Abstract
Peroxidases (POXs) are the key extracellular enzymes produced by crude oil degrading microbes. Knowledge of optimum conditions for POXs activity is crucial for providing effective environment for bioremediation. In this study, physicochemical properties of POXs produced by Actinomyces israelii and Actinomyces viscosus during growth on crude oil were studied. The POXs exhibited similarities in activity and stability with striking differences in response to two divalent metal ions. The POXs from both species had optimum pH of 7.0 and were very stable over a narrow pH range (6.0 - 8.0). The POXs demonstrated similar thermostability exhibiting relative residual activity of 62% at 50°C after 30 min incubation and 45% residual activity at the same temperature after 60 min despite the fact that POXs from A. viscosus and A. israelii had optimum temperatures of 50°C and 40°C, respectively. The POXs from A. viscosus and A. israelii were greatly activated by Fe2+ at 5.0 and 10.0 mM. The enzymes were both strongly inhibited by Cu2+, Mg2+ and Hg2+. Surprisingly, these congeneric POXs demonstrated striking differences in their response to Ca2+ and Mn2+. POX from A. viscosus was activated by Ca2+ and Mn2+ exhibiting relative activity of 136% and 106% at 5 mM, respectively. In contrast, POX from A. israelii was strongly inhibited by Ca2+ and Mn2+ exhibiting 62.5% relative activity in the presence of 5 mM of each metal ion. Increasing the concentration of Ca2+ and Mn2+ led to further activation of POX from A. viscosus and inhibition of POX from A. israelii. Results provide deeper insights into functional properties of studied POXs from closely related microbes. The physicochemical properties are very similar; however, notable differences provide a strong basis for structural characterization of these congeneric enzymes.
Highlights
Crude oil spills have caused destruction of farmlands, contamination of water bodies and loss of biodiversity [1] [2]
Actinobacteria have been reported as useful microbes in the bioremediation of soils contaminated with crude oil and derivatives [8]-[10]
The total POXs from A. viscosus and A. israelii were active over pH range of 4.0 to 9.0 exhibiting above 60% relative activity over pH 6.0 to 8.5 with optimum pH at 7.0 (Figure 1(a))
Summary
Crude oil spills have caused destruction of farmlands, contamination of water bodies and loss of biodiversity [1] [2]. Among the different remediation techniques, bioremediation is the most advantageous due to its cost effectiveness and environmental friendliness [6] [7]. This has led to screening for various microorganisms with high crude oil degrading efficiency. Actinobacteria have been reported as useful microbes in the bioremediation of soils contaminated with crude oil and derivatives [8]-[10]. A wide variety of microbes initiate biodegradation of polyaromatic and aliphatic hydrocarbons by co-metabolism using extracellular and intracellular enzymes of which peroxidases play a crucial role [11] [12]. It is quite surprising that peroxidases isolated from fungi and plants have been extensively studied while actinobacterial peroxidases have been unexplored despite their diverse metabolic capabilities which represent large resource for biocatalysis [17]
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