The biopolymer in activated sludge plays an important role in the removal of heavy metals, and it can also be extracted to serve as biosorbent for heavy metals. It is of big interest to understand the key constituents and feature of the biopolymer. In this study, the biopolymers with quite different proportions of constituents were extracted from activated sludge after the conditioning by cetyl trimethyl ammonium bromide (CTAB) and linear alkylbenzene sulfonate (LAS), and then were used to test their adsorbability for metal ions. CTAB was prominent in increasing proteins (PN) content, and LAS was prominent in increasing polysaccharides (PS) and nucleic acids (NA) content. The biopolymers with different proportions of PN, PS and NA showed different adsorption capacities for Cu2+ (ranging from 258.94 to 389.92 mg/g). Their adsorption capacities significantly correlated with proteins (R-2 = 0.9225, P = 0.00), proteins/polysaccharides (R-2 = 0.9797, P = 0.00), polysaccharides (R-2 = 0.6739, P = 0.01), and nucleic acids (R-2 = 0.6960, P = 0.00). The adsorption capacities showed significant positive correlation with proteins and proteins/polysaccharides, but negative correlation with polysaccharides and nucleic acids. Proteins and high proteins/polysaccharides were identified to be the key constituents and feature of the biopolymer responsible for binding heavy metals, respectively. The findings denote that all biomass with high protein content could be an excellent adsorbent for metal ions. (C) 2016 Elsevier B.V. All rights reserved.