Ribosomes: Manufactured by Design, Part 1
Before joining Reasons to Believe in 1999, I spent seven years working in R&D at a Fortune 500 company, which meant that I spent most of my time in a chemistry laboratory alongside my colleagues trying to develop new technologies with the hope that one day our ideas would become a reality, making their way onto store shelves.
From time to time, my work would be interrupted by an urgent call from one of our manufacturing plants. Inevitably, there was some crisis requiring my expertise as a chemist to troubleshoot. Often, I could solve the plant’s problem over the phone, or by analyzing a few samples sent to my lab. But, occasionally, the crisis necessitated a trip to the plant. These trips weren’t much fun. They were high pressure, stressful situations, because the longer the plant was offline, the more money it cost the company.
But, once the crisis abated, we could breathe easier. And that usually afforded us an opportunity to tour the plant.
It was a thrill to see working assembly lines manufacturing our products. These manufacturing operations were engineering marvels to behold, efficiently producing high-quality products at unimaginable speeds.
The Cell as a Factory
Inside each cell, an ensemble of manufacturing operations exists, more remarkable than any assembly line designed by human engineers. Perhaps one of the most astounding is the biochemical process that produces proteins—the workhorse molecules of life. These large complex molecules work collaboratively to carry out every cellular operation and contribute to the formation of all the structures within the cell.
Subcellular particles called ribosomes produce proteins through an assembly-line-like operation, replete with sophisticated quality control checkpoints. (As discussed in The Cell’s Design, the similarity between the assembly-line production of proteins and human manufacturing operations bolsters the Watchmaker argument for God’s existence.)
About 23 nanometers in diameter, ribosomes play a central role in protein synthesis by catalyzing (assisting) the chemical reactions that form the bonds between the amino acid subunits of proteins. A human cell may contain up to half a million ribosomes. A typical bacterium possesses about 20,000 of these subcellular structures, comprising one-fourth the total bacterial mass.
Two subunits of different sizes (comprised of proteins and RNA molecules) combine to form a functional ribosome. In organisms like bacteria, the large subunit (LSU) contains 2 ribosomal RNA (rRNA) molecules and about 30 different protein molecules. The small subunit (SSU) consists of a single rRNA molecule and about 20 proteins. In more complex organisms, the LSU is formed by 3 rRNA molecules that combine with around 50 distinct proteins and the SSU consists of a single rRNA molecule and over 30 different proteins. The rRNAs act as scaffolding that organizes the myriad ribosomal proteins. They also catalyze the chain-forming reactions between amino acids.
Ribosomes Make Ribosomes
Before a cell can replicate, ribosomes must manufacture the proteins needed to form more ribosomes—in fact, the cell’s machinery needs to manufacture enough ribosomes to form a full complement of these subcellular complexes. This ensures that both daughter cells have the sufficient number of protein-manufacturing machines to thrive once the cell division process is completed. Because of this constraint, cell replication cannot proceed until a duplicate population of ribosomes is produced.
Is There a Rationale for Ribosome Structure?
Clearly, ribosomes are complex subcellular particles. But, is there any rhyme or reason for their structure? Or are ribosomes the product of a historically contingent evolutionary history?
New work by researchers from Harvard University and Uppsala University in Sweden provides key insight into the compositional make up of ribosomes, and, in doing so, help answer these questions.1
As part of their research efforts, the Harvard and Uppsala University investigators were specifically trying to answer several questions related to the composition of ribosomes, including:
- Why are ribosomes made up of so many proteins?
- Why are ribosomal proteins nearly the same size?
- Why are ribosomal proteins smaller than typical proteins?
- Why are ribosomes made up of so few rRNA molecules?
- Why are rRNA molecules are so large?
- Why do ribosomes employ rRNA as the catalyst to form bonds between amino acids, instead of proteins which are much more efficient as enzymes?
Ribosome Composition Is Optimal for Efficient Production of Ribosomes
Using mathematical modeling, the Harvard and Uppsala University investigators discovered that if ribosomal proteins were larger, or if these biomolecules were variable in size, ribosome production would be slow and inefficient. Building ribosomes with smaller, uniform-size proteins represents the faster way to duplicate the ribosome population, permitting the cell replication to proceed in a timely manner.
These researchers also learned that if the ribosomal proteins were any shorter, inefficient ribosome production also results. This inefficiency stems from biochemical events needed to initiate protein production. If proteins are too short, then the initiation events take longer than the elongation processes that build the protein chains.
The bottom line: The mathematical modeling work by the Harvard and Uppsala University research team indicates that the sizes of ribosomal proteins are optimal to ensure the most rapid and efficient production of ribosomes. The mathematical modeling also determined that the optimal number of ribosomal proteins is between 50 to 80—the number of ribosomal proteins found in nature.
