Lipid World

This Lipid World paper is must reading for me. From 2001, the authors lay out the case for lipids being involved in the origin of life. The title is in contrast to RNA World which is the common theory that life started with RNA molecules that were autocatalytic.

Summary & Review

Section 1: Introduction: Did Life Processes Necessarily Begin with Biopolymers?

• Biopolymers means RNA, DNA and proteins
• One advantage of RNA World is that it seems to have the evolution and information storage problem solved already
• On the other hand, RNA World is quite complex and so the chances of life starting from it become increasingly small

Section 2: Lipid-like Amphiphiles Are Highly Diverse

• Current lipids are highly diverse and have multiple functions rather than simply being a barrier between inside and outside
• This would be expected if lipid evolution was involved in the origin of life (but it could have also evolved later, so it's a chicken and egg argument)

Section 3: Amphiphiles Undergo Self-Assembly

• This is one of the most compelling arguments for Lipid World--the cells form by themselves once concentrations of lipid are high enough
• The problem is reduced to finding building blocks and reactions that can form lipids
• The lipids also don't have to be the same type in order to form; however, some combinations will be more stable than others and this would create a selective pressure enabling the early proto-cells to evolve

Section 4: Lipid-Like Amphiphilic Molecules Predated the Origin of Life on the Earth

• Both prebiotic reactions and source from space (meteors, comets, space dust) could have created lipids
• Here the paper diverges from my hypothesis which is that the first life forms found a way to efficiently synthesize lipid molecules and only the basic building blocks needed to be available

Section 5: Prebiotic Synthesis and Assembly of Membranes

• Quote: "One aspect of early cellular life that is often disregarded is that primordial membranes would need to continuously add amphiphilic components in order to accommodate the growth and replication of the encapsulated macromolecular system or of the lipid aggregate itself." Exactly!
• Studies done in 1977-78 discovered the lipid properties for self-assembly of micelles. They formed with a minimum of 10 carbons. A variety of polar groups were used such as carboxylic acids, alcohols, amines, alkyl phosphates, and alkyl sulfates (some of these may have required more than 10 carbons). They also studied mixtures and found some were more stable than the single components.
• Experiments have shown that PAH (polyaromatic hydrocarbons, e.g. phenanthrene) can photochemically react with CO2 to form aliphatic acids. These hydrocarbons could float on top of the water and be exposed to UV light from the sun.
• "There are two challenges for future research in this area. The first is to find a plausible synthetic pathway for hydrocarbons with 10 or more carbons in their chains. [...] Second, reactions must be established by which both polar and ionic character can be added to the hydrocarbon chains." Yes, these are what I need to find with an autocatalytic set of molecules.
• Alternatively, a polar building block could be extended to a long chain hydrocarbon which is how lipids are formed in our current cells (this makes much more sense)

Section 6: Lipozymes: Non-covalent Amphiphilic Aggregates with Catalytic Properties

• Many reactions are catalyzed by the presence of micelles (apparently by the concentration of greasy reagents into the hydrophobic portions, or by changes in reactivity similar to solvent effects)
• One useful reaction is the hydrolysis of esters with n-heptyl-SH
• Keep in mind: Catalysis can take place in the aqueous cytosol and in the lipid bilayer
• They introduce the term "lipozyme" to describe lipid aggregates that enhance the rate of a chemical reaction (this is a lousy term because an enzyme refers to a specific molecule but lipozyme refers to an aggregate state of molecules)

Section 7: Autocatalytic Lipozymes May Self-reproduce

• Here they get to the idea of autocatalytic sets!
• Criticism of this idea: "But they have been argued not to embody some of the properties essential for initiating an evolutionary process, since they lack information carriers such as nucleic acids or peptides." I need to come up with a mechanism to add information carriers.
• "autocatalytic lipozymes would belong to the class of phenotypic replicators, because of their functional rather than digital inheritance." Good to know the term for what I'm proposing.

