Chapter 17. Biological Transformations

Summary

Biologically mediated transformations of xenobiotic organic chemicals are important in both natural and engineered systems, since many thermodynamically feasible reactions do not occur abiotically on useful timescales. Focusing on the “microbial organic chemists”, the chief biochemical attacks used to initiate organic compound transformations (hydrolyses, oxidations, reductions, and additions) are depicted so as to facilitate recognition of the structural features of a xenobiotic compound of interest that are likely to lead to its microbial transformation under particular conditions. Various phenomena may limit the rate of microbial biotransformation; these include transfer from the environment to the cells, biouptake mechanisms, microbial growth, and enzymatic processing. In many cases, rates of biotransformation of labile and abundant organic compounds can be understood in light of Monod expressions linking the rate of microbial population growth to the use of the compound as a growth-limiting substrate. In others cases involving recalcitrant and/or trace organic chemicals, the rate of biotransformation can be understood using Michaelis-Menten models of enzymatic processes that are responsible for the initial transformation of the compound. Such cases are sometimes called co-metabolism of the xenobiotic compound since they involve incidental use of enzymatic systems naturally present to transform other substrates like methane.