Methods and mechanisms of the interactions between biomacromolecules and heavy metals

Heavy metal pollution is a serious threat to both human health and the economic development of China. In soil environments, microbial derived organic substances contribute more than 50% of soil organic matter, which controls the migration and transformation of heavy metals. Over time, the microorganisms have developed a range of resistance and detoxification strategies to deal with the toxicity of heavy metals. Microorganisms play important roles in pollution remediation, hence, a comprehensive understanding of the underlying interaction mechanisms between biomacromolecules and heavy metals is fundamental for the development of microbial remediation methods. In recent years, the application of spectroscopic, thermodynamic, and omics approaches has expanded our understanding of the binding strength, coordination structure, and redox mechanisms of heavy metals and biomacromolecules. X-ray absorption fine structure (XAFS) and X-ray emission spectroscopy (XES) measured the valence state, coordination atom, and the structure of heavy metals in biomacromolecules. Three-dimensional excitation-emission matrix (3D-EEM), Fourier transfonn infrared spectroscopy (FTIR) and Raman spectroscopy detected the reactive groups in the biomacromolecules through changes in the fluorescent and vibrational spectrum. The thermodynamic information for the reactions between biomacromolecules and heavy metals was detected using isothermal titration calorimetry (ITC) and the surface complexation model (SCM). Most of the models assumed the interactions between carboxyl, phosphoryl, sulfhydryl groups, and heavy metal ions through a monodentate complex. The synthesis and secretion of extracellular polymeric substance (EPS) is the universal mechanism through which microorganisms resist heavy metal stress, though the composition of EPS varied among microorganisms and heavy metals. Furthermore, the microorganisms have evolved functional proteins to fix and detoxify heavy metals, mainly through the sulfhydryl groups in metallothionein, flagellin, and amyloid. The hemiacetal groups, c-type cytochromes, and related functional sites in biomacromolecules promoted the redox of As, Cr, Cu, Ag, and Hg. Meanwhile, the microorganisms regulate the redox processes through a variety of genetically encoded pathways. These indepth understanding of the interactions between biomacromolecule and heavy metal is essential for the treatment of heavy metal pollution using microbial resources. Despite the salient accomplishments cited above, there is a significant knowledge gap regarding the molecular level interactions between microorganisms and heavy metals. The structure and physiological regulation of the functional biomolecules involved in the stabilization and transformation of heavy metals are still unclear. Additionally, the behavior of heavy metals in combined pollution systems and the role of biofilms in their transformation is also not clear. Furthermore, the majority of available literature mainly adopted methodologies from either the chemical or biological field, and the absence of interdisciplinary studies may account for these knowledge gaps. Therefore, it is important to combine the biological and chemical technologies and focus on the interactions between the microbial community, function, and heavy metals in composite biofilms. It is paramount to develop methods for the extraction and characterization of the biomolecules and to investigate their electron transfer and complexation reactions with heavy metals. More attention should be given to combined pollution systems and the interactions between metal-small molecular complexes and biomacromolecules. These studies will promote the understanding of the interactions between heavy metals and functional biomolecules at the molecular level, and improve the effectiveness of microbial remediation for Ixavy metal pollution. Overall, this review summarizes the latest research methodologies and the underlying interaction mechanisms between biomacromolecules and heavy metals and suggests the directions for further research regarding the methodology and systems. The integration of the methodologies, innovation, and a vivid comprehension of the underlying mechanisms will contribute significantly to the development of microbial remediation strategies for heavy metal pollution in water and soil.