Biodegradation of 2,4,6-trinitrotoluene (TNT): An enzymatic perspective

Enzymatic degradation of TNT by aerobic bacteria is mediated by oxygen insensitive (Type 1) or by oxygen sensitive nitroreductases (Type II nitroreductases). Transformation by Type I nitroreductases proceeds through two successive electron reductions either by hydride addition to the aromatic ring or by direct nitro group reduction following a ping pong kinetic mechanism. TNT is reduced to the level of hydroxylaminodinitrotoluenes and aminodinitrotoluenes by pure enzyme preparations without achieving mineralization. Interestingly, database gene and amino acid sequence comparisons of nitroreductases reveal a close relationship among all enzymes involved in TNT transformation. They are all flavoproteins which use NADPH/NADH as electron donor and reduce a wide range of electrophilic xenobiotics. TNT degradation by fungi is initiated by mycelia bound nitroreductases which reduce TNT to hydroxylaminodinitrotoluenes and aminodinitrotoluenes. Further degradation of these products and mineralization is achieved through the activity of oxidative enzymes especially lignin degrading enzymes (lignin and manganese peroxidases).