As in virtually every metabolic reaction on Earth, the energy produced by these reactions is stored in a molecule called ATP. The Ack enzyme catalyzes the synthesis of ATP directly. On the other hand, most ATP molecules--including those that this microbe makes by converting carbon monoxide into methane--are produced by multi-enzyme protein machines within the cell membrane that get their energy indirectly, from yet another protein machine that pumps an osmotic imbalance across the membrane. "It's difficult to imagine that something so complex could have emerged all at once," Ferry says, as the chemoautotrophic theory requires.
The acetate-producing species appears to be the direct descendant of one of the earliest true microbes. "We know that this bug is very ancient indeed," Ferry told the Penn State Astrobiology Research Center's annual meeting earlier this week. "There is strong phylogenetic evidence that acetate kinase is a very ancient enzyme." No such evidence can pinpoint the age of Pta, "but these two enzymes always work together," suggesting that they evolved together. The two enzymes' primeval genetic provenance and the simplicity of the three-step cycle, House says, "are absolutely central to the idea."
"This long-standing debate between the heterotrophic and chemotrophic theories," House continues, "revolved around carbon fixation." The new thermodynamic theory inverts the focus, Ferry says. "All these pathways evolved first to make energy. Afterwar
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Contact: Barbara K. Kennedy
science@psu.edu
814-863-4682
Penn State
12-May-2006