Is the Origin of Life in Hot Water?
Is origin of life chemistry in hot water? So it seems according to a new paper in the Proceedings of the National Academy of Sciences. The authors address the conundrum of origin of life chemists between the rate of (un-catalyzed) organic reactions and the lack of time available for these reactions to occur. From the article (note: an enzyme is a biological catalyst):
Whereas enzyme reactions ordinarily occur in a matter of milliseconds, the same reactions proceed with half-lives of hundreds, thousands, or millions of years in the absence of a catalyst. Yet life is believed to have taken hold within the first 25% of Earth's history. How could cellular chemistry and the enzymes that make life possible, have arisen so quickly?" [Internal citations omitted]
Indeed this is one of the problems with origin of life scenarios, particularly those scenarios that presume a metabolism-first world (as opposed to an RNA-first world). The half-life of certain reactions without a catalyst can be millions of years, but studies show that the emergence of early bacteria could be dated as far back as 3.5 billion years (see ENV post on a cold origin of life and Schopf, J. William, "The First Billion Years: When Did Life Emerge?" Elements vol 2:229 (2006) for more on this). This means there was a limited amount of time for fundamental biological reactions to occur. Reaction kinetics can be prohibitive. However, the authors of this paper have a theory to solve the reaction kinetics problem.
A little chemistry review: Reaction kinetics has to do with the rate of a reaction, or how long a reaction takes to produce products. The products of a particular reaction may be perfectly stable, but that doesn't necessarily mean the reaction is going to proceed any faster, especially if it has a large energy barrier to overcome before getting to those stable products. So if you want to speed up a reaction, you have two options: 1) Add energy, or 2) add a catalyst which lowers the energy barrier.
This article mentions, by way of example, several biologically significant reactions. Normally, these reactions, either done in the lab or in nature, require a catalyst. Catalysts usually serve to stabilize intermediates in a reaction, which lowers the energy barrier. By lowering the energy barrier, the reaction can complete much more quickly -- as in a matter of seconds, instead of a matter of millions of years.
But we don't have the luxury of catalysts in an origin of life scenario because catalysts for many biological reactions are specific to those reactions and too complex for early earth reactions. Another way to speed up a reaction is to add energy, usually in the form of heat. The authors of this article propose that many of these biological reactions which are prohibitively slow are sped up if they are in a hot environment, such as boiling water. They justify their theory by showing how reaction rates of certain biologically essential reactions, such as OMP decarboxylation or DNA phosphodiester hydrolysis decreases significantly at 100oC compared to 25oC. Furthermore, the authors point out that the slowest uncatalyzed reactions are most sensitive to temperature. Some examples that they report from the literature:
- Urea hydrolysis (half life of 500 years at 25oC) increases about 3,000-fold when the temperature is raised from 25 to 100oC
- Hydrolysis of O-glycosidase bonds (half-life of 18 million years at 25oC) increases about 190,000-fold when the temperature is raised from 25 to 100oC
- Hydrolysis of aliphatic phosphate monoester dianions (half life of 1.1 x 1012 years at 25oC) is accelerated about 10,000,000-fold when the temperature is raised from 25 to 100oC
The authors provide no explanation for their choice of reactions, only that these are important biological reactions. How these reactions contribute to the formation of a self-assembled metabolic cycle is not covered in this paper; neither is how the information system of a cell formed, for that matter. Understandably, origin-of-life chemistry has many steps, and these authors are addressing one issue, but for the most part, this article is a lot more about fundamental principles of kinetics and thermodynamics than anything that adds to the study of the origin-of-life. Yes, heat makes many kinetically prohibitive reactions speed up. Yes, enzymes operate by lowering the enthalpy of reaction which does not come into play until temperatures are lowered. As important as these things are for chemistry, where are the starting reactions, how do they assemble, what do they result in, how do the enzymes form, what about the heat degrading products or accelerating side reactions? These are important questions when the premise of a paper is that the geological clock is ticking.
The most interesting idea in this paper is the proposition that enzymes evolved to operate by lowering the enthalpy of reaction because these reactions are so temperature dependent:
From an evolutionary standpoint, it is unlikely that the common enthalpy-lowering effect of present-day enzymes is fortuitous. As the environment cooled, a primitive catalyst that reduced delta H (double dagger) would have offered a selective advantage over a catalyst that raised T(delta S) by an equivalent amount...We propose that enthalpy-lowering mechanisms became common because they are so temperature-dependent; and because there is almost no limit -- at least in principle -- to the benefit that might arise from the action of a purely 'enthalpic' catalyst. Natural selection has presumably resulted in the evolution of enzymes toward greater catalytic power and specificity...
However, there is very little specific information or data to go along with this statement. It is really speculation, which is interesting, but not compelling without additional explanation for how exactly these catalysts were formed, how the original biological reactions happened, and how the energy added through heat is harnessed and controlled in such a way as to protect the intermediates and products of the reactions from degradation or unhelpful side reactions.
Only two months ago Evolution News and Views reported on a paper that came out about a cold origin of life scenario. That article dismissed the problem of kinetics and addressed the problem of product stability and concentration, which they claimed was prohibitive in origin-of-life scenarios. If origin-of-life chemists are going to address each and every step in such a piecemeal fashion, hopefully, they will take a step back to realize that they have some competing problems in their current scenarios. Fixing one problem makes the other worse. Heating a reaction does nothing for product stability. Cooling a reaction makes the reaction rate problems worse. Furthermore, neither of these accounts for 1) how DNA became the information-carrying powerhouse that it is nor 2) how the initial cycle of a metabolic cycle came together without competing side reactions and without relying on a previous step in the cycle.
Dembski and Wells state the overall problem best in their work, Design of Life (2008):
Whenever origin-of-life researchers accept plausibility rather than evidence as their standard for scientific truth, they in effect give up the search for what really happened or for what with reasonable probability could have happened. Plausibility, as Stewart and many origin-of-life researchers understand the term, implies no effort to estimate probability. Instead, they settle for what they can imagine was possible or could have happened. In this way, they substitute opinion and prejudice for experiments and data. (241)