By Marlene Cimons, National Science Foundation
In the early part of the 20th century, German-born biochemist Hans Krebs figured out one of the most universal ways living things use nutrients in order to produce energy, grow and reproduce. His discovery of the cycle, which was named for him, led to a Nobel Prize in 1953.
The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle, is a process where dietary sugars in the presence of oxygen burn off carbon, which is released as carbon dioxide. This process—respiration—makes much of the energy that drives metabolism. Scientists have found some version of the Krebs cycle in almost all living things, from animals and plants to yeast and bacteria.
Recently, however, researchers have discovered at least one exception: the parasite that causes malaria. Unlike almost every other organism yet studied, the breakdown of sugar by the malaria parasite is completely disconnected from the TCA cycle—it is instead fed by the amino acids glutamine and glutamate—and, in fact, is not even a cycle at all.
“The parasite has basically take the standard textbook circular cycle and broken it in half, running one half in the normal direction and the other backwards,” said Kellen Olszewski, a graduate student on the laboratory team of Manuel Llinas at Princeton University. “This turns the textbook model on its head.”
The research, published recently in the journal Nature, was funded by the National Science Foundation with several grants, including an estimated $2.3 million from the American Recovery and Reinvestment Act of 2009.
Malaria is a major global public health problem--one of the top ten killers in the world, according to the World Health Organization (WHO). It exacts the heaviest toll of disease and death among the poorest nations in the world, particularly in children and pregnant women.
WHO estimates between 300 to 500 million cases occur annually, with between 1.5 to 2.7 million deaths, 90 percent of them in tropical Sahara. Outside Africa, approximately two-thirds of the remaining cases occur in Brazil, India and Sri Lanka, although the disease exists in about 100 countries, according to WHO.
Malaria is caused by Apicomplexan parasites of the genus Plasmodium, with four species that affect humans. Transmission occurs through the female Anopheles mosquito. Llinas’ lab studies the deadliest form of the parasite, Plasmodium falciparum.
Thus far, “a rewiring of the TCA cycle as significant as this one hasn’t been observed in any other parasite,” said Olszewski, lead author of the paper. “However, a few parasites, like Cryptosporidium, have lost the pathway entirely during their evolution. There are quite a few parasites related to the Plasmodium malaria parasites, but we doubt that any of them have evolved a pathway similar to this one.”
A clearer understanding of how the parasite functions ultimately could present possible new drug targets. “We’re investigating that avenue, but it’s too early to say anything definitive,” Olszewski said. “At best, it clarifies our understanding of how a lot of other pathways that are drug targets work, since they are connected to TCA metabolism.
“Why did the parasite change such a fundamental pathway in such a weird way?” he added. “We think it goes back again to the fact that it’s a parasite. Most free-living organisms have to worry about starving, or about their environment changing suddenly on them, and the normal TCA cycle is a very versatile hub that lets creatures eat a wide variety of nutrients and generate energy in a very efficient way. The parasite, however, is floating around in your bloodstream with a pretty stable environment and a constant supply of glucose and glutamine. If these ever run out, the host and everything inside it is dead or soon will be, so it doesn’t need to worry about finding another food source and can burn sugar for energy in a much less efficient way—fermentation—and still be fine.”
Instead, this new branched pathway seems to serve at least two other purposes, he said. One arm produces a molecule the parasites need to make heme, a substance that allows the parasite to make a protein necessary to transport electrons, and the other produces acetyl-CoA, which is used to regulate protein function, and possibly gene expression.