title: Metabolic flux-based modularity using shortest retroactive distances.
abstract: BACKGROUND
  Graph-based modularity analysis has emerged as an important tool to study the functional organization of biological networks. However, few methods are available to study state-dependent changes in network modularity using biological activity data. We develop a weighting scheme, based on metabolic flux data, to adjust the interaction distances in a reaction-centric graph model of a metabolic network. The weighting scheme was combined with a hierarchical module assignment algorithm featuring the preservation of metabolic cycles to examine the effects of cellular differentiation and enzyme inhibitions on the functional organization of adipocyte metabolism.
  
  
  RESULTS
  Our analysis found that the differences between various metabolic states primarily involved the assignment of two specific reactions in fatty acid synthesis and glycerogenesis. Our analysis also identified cyclical interactions between reactions that are robust with respect to metabolic state, suggesting possible co-regulation. Comparisons based on cyclical interaction distances between reaction pairs suggest that the modular organization of adipocyte metabolism is stable with respect to the inhibition of an enzyme, whereas a major physiological change such as cellular differentiation leads to a more substantial reorganization.
  
  
  CONCLUSIONS
  Taken together, our results support the notion that network modularity is influenced by both the connectivity of the network's components as well as the relative engagements of the connections.
authors.affiliation: Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Room 150, Medford, MA 02155, USA.
authors.forename: Gautham Vivek
authors.initials: GV
authors.lastname: Sridharan
authors.forename: Michael
authors.initials: M
authors.lastname: Yi
authors.forename: Soha
authors.initials: S
authors.lastname: Hassoun
authors.forename: Kyongbum
authors.initials: K
authors.lastname: Lee
mesh_terms: D017667:Adipocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D019295:Computational Biology / Q000379:methods*; D003196:Computer Graphics; D004791:Enzyme Inhibitors / Q000494:pharmacology; D053858:Metabolic Networks and Pathways / Q000187:drug effects; D008954:Models, Biological
chemical_list: D004791:Enzyme Inhibitors