THERMODYNAMIC ANALYSIS OF TRINITROTOLUENE BIODEGRADATION AND MINERALIZATION PATHWAYS

Biodegradation of 2,4,6-trinitrotoluene (TNT) proceeds through several different metabolic pathways. However, the reaction steps which are c onsidered rate-controlling have not been fully determined. Glycolysis and other biological pathways contain biochemical reactions which are acutely rate-limiting due to enzyme control. These rate-limiting steps also have large negative Gibbs free energy changes. Because xenobioti c compounds such as TNT can be used by biological systems as nitrogen, carbon, and energy sources, it is likely that their degradation pathw ays also contain acutely rate-limiting steps. Identification of these rate-controlling reactions will enhance and better direct genetic engi neering techniques to increase specific enzyme levels. This article id entifies likely rate-controlling steps (or sets of steps) in reported TNT biodegradation pathways by estimating the Gibbs free energy change for each step and for the overall pathways. The biological standard G ibbs free energy change of reaction was calculated for each pathway st ep using a group contribution method specifically tailored for biomole cules. The method was also applied to hypothetical ''pathways'' constr ucted to mineralize TNT using several different microorganisms. Pathwa ys steps that have large negative Gibbs free energy changes are postul ated to be potentially rate-controlling. The microorganisms which util ize degradation pathways with the largest overall (from TNT to citrate ) negatiave Gibbs free energy changes were also determined. Such micro organisms can extract more energy from the starting substrate and are thus assumed to have a competitive advantage over other microorganisms . Results from this modeling-based research are consistent with much e xperimental work available in the literature. (C) 1996 John Wiley & So ns, Inc.