Section 8: Statistical Properties of Lipozyme Catalysis: the Importance of Diversity

• They introduce their Graded Autocatalysis Replication Domain (GARD) model which is essentially a model of molecular reactions where the molecules have a range of catalytic abilities to not only replicate themselves but also other members in the set

Section 9: Mutually Catalytic Networks Within Complex Lipozymes

• This section outlines the same series of steps I proposed for the origin of life in this post
• Here they explain why they started with simulations instead of wet chemistry experiments: "Future experiments could be initiated with a mixture resembling a possible prebiotic amphiphile repertoire, and while keeping the system far from equilibrium, the composition of individual assemblies would be monitored. Such experiments are still very challenging and could be characterized by utterly slow rate constants of months and years. For this reason we have resorted to a mathematical modeling approach, based on computer simulations of the kinetics of self-assembly in complex molecular mixtures." I agree.
• They also make the very interesting point that catalysis could happen in the membrane. I like this idea because the membrane concentrates molecules (easier for them to meet) and also provides another dielectric constant for reactions which will increase the speed of certain processes (e.g. hydrolysis by aliphatic thiols)

Section 10: Compositional Information in Lipozymes

• Discusses how these Lipid World protocells would legitimately evolve with collections of molecules being passed on to daughter cells
• They make the important point that our modern day cells to the same thing with structures of membranes and the collection of molecules in the cell, including RNA and proteins
• Questions I have: What parts of the cell aren't encoded by DNA? Are any molecules in modern day cells replicated by an autocatalytic network not driven by DNA and proteins?

Section 11: Evolution of Autocatalytic Lipozymes

• More details of their simulations which supports Lipid World
• Reference The Major Evolutionary Transitions which discusses a nucleic acid takeover leading to the modern genetic code. I'll read this next

Section 12: Summary

• RNA world has problems. Nucleotides are complex and not present in prebiotic soup experiments. They also don't have a way to carry out metabolism.
• They offer an interesting theory on the evolution of protein and nucleotide biopolymer. Amino acids and nucleotides could have formed some head groups on lipids. Then, being in the 2-dimensional lipid layer would aid their oligomerization. These could then be detached and wind up in the cytosol as further catalysts.
• "An important goal for future research will be to provide an additional experimental basis for the Lipid World scenario." I agree.  I'll get on it.

I believe this will end up being one of the seminal papers on the origin of life.

Late to the Party

I was just reading this paper called "The early evolution of lipid membranes and the three domains of life" from 2012 in Nature Reviews. It talks about the dominant research areas on the origin of life:

Despite the crucial role of membranes in allowing the genetic and metabolic systems to interact and evolve together, most studies on the origin and early evolution of life have focused on the emergence of either the genetic system or energy and carbon metabolism

 But they go on to say that a focus on membranes as an early key component is changing:

This avenue of research is progressing quickly, to the point that the traditional focus on self-maintenance (metabolism) as a major property of life, together with self-replication (a genetic system), is shifting towards a focus on self-assembly (membranes) in contemporary origin-of-life thinking.

And they give references that I'll have to look up:

• Mansy, S. S. et al. Template-directed synthesis of a genetic polymer in a model protocell. Nature 454, 122–125 (2008).
• Segre, D., Ben-Eli, D., Deamer, D. W. & Lancet, D. The lipid world. Orig. Life Evol. Biosph. 31, 119–145 (2001).
• Mansy, S. S. & Szostak, J. W. Reconstructing the emergence of cellular life through the synthesis of model protocells. Cold Spring Harb. Symp. Quant. Biol. 74, 47–54 (2009).

It looks like I'm late to the party with my idea about focusing on membranes. However, the article doesn't mention autocatalytic sets. I'll have to see if any of the references mention them.

Hanczyc's Oil Drop Life

I found a Nature News article discussing Hanczyc's oil drop life which was also the subject of the TED talk I saw. My opinion is that this is a system that has life-like behavior but is not relevant to the origin of life. They are nitrobenzene droplets in basic aqueous solution (with a pH indicator to make the motion more interesting) and a greasy anhydride as "fuel". Differing gradients of pH allow the drops to skim across the surface and move like cells.

