Shelley D. Copley
Ph.D. Harvard University, 1987
Professor; Molecular, Cellular and Developmental Biology
Biodegradation of recalcitrant pollutants; mechanistic studies of enzymes involved in biodegradation.
Current Research: Evolution of enzymes and metabolic pathways
The Copley laboratory studies the evolution of enzymes and metabolic pathways in the context of the complex metabolic and regulatory networks in cells. Enzymes in extant organisms are highly efficient catalysts for specific reactions. However, they are not perfectly specific. Most, and probably all, enzymes also have low-level “promiscuous” activities that arise because of the highly reactive environment at their active sites. Although these activities are inefficient, they can accelerate reactions by several orders of magnitude relative to the rates of uncatalyzed reactions. Thus, a promiscuous activity provides an excellent starting place for evolution of a new enzyme if that activity becomes important for growth or survival. The presence of hundreds of enzymes, each of which has a number of promiscuous activities, provides the possibility of patching together multiple promiscuous activities to generate a novel metabolic pathway. Microbes in contaminated environments can evolve new metabolic pathways for
degradation of pollutants by recruiting enzymes with promiscuous activities to serve new roles. These newly evolved pathways
are often quite inefficient. This is the case for the degradation of pentachlorophenol (PCP) by Sphingobium chlorophenolicum,
a bacterium isolated from a site that was heavily contaminated with PCP. The ratedetermining step for degradation is the first
step in the pathway; see Figure 1 (McCarthy et al. 1997). The enzyme responsible for this reaction, PCP hydroxylase, turns over at a rate of only 0.02 s-1 (Hlouchova et al. 2012).
For comparison, well-evolved enzymes that hydroxylate naturally occurring phenols turn over their substrates at 25 to 50 s-1. We
have recently shown that the chemical steps catalyzed by PCP hydroxylase are, in fact, reasonably efficient. Turnover of the enzyme is slow due to a step that occurs after the chemistry is over that is probably involved in release of the products (Rudolph and Copley, in preparation). The initial step in the degradation of PCP by S. chlorophenolicum generates a particularly toxic intermediate, tetrachlorobenzoquinone (TCBQ). We have discovered how the bacterium is protected from the toxic effects of TCBQ. A transient interaction between the enzyme that forms it (PCP hydroxylase) and the enzyme that converts it to tetrachlorohydroquinone (TCHQ) prevents release of TCBQ to the cytoplasm where it could damage small molecules and
macromolecules (Yadid et al. 2013). References Hlouchova, K, J Rudolph, JM Pietari, LS Behlen, and SD Copley. Pentachlorophenol hydroxylase, a poorly functioning enzyme required for degradation of pentachlorophenol by Sphingobium chlorophenolicum. Biochemistry. 51:3848-60. McCarthy, DL, A Claude, and SD Copley. 1997. In vivo levels of chlorinated
hydroquinones in a pentachlorophenoldegrading bacterium. Appl. Env. Microbiol. 63:1883-1888. Yadid, I, J Rudolph, K Hlouchova, and SD Copley. 2013. Sequestration of a highly reactive intermediate in an evolving pathway for degradation of pentachlorophenol. Proc. Natl. Acad. Sci. USA. 110:E2182-90.
Shelley Copley is a member of the CIRES Professor.