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BIOCHEMISTRY
Universality in intermediary metabolism

Eric Smith ^*, , and Harold J. Morowitz 

^*Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501; and Krasnow Institute for Advanced Study, George Mason University, Fairfax, VA 22030

Communicated by Murray Gell-Mann, Santa Fe Institute, Santa Fe, NM, July 14, 2004 (received for review February 6, 2004)

We analyze the stoichiometry, energetics, and reaction concentration dependence of the reductive tricarboxylic acid (rTCA) cycle as a universal and possibly primordial metabolic core. The rTCA reaction sequence is a network-autocatalytic cycle along the relaxation pathway for redox couples in nonequilibrium reducing environments, which provides starting organic compounds for the synthesis of all major classes of biomolecules. The concentration dependence of its reactions suggests it as a precellular bulk process. We propose that rTCA is statistically favored among competing redox relaxation pathways under early-earth conditions and that this feature drove its emergence and also accounts for its evolutionary robustness and universality. The ability to enhance the rate of core reactions creates an energetic basis for selection of subsequent layers of biological complexity.

We intend that "biomass" should be understood as something like a thermodynamic state of matter defined by a particular transport channel for energy and entropy through molecular bond distributions. This feature characterizes modern biota but also earlier levels of organization sometimes distinguished as "proto-life."

To whom correspondence should be addressed. E-mail: desmith{at}santafe.edu.

© 2004 by The National Academy of Sciences of the USA

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