Human-built nanotechnology can only hope to slowly catch up with what natural evolutionary processes have already invented. As I’ve said here before, viruses and bacteria are naturally-occurring nanomachines, self-constructed from common materials and powered by ambiently-available chemical energy. One big reason that I don’t fear a globally destructive nanotech invasion – a “grey goo” scenario – is that humans are very unlikely to discover anything that four billion years of mutation/selection cycles haven’t already discovered. Case in point: viral motors.
An article by Jeanna Bryner on the LiveScience blog showcases the latest discovery in natural nanotechnology… an ATP-driven motor used by viruses to package their DNA in preparation for injection into a host cell. The viral motor is constructed from two rings of protein molecules, each ring consisting of five individual molecules chained together to form a physical loop. The molecular rings turn in response to a cascade of chemical bonding reactions, and the resulting action pulls a strand of viral DNA into a tight spool inside the virus’s protein sheath.
Proteins make good machines. A given protein molecule is made up of a few simple parts – amino acids – that can link together to form long and complicated arrangements. Each amino acid can link with another through a chemical reaction that releases a water molecule and produces a strong bond, and often a second linkage can be made using one of many types of side-chains… molecular components of amino acids that vary in the strength, covalency and polarity they can exhibit. Amino acids can link together to produce immensely long chains, and because different amino acids have slightly different properties those chains can bend easily in some places and be rigid in others. Protein chains can fold in upon themselves – they can even be tied into knots – resulting in a nearly infinite variety of shaped, functional machine bits. Because of side-chains, proteins don’t have to be linear. They can branch, interlink, and anastomose with staggering complexity… they can even use individual atoms of metals as focal points where many individual chains meet up.
How many different protein machines can be made? Well, how many things can you make with an infinite pile of tinkertoys? Yes, that’s right… an infinite variety. Proteins bent into long, rigid lengths with a solid knot at one end can act like a hammer. Proteins bent into rings can act like gears. Proteins can form scaffolds that support and arrange other protein machine parts. Proteins can have open binding points where an electrically-charged side-chain sits exposed, so that when a particular metal atom comes along and binds there the entire protein might be forced by the laws of chemical bonding to kink in a particular way. When the metal atom is lost or pulled away by another molecule, the protein unkinks… forming a moving machine part.
Evolution constantly tries new arrangements of parts and only keeps the arrangements that work, and by work I mean do anything that keeps the machine running. A new part – a new protein configuration – tossed by mutation into the mix might find a useful role or it might destroy the entire contraption. There’s no plan, only constant tinkering and kluging, with results that appear subtle and elegant only because one never sees the failed attempts. Those are just thrown away. We only see the winners, which may or may not be the absolutely and ultimately best way of doing something… but evolution doesn’t hand out awards based on comparison with perfection, only based on the question: are you still alive?
Microbes use proteins because proteins are probably the most reliable and most easily assembled core components that one can produce naturally from the constituent elements of the Earth’s crust. Bacteria and viruses have had four billion years to test random variations in molecular machinery, and it shouldn’t surprise anyone that the results we see today work pretty well. Not perfectly, but well enough. Those interested in building tiny machines would probably save a lot of time by looking first at how living organisms solve similar problems and copying the robust solutions evolution has produced already.
I suspect that today’s separate fields of molecular biology and industrial nanotechnology will eventually collide, much as chocolate and peanut butter once collided, yielding results that are both delicious and revolutionary.
~ by Planetologist on January 12, 2009.