Elsevier

Experimental Gerontology

Volume 46, Issues 2–3, February–March 2011, Pages 164-169
Experimental Gerontology

Respiratory chain cysteine and methionine usage indicate a causal role for thiyl radicals in aging

https://doi.org/10.1016/j.exger.2010.08.034Get rights and content

Abstract

The identification of longevity-related structural adaptations in biological macromolecules may yield relevant insights into the molecular mechanisms of aging. In screening fully sequenced animal proteomes for signals associated with longevity, it was found that cysteine depletion in respiratory chain complexes was the by far strongest predictor on the amino acid usage level to co-vary with lifespan. This association was though restricted to aerobic animals, whereas anaerobic animals showed variable cysteine accumulation. By contrast, methionine accumulation, a prominent feature of mitochondrially encoded proteins affording competitive antioxidant protection, was not predictive of longevity, but rather paralleled aerobic metabolic capacity. Hence, the easily oxidized sulfur-containing amino acids cysteine (a thiol) and methionine (a thioether) show doubly diametrical behaviour in two central paradigms of respiratory oxidative stress. From this comparison, it is concluded that only the one–electron oxidation of thiols to thiyl radicals contributes to aging, whereas other forms of sulfur oxidation, especially even-electron oxidation of both thiols and thioethers, are less critically involved, presumably as their consequences may be much more easily repaired. Thiyl radicals may yet act as chain-transfer agents to entail an irreversible intramembrane cross-linking (“plastination”) of some of the a priori most hydrophobic and insoluble proteins known, the respiratory chain complexes.

Research Highlights

►Cysteine avoidance in mitochondrially encoded proteins predicts longevity in aerobes. ►Methionine accumulation in these proteins predicts aerobic capacity, but not longevity. ►Thus, a specific facet of cysteine's redox chemistry must be prejudicial to longevity. ►This facet is likely to be the formation of thiyl radicals acting as chain-transfer agents. ►Thiyl radicals may lead to inner mitochondrial membrane plastination.

Introduction

Exceptional longevities in the animal kingdom of 100 years or more have long attracted the interest of the scientific biomedical community. Various aspects about these extreme longevities appear to be astonishing: first, they have been realized several times independently during evolution, in numerous phylogenetically unrelated species, e.g. among mammals, fish, or reptiles. Second, they are often accompanied by proportionately extended developmental periods without reproduction, making the survival for such extended timeframes an evolutionary necessity rather than an individual luxury. Third, even the exceptionally long-lived animals do one day show signs of aging that very closely resemble those signs to be found in short-lived animals just much earlier. These and other observations pose the question whether it is actually “simple” for a species to acquire exceptional longevity, which one might feel to infer, or whether longevity and thus long developmental and reproductive periods rather constitute the outcome of a perennial, fierce interplay of mutation and selection for this very trait.

There are essentially two alternatives on how increased longevity might be accomplished. It might either result from an investment into improved replacement and repair capacity, which also includes the restricted use of terminal differentiation (as in sponges, cnidaria, or plants, for instance), or it might result from a stepwise structural optimization of biochemical building blocks that for some reason cannot be replaced or repaired with the required fidelity or velocity or at an affordable energetic cost.

In recent years, fascinating experimental results have demonstrated once again the importance of genetically encoded mechanisms of replacement and repair on various biological levels. First, on a cellular level, immortality is basically possible. Obviously, there must be mechanisms that enable germ cells to fully escape the forces of temporal attrition. Moreover, differentiated cells that have reached their replicative limit and show numerous signs of senescence can still be rejuvenated by transformation with oncogenes. In addition, a multitude of middle-aged, differentiated cells types have been experimentally rejuvenated by transgenic protein expression of various reprogramming factors, resulting in their metamorphosis into induced pluripotent stem cells (Yamanaka, 2008). Above all, highly differentiated cells of the immune system, human B cells, have been shown to become conditionally immortal in culture by simple stimulation with an extracellular ligand (CD40L), without the need of oncogenic transformation or other types of genetic manipulation (Wiesner et al., 2008). Lacking any signs of senescence, these cells maintained their functional capacity in terms of antigen presentation and T cell activation for more than 4 years in continuous culture and in continuous need of the extracellular ligand. Hence, the achieving of immortality on a cellular level appears to be a rather straightforward task. But also on the organismal level, numerous strategies to significantly extend longevity in different laboratory animal species have been described. For instance, the genetic manipulation of signalling pathways such as the IGF-1 cascade has led to surprisingly durable and long-lived animals (Bartke, 2008, Kuningas et al., 2008). Interestingly, many of these manipulations have just brought about a simple impairment of the targeted signalling pathway and may thus be considered to constitute rather artless alterations in a functional sense. Combinedly, these results seem to indicate that longevity was a modifiable biological property like many others and thus not fundamentally different from simple anatomical characteristics such as body size or adipose tissue mass, the default targets of the IGF-1 cascade.

