Cars and Cells: How Trade-offs Make Good Designs
I have spent the last few days shopping for a car: decisions, decisions! And of course one of the questions I have to answer is, Do I want horsepower or fuel efficiency?
The trade-off between power and efficiency not only constrains automotive designs but also it impacts a key energy-harvesting metabolic pathway in the cell, as recent work by researchers from Israel and Germany demonstrates.1 This important new insight highlights the exquisite molecular logic of biochemical pathways, providing even more evidence that life stems from the work of a Creator.
Just like automobiles, cells need fuel to run. One of the most important sources of cellular energy is the carbohydrate, glucose. The cell’s biochemical machinery breaks down this six-carbon sugar into smaller three-carbon compounds and in the process uses the released energy to produce ATP (adenosine triphosphate). This molecule, in turn, serves as a fuel, powering virtually all of the cell’s operations.
In eukaryotic cells, the route for glucose breakdown is called the Embden–Meyerhof–Parnas (EMP) pathway. The proteins that make up this pathway sequentially break down each molecule of glucose into two pyruvate molecules. This process generates a net of two ATP molecules.
Many bacteria also employ the EMP pathway. But some make use of alternative pathways to break down glucose. One of the most common alternatives is the Entner–Doudoroff (ED) pathway. While this metabolic route shares many of the features of the EMP pathway, it also displays key differences that impact its yield. Instead of generating two molecules of ATP per glucose molecule, the ED pathway produces only one.
Biochemists wonder, “Why is the less-efficient ED pathway so prevalent?”2 Skeptics often point to the occurrence of what appear to be poor designs, such as the ED pathway, as evidence that life must have arisen via evolutionary processes. If the Creator is all-good, all-powerful, and all-knowing, why would inefficient systems like the ED pathway exist in nature?
Yet, the international team’s work explains the prevalence of the ED pathway. They learned that while this metabolic route is less efficient than the EMP pathway, it releases more free energy, propelling the pathway forward. Conversely, the EMP pathway operates with greater efficiency, but has a reduced driving force behind it. This property makes the EMP pathway susceptible to bottlenecks. To overcome potential backlogs, cells that rely on the EMP pathway produce a greater quantity of the proteins that contribute to this metabolic process than is required for the ED route. Such greater protein demands for the EMP pathway represent an added cost for the cell.
In other words, the ED pathway is not a poor design. Instead, the EMP and ED routes represent two different ways to resolve the trade-off between power and efficiency––both approaches elegant in their own right.
- Avi Flamholz et al., “Glycolytic Strategy as a Tradeoff between Energy Yield and Protein Cost,” Proceedings of the National Academy of Sciences, USA 110 (June 11, 2013): 10039–44, doi: 10.1073/pnas.1215283110.
- Arion I. Stettner and Daniel Segrè, “The Cost of Efficiency in Energy Metabolism,” Proceedings of the National Academy of Sciences, USA 110 (June 11, 2013): 9629–30, doi: 10.1073/pnas.1307485110.