Electron transport chains as a window into the earliest stages of evolution

The origin and early evolution of life is generally studied under two different paradigms: bottom up and top down. Prebiotic chemistry and early Earth geochemistry allow researchers to explore possible origin of life scenarios. But for these "bottom-up" approaches, even successful experiments only amount to a proof of principle. On the other hand, "top-down" research on early evolutionary history is able to provide a historical account about ancient organisms, but is unable to investigate stages that occurred during and just after the origin of life. Here, we consider ancient electron transport chains (ETCs) as a potential bridge between early evolutionary history and a protocellular stage that preceded it. Current phylogenetic evidence suggests that ancestors of several extant ETC components were present at least as late as the last universal common ancestor of life. In addition, recent experiments have shown that some aspects of modern ETCs can be replicated by minerals, protocells, or organic cofactors in the absence of biological proteins. Here, we discuss the diversity of ETCs and other forms of chemiosmotic energy conservation, describe current work on the early evolution of membrane bioenergetics, and advocate for several lines of research to enhance this understanding by pairing top-down and bottom-up approaches.

On the principle of ion selectivity in Na+/H+-coupled membrane proteins: experimental and theoretical studies of an ATP synthase rotor

Numerous membrane transporters and enzymes couple their mechanisms to the permeation of Na(+) or H(+), thereby harnessing the energy stored in the form of transmembrane electrochemical potential gradients to sustain their activities. The molecular and environmental factors that control and modulate the ion specificity of most of these systems are, however, poorly understood. Here, we use isothermal titration calorimetry to determine the Na(+)/H(+) selectivity of the ion-driven membrane rotor of an F-type ATP synthase. Consistent with earlier theoretical predictions, we find that this rotor is significantly H(+) selective, although not sufficiently to be functionally coupled to H(+), owing to the large excess of Na(+) in physiological settings. The functional Na(+) specificity of this ATP synthase thus results from two opposing factors, namely its inherent chemical selectivity and the relative availability of the coupling ion. Further theoretical studies of this membrane rotor, and of two others with a much stronger and a slightly weaker H(+) selectivity, indicate that, although the inherent selectivity of their ion-binding sites is largely set by the balance of polar and hydrophobic groups flanking a conserved carboxylic side chain, subtle variations in their structure and conformational dynamics, for a similar chemical makeup, can also have a significant contribution. We propose that the principle of ion selectivity outlined here may provide a rationale for the differentiation of Na(+)- and H(+)-coupled systems in other families of membrane transporters and enzymes.

Methylcitrate cycle defines the bactericidal essentiality of isocitrate lyase for survival of Mycobacterium tuberculosis on fatty acids

Few mutations attenuate Mycobacterium tuberculosis (Mtb) more profoundly than deletion of its isocitrate lyases (ICLs). However, the basis for this attenuation remains incompletely defined. Mtb's ICLs are catalytically bifunctional isocitrate and methylisocitrate lyases required for growth on even and odd chain fatty acids. Here, we report that Mtb's ICLs are essential for survival on both acetate and propionate because of its methylisocitrate lyase (MCL) activity. Lack of MCL activity converts Mtb's methylcitrate cycle into a "dead end" pathway that sequesters tricarboxylic acid (TCA) cycle intermediates into methylcitrate cycle intermediates, depletes gluconeogenic precursors, and results in defects of membrane potential and intrabacterial pH. Activation of an alternative vitamin B12-dependent pathway of propionate metabolism led to selective corrections of TCA cycle activity, membrane potential, and intrabacterial pH that specifically restored survival, but not growth, of ICL-deficient Mtb metabolizing acetate or propionate. These results thus resolve the biochemical basis of essentiality for Mtb's ICLs and survival on fatty acids.