Given these observations, what is the role of primary building block stability in attaining longevity? In contrast to modifications in individual signal transduction pathways, which might already become optimized by a single amino acid substitution, structural stability optimizations have to work on a much more collective level. For a single cell to become resistant to heat, radiation, or starvation, numerous proteins, lipids, and other cellular constituents have to be optimized in a stepwise fashion, by variation and selection. Hence, if collective structural adaptations were detectable in higher animals in definite relation to increased longevity, such a finding would strongly argue for the indispensable importance of the corresponding structural stability aspect, because in such a case, one would have to conclude that all other conceivable adaptations on more easily accessible levels have failed to achieve the same result. In other words, why should tedious and time-consuming bit-by-bit evolutionary optimizations on a structural stability level have taken place if simpler, more straightforward mechanisms of signal transduction modulation (e.g., impairment of the IGF-1 cascade), enhanced cellular replacement (e.g., from stem cells) or accelerated molecular repair (e.g., by faster re-synthesis) were already sufficient to extend longevity in real life in the wild?

In the following, one such step-by-step evolutionary optimization on the structural stability level is described, i.e. the loss of the amino acid cysteine in respiratory chain complexes with increasing species longevity. Regarding humans as a case-in-point, this adaptation comprises approximately 80 different amino acid substitutions, demonstrating the immense evolutionary pressure against cysteine in the inner mitochondrial membrane of long-lived animals.

Section snippets

Mitochondrial cysteine depletion and longevity

In the animal kingdom, global cysteine usage is characterized by little variation. Irrespective of phylogeny or particular life history traits, nuclear-encoded cysteine usage amounts to approximately 2.0–2.5% of all encoded amino acids (Moosmann and Behl, 2008). Mitochondrially encoded cysteine usage, however, varies substantially, and accounts for only ~ 0.5% of all amino acids in certain chordates, while contributing ~ 4.5% of all amino acids in some platyhelminthes. It was recently

Mitochondrial methionine accumulation and aerobic metabolic capacity

Similarly as with cysteine, global methionine usage in animals falls within a well defined range of 2.2–2.8% of all encoded amino acids (Bender et al., 2008). Methionine usage in mitochondrially encoded proteins yet ranges from approximately 2% in echinoderms to up to more than 10% in many insects and is strikingly higher than in nuclear-encoded proteins in the majority of animals. The very unusual variability in the fractional use of an amino acid between species was accounted for by the use a

What is the decisive difference between cysteine's and methionine's redox chemistry?

A very relevant redox biochemical difference between cysteine and methionine has to exist regarding the aging process, as might be diagnosed from a comparison of Fig. 2A and B, or from the structural sketch of bovine cytochrome c oxidase depicted in Fig. 3. In nuclear-encoded proteins, cysteine and methionine are similarly frequent and similarly distributed between hydrophilic and hydrophobic environments. In general terms, both amino acids are easily oxidized, and some types of oxidation are

Conclusion: “The Plastination Hypothesis of Aging”

The unique free radical chain-transfer activity of thiols in nonpolar environments might pinpoint to the most likely reason for the specific and longevity-dependent avoidance of cysteine in inner mitochondrial membrane proteins: thiols in this lipophilic environment may become radicalized by various diffusible initiators from outside the protein and may then transfer and translate this event into the radicalization of a nearby part of the protein backbone (Schoneich, 2008). In consequence,

Acknowledgments

This work was supported by the Hans–Gottschalk–Stiftung.

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