Ribosome Composition Is Optimal to Produce a Varied Population of Ribosomes
The insights of this work have bearing on the recent discovery that within cells a heterogeneous population of ribosomes exists, not a homogeneous one as biochemists have long thought.2 Instead of every ribosome in the cell being identical, capable of producing each and every protein the cell needs, a diverse ensemble of distinct ribosomes exists in the cell. Each type of ribosome manufactures characteristically distinct types of proteins. Typically, ribosomes produce proteins that work in conjunction to carry out related cellular functions. The heterogeneous makeup of ribosomes contributes to the overall efficiency of protein production, and also provides an important means to regulate protein synthesis. It wouldn’t make sense to use an assembly line to make both consumer products, such as antiperspirant sticks, and automobiles. In the same manner, it doesn’t make sense to use the same ribosomes to make the myriad proteins, performing different functions for the cell.
Because ribosomes consist of a large number of small proteins, the cell can efficiently produce heterogeneous populations of ribosomes by assembling a ribosomal core and then including and excluding specific ribosomal proteins to generate a diverse population of ribosomes.3 In other words, the protein composition of ribosomes is optimized to efficiently replicate a diverse population of these subcellular particles.
The Case for Creation
The ingenuity of biochemical systems was one of the features of the cell’s chemistry that most impressed me as a graduate student (and moved me toward the recognition that there was a Creator). And the latest work by researchers on ribosome composition from Harvard and Uppsala Universities provides another illustration of the clever way that biochemical systems are constructed. The composition of these subcellular structures doesn’t appear to be haphazard—a frozen accident of a historically contingent evolutionary process—but instead is undergirded by an elegant molecular rationale, consistent with the work of a mind.
The case for intelligent design gains reinforcement from the optimal composition of ribosomal proteins. Quite often, designs produced by human beings have been optimized, making this property a telltale signature for intelligent design. In fact, optimality is most often associated with superior designs.
As I pointed out in The Cell’s Design, ribosomes are chicken-and-egg systems. Because ribosomes are composed of proteins, proteins are needed to make proteins. As with ingenuity and optimality, this property also evinces for the work of intelligent agency. Building a system that displays this unusual type of interdependency requires, and hence, reflects the work of a mind.
On the other hand, the chicken-and-egg nature of ribosome biosynthesis serves as a potent challenge to evolutionary explanations for the origin of life.
The Challenge to Evolution
Because ribosomes are needed to make the proteins needed to make ribosomes, it becomes difficult to envision how this type of chicken-and-egg system could emerge via evolutionary processes. Protein synthesis would have to function optimally at the onset. If it did not, it would lead to a cycle of auto-destruction for the cell.
Ribosomes couldn’t begin as crudely operating protein-manufacturing machines that gradually increased in efficiency—evolving step-by-step—toward the optimal systems, characteristic of contemporary biochemistry. If error-prone, ribosomes will produce defective proteins—including ribosomal proteins. In turn, defective ribosomal proteins will form ribosomes even more prone to error, setting up the auto-destruct cycle. And in any evolutionary scheme, the first ribosomes would have been error-prone.
The compositional requirement that ribosomal proteins be of the just-right size and uniform in length only exacerbates this chicken-and-egg problem. Even if ribosomes form functional, intact proteins, if these proteins aren’t the correct number, size, or uniformity then ribosomes couldn’t be replicated fast enough to support cellular reproduction.
In short, the latest insights in the protein composition of ribosomes provides compelling reasons to think that life must stem from a Creator’s handiwork.
So does the compositional makeup of ribosomal RNA molecules, which will be the topic of my next blog post.
- The Cell’s Design: How Chemistry Reveals the Creator’s Artistry by Fazale Rana (book)
- Proteins Made by Design, Part 1 by Fazale Rana (article)
- Proteins Made by Design, Part 2 by Fazale Rana (article)
- Proteins Made by Design, Part 3 by Fazale Rana (article)
- Shlomi Reuveni et al., “Ribosomes Are Optimized for Autocatalytic Production,” Nature 547 (July 20, 2017): 293–97, doi:10.1038/nature22998.
- Zhen Shi et al., “Heterogeneous Ribosomes Preferentially Translate Distinct Subpools of mRNAs Genome-Wide,” Molecular Cell 67 (July 6, 2017): 71–83, doi:10.1016/j.molcel.2017.05.021.
- Jeffrey A. Hussmann et al., “Ribosomal Architecture: Constraints Imposed by the Need for Self-Production,” Current Biology 27 (August 21, 2017): R798–R800, doi:10.1016/j.cub.2017.06.080.