I've noticed something similar while watching crystallization of an organic compound. The first crystals formed on top of the solution and as the crystals grew, they scooted across the surface, either from the impact of molecules being added to the crystal or from the temperature of the solution heating up (or cooling) on the side that was growing. (At a molecular level, are these two explanations really different?)

So while Hanczyc's oil droplets are fascinating to watch, they are a physical phenomenon and not a useful step in the direction of autocatalytic sets which could explain the origin of life. However, the article mentioned research by Jack Szostak at Harvard who is doing research on protocells focusing on the lipid portion so I'll look in to that next.

Why DROOL?

My interest in the Origin of Life was inspired by my discovery of self-replicating cellular automata in simple systems, as described on my Fourier Life page. These systems show that simple components can act together to produce complex emergent behavior, in this case self-replication. The same phenomenon may occur with autocatalytic sets to produce life.

But where to start? Here's what I could do:

1. Create a simulation of a realistic prebiotic soup and run simulations to see if I can find an autocatalytic set of molecules with realistic chemistry
2. Pick a simpler system to explore autocatalytic sets (something in between cellular automata and a realistic prebiotic soup simulation)
3. Research what others have done on autocatalytic sets so I don't have to "re-invent the wheel"

Option 1 is the ultimate goal but I might spend a lot of time programming a realistic simulation only to find out my approach is wrong and I will have wasted a lot of time. Option 2 is interesting, but what model system should I choose? To decide that, I probably need to do option 3. So although option 3 is the most boring, it's probably the best place to start.

I also need to develop my initial hypothesis more. Here are the steps I have in mind:

1. Organic building block molecules are formed in the ocean or isolated pools (I need to look up what molecules are realistic)
2. These molecules react in all possible combinations, making and breaking bonds
3. Some of these molecules ultimately form lipid molecules, perhaps being catalyzed by other molecules in the soup (perhaps by making a key carbon-carbon bond-forming reaction be intramolecular instead of intermolecular which can speed up the reaction 1000-fold)
4. The concentration of lipids builds up until the CMC (critical micelle concentration) is met at which time micelles spontaneously form
5. Some force (wind, lightning, rock slide) occurs which turn the micelles into liposomes, trapping some of the prebiotic soup inside the first cells
6. Some of these cells continue to carry out the same reactions which form more of the lipid molecules inside the cell which then become incorporated in the membrane
7. Once the membrane get large enough, the cell spontaneously splits into two with half of the cytosol going into each new cell

So far, this picture doesn't really rely on an autocatalytic set of molecules. Would the above cells really be alive? Ayn Rand's definition of life is: the process of self-generated and self-sustaining action. They do grow and divide, so that is a type of action. The process is also self-generated because the result of the action come from reactions within the cell. It also appears that the cells could evolve because some cells would have a more efficient set of molecules within them which would allow them to produce new lipid more quickly than others. These would then use up more of the building blocks in the soup and so this type of cell would be more prevalent than other types.

This type of system may not be too difficult to set up in a lab. In fact, something like this may have already been developed. I recall seeing something similar in a TED talk. I should look for that...

DROOL Notebook

Mythbusters have said that the only difference between screwing around and science is writing it down. So this is my notebook for my screwing around--err, I mean scientific studies--on the origin of life.

DROOL stands for Discovering and Recreating the Origin Of Life. Those are my goals. Specifically, I want to test this hypothesis:

Life started with an autocatalytic set of molecules that created lipids which spontaneously formed liposomes and contained the autocatalytic set of molecules. These first cells continued producing lipids which, when added to the liposome, became large enough to spontaneously split in two and enabled the first cells to reproduce themselves.

This blog will be haphazard and non-structured, at least at first. It will be a running commentary on my experiments and, hopefully, results.

I plan to run both computer and wet chemistry experiments. The first will be toward the goal of discovering the origin of life, the second in recreating it.

More to come...