornithine, lysine, histidine, homophenylalanine, tyrosine, glutamine in control brewer yeast (before biosorption) and their profile differs according with the metal ions types. According with the peaks area there are differences in the presence of the amino acids due to the possible coordination with copper, lead and zinc ions.

Keywords

• Brewer yeast
• amino acids
• biosorption
• heavy metals

1 Introduction

Biosorption can be defined as the selective sequestering of metal soluble species that result in the immobilization of the metals by microbial cells. Biosorption is a process with some unique characteristics. It can effectively sequester dissolved metals from very dilute complex solutions with high efficiency. This makes biosorption an ideal candidate for the treatment of high volume low concentration complex waste-waters (Gadd, 1993; Wang, 2006) . The selective sequestering of metal soluble species that result in the immobilization of the metals by microbial cells is defined as biosorption. It refers to physicochemical mechanisms of inactive (i.e. non-metabolic) metal uptake by microbial biomass (Volesky 2001) . Metal sequestering by different parts of the cell can occur via various processes: complexation, chelation, coordination, ion exchange, precipitation, reduction (Wang 2009, Alluri, 2007) . Immobilization may be the result of more than one mechanism, for example, metal complexation may be followed by metal reduction or metal precipitation. These biopolymers, constituents of the cell wall and the other parts of the cell possess functional groups that have a significant potential for metal binding (Dakiky, 2002) .

Metal uptake by non-living cells is mainly a passive biosorption and consists in an adsorption of metal ions to the cells surface by interactions between metal and functional groups displayed on the surface of the cells (Paknikar, 2003) .

Brewer's yeast is made from a one-celled fungus called Saccharomyces cerevisiae and is used to make beer. It also can be grown to make nutritional supplements. The yeast biomass has been successfully used as biosorbent for removal of Ag, Au, Cd, Co, Cr, Cu, Ni, Pb, U, Th and Zn from aqueous solution. Yeasts of genera Saccharomyces, Candida, Pichia are efficient biosorbents for heavy metal ions. A number of literatures have proved that S. cerevisiae can remove toxic metals, recover precious metals and clean radionuclides from aqueous solutions to various extents (Podgorskii, 2004; Tálos, 2009) 2

Materials and Methods

Saccharomices cerevisae biomass was supplied as a lyophilized by-product from industrial ethanol production. Prior use as a biosorbent, the biomass was pretreated in order to remove fine particles and to displace any metals already bound to the sorption sites. The waste biomass as washed with deionized water by stirring followed by centrifugation at 3000 rpm for 20 minutes. The supernatant was discarded and the pellet was reslurried in deionized water. The procedure was repeated for three times until the supernatant was clear. Metals solution were prepared using the mixture of their salts CuSO4·5H2O, PbNO3·2H2O, ZnSO4 of analytical reagent grade. The concentrations of metals ions established were 1 mg/L for Cu2+, Zn2+, Pb2+ and were obtained by dissolving the appropriate salts in deionized water.

Experimental procedure:

Metal ion binding experiments were performed by incubation of 25 mg biomass (dry weight) with 50 ml mixture of metals ions-containing solution in 125-ml Erlenmeyer flask on an orbital rotary shaker at 120 rpm for 120 minutes. The experiment was conducted at pH=6 and was established by adjusting it with HCl 0.1M or NaOH 0.1M solutions. In order to identify the free amino acids from biomass by HPLC chromatography the samples were centrifuged at 2500 rpm for 15 minutes, the supernatant was discarded and the pellets were analysed.

The free amino acids were extracted from the pellets using two types of solutions: HCl 0,05M/deionized water (v/v) 1/1. HPLC analysis was performed on a Agilent 1200 system equipped with a binary pump delivery system LC-20 AT (Prominence), a degasser DGU-20 A3 (Prominence), diode array SPD-M20 A, UV–VIS detector (DAD). Amino acids (100 µl) from control brewer’s yeast and samples provided after biosorption of metal ions were identified using an EZ:Faast Kit for free amino acids, provided by Phenomenex (USA). The results are presented in the next table and figures.

3 Results and Discussions

The absorption mechanisms of metals are different and depend on cellular metabolism. In non-living brewer yeast this mechanism is one of physical-chemical interaction between metal ions and some functional groups, especially amino and carboxyl. The aim of this study was to establish the amino acids from brewer yeast involvement in metal ions biosorption, as it is known that amino and carboxyl groups could be responsible for complexation depending on pH intervals and metal:ligand ratio. In the HPLC chromatograms these involvement could be observed by changing the absorption intensities between amino acids from brewer yeast control and brewer yeast charged with metal ions by biosorption.

