How to kill a parasite : a report : an inhibitor study of ribose-5-phosphate isomerase B from Giardia lamblia as a possible drug target

Abstract: The parasite Giardia lamblia is known to cause the disease giardiasis, which is a major threat against the public health, especially in developing countries. A recent study has shown that over 200 million people have giardiasis now, and about 500 000 more people are infected every year. A G. lamblia infection is usually treated with the drugs metronidazole, albendazole, tinidazole and furazolidone. These drugs are in some cases toxic and have severe side effects. Therefore new methods of killing the parasite are of great importance. Ribose-5-phosphate isomerase (Rpi) is an enzyme active in the pentose phosphate pathway, and it catalyses the conversion of ribose 5-phosphate to ribulose 5-phosphate and vice versa. Some enzymes in this class can also act on 6-carbon substrates, interconverting allose 6-phosphate and allulose 6-phosphate. To find which inhibitor works best on the enzyme from G. lamblia (GlRpiB), the enzyme was expressed as a His-tagged construct in E. coli was purified then tested to check its purity. When it was confirmed to be the right protein, the protein was used for crystallization and kinetic assays. Allose-6-phosphate was added before crystallization. Crystals formed in the presence of allose-6-phosphate. The Km, Vmax and kcat were determined and inhibitory tests were performed. It was determined that the Km for ribose-5-phosphate is about 10 mM, and the kcat is around 260sec-1. The Km of for allose-6-phosphate is 0.41 mM, and kcat 0.008sec-1. Among 6 substrate-like compounds tested, only 4PEH (4-phospho-D-erythronohydroxamic acid) showed significant inhibition of GlRpiB.