1Background 2 Metabolic models are indispensable in guiding cellular engineering and in advancing our 3 understanding of systems biology. As not all enzymatic activities are fully known and/or 4 annotated, metabolic models remain incomplete, resulting in suboptimal computational analysis 5 and leading to unexpected experimental results. We posit that one major source of unaccounted 6 metabolism is promiscuous enzymatic activity. It is now well-accepted that most, if not all, 7 enzymes are promiscuous -i.e., they transform substrates other than their primary substrate. 8However, there have been no systematic analyses of genome-scale metabolic models to predict 9 putative reactions and/or metabolites that arise from enzyme promiscuity. 10
Results 11Our workflow utilizes PROXIMAL -a tool that uses reactant-product transformation patterns 12 from the KEGG database -to predict putative structural modifications due to promiscuous 13 enzymes. Using iML1515 as a model system, we first utilized a computational workflow, 14 referred to as Extended Metabolite Model Annotation (EMMA), to predict promiscuous 15 reactions catalyzed, and metabolites produced, by natively encoded enzymes in E. coli. We 16 predict hundreds of new metabolites that can be used to augment iML1515. We then validated 17 our method by comparing predicted metabolites with the Escherichia coli Metabolome Database 18 (ECMDB). 19
Conclusions 20We utilized EMMA to augment the iML1515 metabolic model to more fully reflect cellular 21 metabolic activity. This workflow uses enzyme promiscuity as basis to predict hundreds of 22 reactions and metabolites that may exist in E. coli but may have not been documented in 23 3 iML1515 or other databases. We provide detailed analysis of 23 predicted reactions and 16 1 associated metabolites. Interestingly, nine of these metabolites, which are in ECMDB, have not 2 previously been documented in any other E. coli databases. Four of the predicted reactions 3 provide putative transformations parallel to those already in iML1515. We suggest adding 4 predicted metabolites and reactions to iML1515 to create an Extended Metabolic Model (EMM) 5 for E. coli. 6 7 Keywords 8 Metabolic engineering, enzyme promiscuity, extended metabolic model, systems biology, 9 enzyme activity prediction 10 11 Background 12The engineering of metabolic networks has enabled the production of high-volume commodity 13 chemicals such as biopolymers and fuels, therapeutics, and specialty products [1][2][3][4][5]. Producing 14 such compounds requires transforming microorganisms into efficient cellular factories [6][7][8][9]. 15 Biological engineering has been aided via computational tools for constructing synthesis 16 pathways, strain optimization, elementary flux mode analysis, discovery of hierarchical 17 networked modules that elucidate function and cellular organization, and many others (e.g., [10-18 14]). These design tools rely on organism-specific metabolic models that represent cellular 19 reactions and their substrates and products. Model recons...