The standard solutions of amino acids used for analysis were selected according with the literature data regarding the presence of these compounds in brewer yeast (Podgorskii, 2004) . The main amino acids identified by HPLC method were glycine (Gly), asparagine (ASN), glutamic acid (GLU), leucine (LEU), isoleucine (ILE), ornithine (ORN), lysine (LYS), histidine (HYS), homophenylalanine (HPHE), tyrosine (TYR), glutamine GLN), alanine (ALA), valine (VAL), triptophan (TRP), phenylalanine (PHE), α-aminobutyric acid (ABA). The separation and identification of amino acids were performed on brewer yeast uncharged with metal ions (control) and brewer yeast charged with metal ions after their biosorption at different pH values. The results are presented in the next table and figures. The profiles of chromatograms are different between control brewer yeast and samples charged with metal ions due to the absorption of copper, zinc and lead after incubation. It is supposed that amino acids from brewer yeast has the capacity to coordinate these metal ions and the complexes resulted have different retention times and peaks area than free amino acids as it can be seen in the Table 1. From the data above it can be presumed that amino acids are involved in metal coordination due to their capacity of metal binding through carboxyl and amino groups. Another explanation regarding the changes in amino acids profiles and content is that that it could be affected by autolysis and fermentation conditions like time, temperature, pH, moisture content (Cabuk, 2005) . The metal binding capacity differs from an amino acids to other: GLN, Val and AAA which are present in uncharged brewer yeast sample disappear in the chromatograms of charged brewer’s yeast with all metal ions; ASN and TRP have good affinity for copper ions, GLU for lead ions, and PHE for copper and zinc ions. The others amino acids present different affinity for metal ions according with their molecular structure and the capacity to coordinate copper, lead and zinc. According to covalent index Pb(II) ion is classified as a class b ion, while Cu(II) and Zn(II) are classified as borderline ion, so the behavior of coordination is not full the same and the binding capacity is different (Iqbal, 2004; Niebeer, 1973) . This study represents a first step in elucidation of the absorption mechanism of metal ions from aqueous solution by brewer yeast. Thus, the preliminary results lead to the conclusion that free amino acids, above other constituents from yeast, could be involved in metal biosorption. This process is strongly dependent on experimental conditions, especially on pH level, as it is known that each metal ions have characteristic pH limits for coordination with different organic ligands. This study reveal the presence of 16 free amino acids in brewer’s yeast and significant differences in their profile and content after biosorption of copper, lead and zinc, which could be explained by the coordination of these ions by carboxyl and amine groups of the ligands.

Acknowledgments. This work was supported by Romanian Ministry of Education and Scientific Research UEFISCDI - PN-II-PT-PCCA-2013-4-0743

References

1. Alluri , HK., Ronda , SR., Setalluri , VS. ( 2007 ). Biosorption: an ecofriendly alternative for heavy metal removal . African Journal of Biotechnology 6 ( 25 ): 2924 - 2931 .

2. Cabuk , A. , Akar , T. , Tunali , S. , Tabak , O. , 2005 . Biosorption characteristics of Bacillus sp . ATS -2 immobilized in silica gel for removal of Pb(II) . J.Hazard Mater 136 : 317 - 323 .

3. Dakiky , M. , Khamis , M. , Manassra , A. , Mereb , M. ( 2002 ). Selective adsorption of chromium (VI) in industrial wastewater using low-cost abundantly available adsorbents . Adv. Environ. Res . 6 : 533 - 540 .

4. Gadd , GM. ( 1993 ). Interactions of fungi with toxic metals . Phytologist 124 : 25 - 60

5. Iqbal , M. , Edyvean , R.G.J. , 2004 . Biosorption of lead, copper and zinc ions on loofs sponge immobilized biomass of Phanerochaete chrysosporium . Miner.Eng . 17 : 217 - 223 .

6. Paknikar , KM., Pethkar , AV., Puranik , PR. ( 2003 ). Bioremediation of metalliferous Wastes and products using Inactivated Microbial Biomass . Indian J. Biotechnol . 2 : 426 - 443 .

7. Podgorskii , VS., Kasatkina , TP., Lozovaia , OG. ( 2004 ). Yeasts-biosorbents of heavy metals . Mikrobiol. Z 66 : 91 - 103 .

8. Tálos , K. , Páger , C. , Tonk , S. , Majdik , C. , Kocsis , B. , Kilár , F. , Pernyeszi , T. ( 2009 ). Cadmium biosorption on native Saccharomyces cerevisiae cells in aqueous suspension . Acta Universitatis Sapientiae, Agriculture and Environment , 1 : 20 - 30 .

9. Volesky , B. ( 2001 ). Detoxification of metal-bearing effluents: biosorption for the next century . Hydrometallurgy 59 : 203 - 16 .

10. Wang , JL., Chen , C. ( 2006 ). Biosorption of heavy metals by Saccharomyces cerevisiae: a review . Biotechnol Advances 24 : 427 - 51 .

11. Wang , JL., Chen , C. ( 2009 ). Biosorbents for heavy metal removal and their future . Biotechnology Advances 27 : 195 - 226 .

12. Niebeer E. , McBryde W.A. , 1973 . Free energy relationships in coordinate chemistry (III): a comprehensive index to complex stability . Can.J.Chem . 51 : 2512 - 2524 .