Kinetics of reaction between Fe(II)-ligand and c-type cytochrome under acidic conditions

YIN Yun-lu; CHENG Kuan; WANG Ying; LIU Tong-xu; LI Xiao-min

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China Environmental Science ›› 2026, Vol. 46 ›› Issue (3) : 1417-1425.

Water Pollution Control

Kinetics of reaction between Fe(II)-ligand and c-type cytochrome under acidic conditions

YIN Yun-lu¹, CHENG Kuan², WANG Ying², LIU Tong-xu², LI Xiao-min³

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Abstract

The c-type cytochrome (c-Cyts) is a key protein in microbial oxidation of Fe(II). However, the reaction process and influencing factors of ligand-bound Fe(II) with c-Cyts under acidic conditions remain unclear. In this study, Fe(II)-EDTA and a model c-type cytochrome (c-Cyts) were used to investigate the mechanism of microbial Fe(II) oxidation under acidic conditions with various environmental factors, including anoxic/oxic conditions, pH, and initial Fe(II)-EDTA concentration. A stopped-flow spectrophotometer was employed to study the reduction kinetics of c-Cyts, while the electrochemical properties of Fe(II) and the distribution of different Fe(II) species were characterized. Under anoxic conditions, the reduction rate of c-Cyts increased with increasing pH and Fe(II)-EDTA concentration. At 20μmol/L Fe(II)-EDTA, the apparent rate constants (k_app) were 0.841 and 6.802s^-1 at pH 3.5 and pH 5.0, respectively. At pH 5.0, the k_app increased to 13.028s^-1 when the initial Fe(II)-EDTA concentration increased to 80μmol/L. The increase in pH and Fe(II)-EDTA concentration enhanced the proportion of FeEDTA^2- and intensified the redox peaks of Fe(II), indicating greater Fe(II) reactivity and thus a faster c-Cyts reduction rate. Compared to anoxic conditions, c-Cyts reduction was inhibited under oxic conditions. This inhibitory effect of oxygen decreased as the pH increased. At pH 3.5 and 4.0, c-Cyts reduction was completely inhibited. At pH 4.5, the k_appdecreased from 5.402s^-1 (anoxic) to 1.658s^-1 (oxic); and at pH 5.0, the k_app slightly decreased from 6.802s^-1 (anoxic) to 6.454s^-1 (oxic). This is attributed to the slower reaction between Fe(II)-EDTA and c-Cyts at lower pH, whereas the competing reaction of Fe(II)-EDTA with oxygen is favored, producing reactive oxygen species that further accelerate the reoxidation of reduced c-Cyts. This study elucidates the critical mechanisms of the reaction between Fe(II)-ligand and c-Cyts, providing deeper molecular-level insight into the microbial Fe(II) oxidation process.

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acid conditions / Fe(II) / EDTA / cytochrome c / oxygen

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YIN Yun-lu, CHENG Kuan, WANG Ying, LIU Tong-xu, LI Xiao-min. Kinetics of reaction between Fe(II)-ligand and c-type cytochrome under acidic conditions[J]. China Environmental Science. 2026, 46(3): 1417-1425

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