Sunday, April 27, 2008

Show me the metabolism! Part Two.

What are the requirements a catalytic system of complex polymers (CSCP) must have in order to be relevant to the origin of life?

The CSCP must be secured from the overwhelming tendency of matter and energy to become more randomly distributed in the universe.

Condition 1: An Edge.

The easy part of securing the CSCP from the tendency of matter and energy to become more randomly distributed is the barrier to separate the system from the surroundings: something to draw a surface around the CSCP and keep it together. Kauffman mentions vesicles and protein coacervates as possible CSCP microcontainers for the early terrestrial environment, and plenty of other possibilities have been canvassed. It is not very hard to think of a plausible container that could possibly arise to keep polymers in.

Condition 2: A Proto-metabolism.

The hard part is allowing the CSCP to increase the disorder of its surroundings in order to persist in time. The energy to maintain the CSCP must be coming from somewhere. The CSCP must be part of an overall system of spontaneous reactions that is converting a relatively unstable chemical reactant (or reactants) into a relatively stable chemical product (or products). The CSCP polymers must be intermediates in this net of spontaneous chemical reactions, somewhere on the path between energy-rich ‘food’ and energy-poor ‘waste’.

Condition 3: A Selectively Permeable Edge.

The energetic requirements of this net of reactions also make the easy part- the physical barrier around the CSCP- less easy. The low molecular weight intermediates have to stay in, not just the polymers. The ‘food’ has to get in. The ‘waste’ has to get out. Some selectivity is therefore required in the barrier separating the system from the surroundings. Biological membranes have evolved extraordinarily complex ways of getting the right things in and keeping the wrong things out. I am having a devil of a time trying to make a non-biological membrane to do just one thing: let ethanol through more readily than water. The tendency of matter and energy to become more randomly distributed makes generation of selective membranes a tricky business.

Condition 4: A Complexifiable Proto-Metabolism.

Not just any thermodynamically favourable driving reaction will do. This central driving reaction must proceed relatively slowly, so there are plenty of intermediate molecules around. This reaction must also have many steps, with many intermediates capable of being transformed in various ways. A great deal of complexification of the net of reactions must take place before we arrive at a CSCP. Before a CSCP can form, all of its constituent parts must be present as intermediates in this net of thermodynamically favourable reactions. I have only shown a few intermediates in the picture, but very many are required...and the relative sizes of the energy barriers and depths of the energy wells must be such that reasonable quantities of all the substrates present for making catalytic polymers are present.

I believe these requirements allowing a CSCP to persist in space and time are very difficult to meet. Nothing approaching them has ever been observed, except in two instances:

(1) Living systems

(2) Systems we have designed ourselves with a great deal of effort.

The question of how systems meeting these requirements can spontaneously arise is the key question for the origin of life.

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