TK1656, a thermostable l-asparaginase from Thermococcus kodakaraensis, exhibiting highest ever reported enzyme activity

Marine Microorganism: An Underexplored Source of l-Asparaginase

l-Asparaginase (EC 3.5.1.1) is an enzyme that catalyzes the hydrolysis of l-asparagine to l-aspartic acid. This enzyme has an important role in medicine and food. l-Asparaginase is a potential drug in cancer therapy. Furthermore, it is also applied for reducing acrylamide, a carcinogenic compound in baked and fried foods. Until now, approved l-asparaginases for both applications are few due to their lack of appropriate properties. As a result, researchers have been enthusiastically seeking new sources of enzyme with better performance. A great number of terrestrial l-asparaginase-producing microorganisms have been reported but unfortunately, almost all failed to meet criteria for cancer therapy and acrylamide reducing agent. As a largest area than Earth, marine environment, by contrast, has not been optimally explored yet. So far, a great challenge facing an exploration of marine microorganisms is mainly due to their harsh, mysterious, and dangerous environment. It is clear that marine environment, a gigantic potential source for marine natural products is scantily revealed, although several approaches and technologies have been developed. This chapter presents the historical of l-asparaginase discovery and applications. It is also discussed, how the marine environment, even though offering a great potency but is still one of the less explored area for l-asparaginase-producing microorganisms.

Pcal_1127, a highly stable and efficient ribose-5-phosphate pyrophosphokinase from Pyrobaculum calidifontis

Analysis of the genome sequence of Pyrobaculum calidifontis revealed the presence of an open reading frame Pcal_1127 annotated as ribose-5-phosphate pyrophosphokinase. To examine the properties of Pcal_1127 the coding gene was cloned, expressed in Escherichia coli, and the purified gene product was characterized. Pcal_1127 exhibited higher activity when ATP was replaced by dATP as pyrophosphate donor. Phosphate and EDTA activated the enzyme activity and equivalent amount of activity was detected with ATP and dATP in their presence. Recombinant Pcal_1127 could utilize all the four nucleotides as pyrophosphate donors with a marked preference for ATP. Optimum temperature and pH for the enzyme activity were 55 °C and 10.5, respectively. A unique feature of Pcal_1127 was its stability against temperature as well as denaturants. Pcal_1127 exhibited more than 95 % residual activity after heating for 4 h at 90 °C and a half-life of 15 min in the boiling water. The enzyme activity was not affected by the presence of 8 M urea or 4 M guanidinium chloride. Pcal_1127 was a highly efficient enzyme with a catalytic efficiency of 5183 mM^-1 s^-1. These features make Pcal_1127, a novel and unique ribose-5-phosphate pyrophosphokinase.

Pcal_1699, an extremely thermostable malate dehydrogenase from hyperthermophilic archaeon Pyrobaculum calidifontis

Two malate dehydrogenase homologs, Pcal_0564 and Pcal_1699, have been found in the genome of Pyrobaculum calidifontis. The gene encoding Pcal_1699 consisted of 927 nucleotides corresponding to a polypeptide of 309 amino acids. To examine the properties of Pcal_1699, the structural gene was cloned, expressed in Escherichia coli and the purified gene product was characterized. Pcal_1699 was NADH specific enzyme exhibiting a high malate dehydrogenase activity (886 U/mg) at optimal pH (10) and temperature (90 °C). Unfolding studies suggested that urea could not induce complete unfolding and inactivation of Pcal_1699 even at a final concentration of 8 M; however, in the presence of 4 M guanidine hydrochloride enzyme structure was unfolded with complete loss of enzyme activity. Thermostability experiments revealed that Pcal_1699 is the most thermostable malate dehydrogenase, reported to date, retaining more than 90 % residual activity even after heating for 6 h in boiling water.

A critical review on properties and applications of microbial l-asparaginases

l-Asparaginase is one of the main drugs used in the treatment of acute lymphoblastic leukemia (ALL), a commonly diagnosed pediatric cancer. Although several microorganisms are found to produce l-asparaginase, only the purified enzymes from E. coli and Erwinia chrysanthemi are employed in the clinical and therapeutic applications in humans. However, their therapeutic response seldom occurs without some evidence of hypersensitivity and other toxic side effects. l-Asparaginase is also of prospective use in food industry to reduce the formation of acrylamide in fried, roasted or baked food products. This review is an attempt to compile information on the properties of l-asparaginases obtained from different microorganisms. The complications involved with the therapeutic use of the currently available l-asparaginases, and the enzyme's potential application as a food processing aid to mitigate acrylamide formation have also been reviewed. Further, avenues for searching alternate sources of l-asparaginase have been discussed, highlighting the prospects of endophytic microorganisms as a possible source of l-asparaginases with varied biochemical and pharmacological properties.