How to Eat PCP

The evolution of an irreducibly complex system:
(from http://www.talkdesign.org/faqs/icdmyst/ICDmyst.html#how2eatpcp)

How to Eat Pentachlorophenol

Pentachlorophenol (PCP) is a highly toxic chemical, not known to occur naturally, that has been used as a wood preservative since the 1930's. It is now recognized as a dangerous pollutant that we need to dispose of. But how?

Evolution to the rescue! A few soil bacteria have already worked out a way to break it down and even eat it. And conveniently for us, they do it in an irreducibly complex way. The best known of these bacteria is called Sphingomonas chlorophenolica (also called Sphingobium chlorophenolicum).

The PCP molecule is a six carbon ring with five chlorine atoms and one hydroxyl (OH) group attached. The chlorines and the ring structure are both problems for bacteria. S. chlorophenolica uses three enzymes in succession to break it down, as follows: the first one replaces one chlorine with OH. The resulting compound is toxic, but not quite as bad as PCP itself. The second enzyme is able to act on this compound to replace two chlorines, one after the other, with hydrogen atoms. The resulting compound, while still bad, is much easier to deal with, and the third enzyme is able to break the ring open. At this point, what is left of PCP is well on its way to being food for the bacterium.

All three enzymes are required, so we have IC. How could this IC system have evolved? First of all, bacteria of this type could already metabolize some milder chlorophenols which occur naturally in small amounts. In fact the first and third enzymes were used for this. As a result the cell is triggered to produce them in the presence of chlorophenols. The second enzyme (called PcpC) is the most interesting one; the cell produces it in sufficient quantity to be effective all the time instead of just when it is needed in its normal metabolic role. Thanks to this unusual situation PcpC is available when it is needed to help eat PCP.

The inefficient regulation of PcpC is evidently the key to the whole process. So far as biologists can tell, a recent mutation that changed the deployment of this enzyme is what made PCP degradation possible for this bacterium. It also happens that both PcpC and the first enzyme in the process are now slightly optimized for dealing with PCP; they handle it better than the corresponding enzymes in strains of S. chlorophenolica that use PcpC only in its normal role, but not nearly as well as would be expected for an old, well adapted system. These factors, combined with the fact that PCP is not known to occur naturally, make a strong circumstantial case that this system has evolved very recently.

The chemistry and probable evolution of this system are explained in much greater detail in Shelly Copley's article "Evolution of a metabolic pathway for degradation of a toxic xenobiotic: the patchwork approach" in Trends in Biochemical Sciences. (see Copley SD. (2000). Evolution of a metabolic pathway for degradation of a toxic xenobiotic: the patchwork approach. Trends in Biochemical Sciences, 25(6):261-265.)

What fun!