4.2. Arsenic binding to specific proteins

## 4.2.1. Arsenic binding to hemoglobin

Arsenic species are cleared from the blood immediately in humans, but the time of clearance of arsenic from animal species varies noticeably. The retention of arsenic in rat blood is longer when compared with other species. Arsenic has been found to bind to transferrin in hemodialysis patients [39]. Hemoglobin in red blood cells (RBCs) were predicted to be the sites of arsenic accumulation, because hemoglobin constitutes 97% of dry weight of RBCs [40]. The affinity of hemoglobin in rat liver is much higher in rats as compared with humans. The rat and human hemoglobins are tetramers, each consisting of two α-chains and two β-chains. The difference lies in the number of the cysteine residues with rat hemoglobin consisting of three cysteines (Cys111, Cys104 and Cys13) in α-chain, while two cysteines (Cys125 and Cys93) in βchain. On the other hand, human hemoglobin has only one cysteine in α-chain and two cysteines in β-chain [3].

4.2.4. Arsenic binding to ArsD As(III) metallochaperone

basal rate of ATPase activity is shown by ArsA [44].

4.2.5. Arsenic binding to other proteins

of arsenic for cysteine residues.

Arsenic being the most common toxic element in the environment has resulted in the evolution of arsenic detoxifying mechanisms in nearly all organisms. In archaea and bacteria, trivalent metalloids like arsenite are pumped out of the cell by ArsAB ATPases encoded by various ars operons. Three conserved cysteine residues (Cys12, Cys13 and Cys18) are required for the chaperone activity of ArsD. ArsD also helps to increase the arsenite affinity of Ars A enabling the detoxification of arsenite, even at low concentrations. In the case of ArsA, there are two cysteines (Cys113 and Cys422) in the high affinity metalloid-binding site along with the third cysteine that participates in activation of ATP hydrolysis. In the absence of arsenite, a low

Mechanisms of Arsenic-Induced Toxicity with Special Emphasis on Arsenic-Binding Proteins

http://dx.doi.org/10.5772/intechopen.74758

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Trivalent arsenic species are also known to bind to other proteins like actin, tubulin, estrogen receptor and glucocorticoid receptors. Arsenite can bind to Kelch-like ECH-associated protein 1 (KEAP 1). This is a major antioxidant-sensing protein which acts at low Kd values. One of the most common motifs present in many proteins consists of two cysteine residues separated by two amino acids (CXXC). The presence of cysteine residues increases with increasing complexity of the organisms, making humans vulnerable to arsenic toxicity because of the high affinity

One such important cell surface protein consisting of highly conserved cysteine residues is connexin 43 (Cx43)—a widely expressed gap junctional protein important for cell death, proliferation and differentiation [45]. Cx43 has nine Cys residues, six of which are in the extracellular domain and three in the intracellular domain. Six connexin monomers form a hemichannel called connexin. In the plasma membrane, one connexin can dock to another connexin in the plasma membrane of an adjacent cell resulting in the formation of complete gap junction channel. A hemichannel formed by single type of connexin isoforms is called homomeric hemichannel or consists of multiple types of isoforms called heteromeric hemichannel. Two identical homomeric or heteromeric hemichannels dock to form a homotypic channel, and two different homomeric or heteromeric hemichannels dock to form a heterotypic channel (Figure 6). Recent in silico studies (Hussain et al., manuscript communicated) in combination with cellular and biochemical analysis revealed insights into the binding modes of arsenite to conserved Cys groups in Cx43. In Cx43, As+3 can be bound to three cysteines in the intracellular domain in a monovalent fashion as they are free cys, while it can bind the extracellular domain cysteines in either monovalent, divalent or trivalent fashion depending on the state and location of the protein in the cell. Arsenite ion (As+3) can attack the free sulfhydryl group until all the valencies of the As+3 are satisfied by covalent bonding to the sulfur from the cys residues. This profoundly affects the Cx43 primary, secondary, tertiary and the quaternary structure. This study is the first of its kind which shows that arsenic can directly bind to Cx43 via its highly conserved cysteine residues causing misfolding of Cx43, which leads to alteration of transportation, localization and oligomerization of Cx43. Further experiments revealed that Cx43 was colocalizing with ER marker (calnexin), revealing the inability of Cx43 to be transported beyond endoplasmic reticulum/endoplasmic reticulum
