Abstract
BackgroundDespite the chloride anion is involved in fundamental biological processes, its interactions with proteins are little known. In particular, we lack a systematic survey of its coordination spheres.ResultsThe analysis of a non-redundant set (pairwise sequence identity?<?30%) of 1739 high resolution (<2 Å) crystal structures that contain at least one chloride anion shows that the first coordination spheres of the chlorides are essentially constituted by hydrogen bond donors. Amongst the side-chains positively charged, arginine interacts with chlorides much more frequently than lysine. Although the most common coordination number is 4, the coordination stereochemistry is closer to the expected geometry when the coordination number is 5, suggesting that this is the coordination number towards which the chlorides tend when they interact with proteins.ConclusionsThe results of these analyses are useful in interpreting, describing, and validating new protein crystal structures that contain chloride anions.
Highlights
Despite the chloride anion is involved in fundamental biological processes, its interactions with proteins are little known
Chloride is essential to maintain cellular and whole body pH, which is mainly buffered by the CO2/ HCO3− equilibrium, since the exchange of bicarbonate across the plasma membrane is coupled with chloride exchange [2]
The chloride anions have 29 Å2 exposed to the solvent, which means about 22% of their surface
Summary
Despite the chloride anion is involved in fundamental biological processes, its interactions with proteins are little known. It is sufficient to open any biochemistry or bioinorganic chemistry book to verify that, chloride is essential for any form of life, its biological chemistry receives less attention than other small ions like for example sodium(I), calcium(II) or magnesium(II). A well-known channel with specificity for anions, in particular chloride and bicarbonate, is the cystic fibrosis transmembrane conductance regulator [3]. For example in many amylases, a chloride anion coordinated by arginine and lysine side-chains is bound close the active site, where it may assist the reaction [4]. In photosystem II two chloride anions have been identified close to the Mn4CaO6 cluster and may contribute to it stability/reactivity [5,6].
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