Coenzymes: live long and prosper at a molecular scale
The intracellular environment is a challenging place for all molecules. Nonetheless, it has been shown that macromolecules such as proteins can be extremely long-lived in vivo. In our paper, we now demonstrate that such longevity, and in turn stability, can even be achieved on the scale of small molecules - coenzymes.
We all know that nothing lasts forever. In living, metabolically active cells, things can get particularly tricky. Molecules are packed in a crowded, highly reactive environment. As "nothing of chemistry disappears in biology" (A. Golubev, ref. 1), biochemists have long been wondering how this potent, chemically reactive surrounding affects the stability of intracellular components. While we owe our very existence to the longevity and stability of DNA (enabled by a set of dedicated repair mechanisms), findings of very long-lived proteins came to a larger surprise. In tissues where cells barely divide, some proteins may stick around for months or even years (see e.g. ref. 2), facing an increased probability of damage accumulation and perhaps acting as a molecular mechanism behind the aging process.
Attending a talk on the stability of such long-lived protein complexes, but working mainly with small molecules (metabolites), we wondered if this could also be possible at an even smaller scale. A promising set of candidate molecules immediately came to mind: coenzymes! Like their much larger counterparts — enzymes — they mainly act catalytically and do not dissipate during a catalytic cycle. However, when browsing the literature, we learned that a lot can go wrong in the challenging life of a coenzyme. Most notably, they are intrinsically reactive and face a plethora of damage reactions that can corrupt them. Although specific coenzyme repair mechanisms (reviewed in ref. 3) have been discovered, little was known about the actual half-life and factors driving the exchange of coenzymes in the cell. Asking colleagues what they would expect, all would start with a rather long pause, a smile, followed by "I don't know". After additional pondering, some would finally conclude: "they might be quite stable". That was our assumption, too; they could be … but are they? We set out to actually determine the half-life of coenzymes in metabolically active cells and were surprised by the answer: Some coenzymes, mostly derived from B vitamins, are not turned over at all; they are essentially only made to compensate for growth. After all, when one cell becomes two, the amount of available coenzyme must also double.
How did we determine coenzyme turnover? We used microorganisms that are able to derive their coenzymes from simple sugar molecules, and changed their menu to a "heavy" sugar where all carbon atoms are replaced by a heavier isotopic form. We then followed the incorporation of "heavy" carbon into coenzymes over time using a state of the art mass spectrometer which allows us to distinguish the associated mass shifts. Next, we measured the fraction of "old" coenzymes with no label, and "new" coenzymes that had obtained at least one labelled carbon. We observed a close match between the rate of renewal of coenzymes and the expected synthesis rate that only compensates for growth. Most notably, this finding then allowed us to infer that these coenzymes must be maximally long-lived in vivo, that is, "within the cell", but potentially also "enabled by the cell", as some coenzymes might well owe their stability to repair systems.
Thus, not only macromolecules such as proteins, but also molecules as small (and reactive!) as coenzymes can achieve exceptional longevity within a cell. In other words - some coenzymes might very well outlive their macromolecular counterparts - enzymes - and are passed on to their grand-grand-… children.
Read the paper in Nature Microbiology: Longevity of major coenzymes allows minimal de novo synthesis in microorganisms.
- Golubev A (2009) How could the Gompertz-Makeham law evolve. J. Theor. Biol. 258:1-17.
- Toyama BH, et al. (2013) Identification of long-lived proteins reveals exceptional stability of essential cellular structures. Cell 154:971-982.
- Linster CL, Van Schaftingen E, & Hanson AD (2013) Metabolite damage and its repair or pre-emption. Nat. Chem. Biol. 9:72-80.